ICEA/NEMA Standards Summary:The National Electrical Manufacturers Association (NEMA) publishes the ICEA wire and cable standards applications, and some of the new standards carry NEMA numbers also. New ICEA/NEMA Standards:ICEA S-93-639/NEMA WC74 Shielded Power Cable 5-46 kVICEA S-94-649 Concentric neutral cables 5-46 kVICEA S-95-658/NEMA WC70 Nonshielded 0-2kV CablesICEA S-96-659/NEMA WC71 Nonshielded 2001-5kV CablesICEA S-97-682 Copper tape shield, LCT, or drain-wire shielded cables 5-46 kV Withdrawn ICEA/NEMA Standards:ICEA S-19-81/NEMA WC3 Rubber-Insulated Wire and CableICEA S-61-402/NEMA WC5 Thermoplastic-Insulated Wire and CableICEA S-66-524/NEMA WC7 Cross-Linked-Thermosetting-Polyethylene Insulated Wire and CableICEA S-68-516/NEMA WC8 Ethylene-Propylene-Rubber Insulated Wire and Cable

ICEA Changes Cable Standard Strategy - January/February 2002

The winds of change are blowing. For years, cable buyers have called out Insulated Cable Engineers Association

(ICEA) standards to specify power and control cables. Now ICEA is replacing those familiar standards, along with

electric utility standards that have also been used for many medium voltage industrial applications.

"If you use references to ICEA standards to buy MV power cable, you'll need to update your specifications," says

Dave Mercier, technical director of Southwire's Electrical Division.

New standards are organized differently

ICEA standards include requirements for conductors, insulation, coverings, and construction details for wires and

cables. Buyers also call out ICEA standards to specify requirements such as dc voltages for field tests, emergency

overloads and minimum bending radii.

The key difference in the new ICEA standards is the way they are organized. The old organization called out separate

standards for polyethylene, ethylene propylene rubber and other insulation materials. The new standards are

organized by application. For example, a single document now covers 5kV - 46kV shielded power cables. "The

advantage is that all insulations suitable for a specific application are now in a single document," Mercier says.

The new ICEA standards also reflect the latest cable construction technologies, including improvements in the ability

to manufacture "round" cables. The new standards define insulation thickness and eccentricity by minimum and

maximum allowable values, which assures a more "round' cable than the old standards. The old nominal average

thicknesses - such as 220 mils - are used only as a reference and for cable identification.

AEIC supplements change also

Many industrial cable specification writers also refer to AEIC cable specifications for the qualification testing those

standards require. Because the new ICEA standards now include some qualification tests, AEIC has rewritten their

standards to supplement the new ICEA standards. A single new specification - AEIC CS-8 - replaces AEIC CS-5 for

XLPE and AEIC CS-6 for EPR insulation.

"If your specifications include CS-8 qualification testing, word the reference to make it clear that the only applicable

part of the CS-8 standard is the qualification-testing portion," Mercier says.

The minimum requirement when using cable under the NEC remains UL 1072, "Safety Standard for Medium-Voltage

Power Cables." The third edition of UL 1072 will incorporate the majority of the changes in the ICEA standards.

"Southwire has always been active in the industry's standards process," says Mercier. "We've participated in

development of the new ICEA standards, we're involved in the ongoing NEC process. It's all part of bringing the best

wire and cable products to our customers."

IEC 60287 "Calculation of the continuous current rating of cables (100% load factor)" is the International

Standard which defines the procedures and equations to be used in determining the current carry

capacity of cable.  The standard is applicable to all alternating current voltages and direct current cables

up to 5kV.

This note will introduce the concepts adopted by the standard, provide some guidance on using the

standard and direct the reader to further resources.

Contents  [hide]

1. Thermal Problem

2. The Standard in More Detail

3. Applying the Standard

4. Other Related Resources

5. Summary

Thermal Problem

Principle- simple wire in homogeneous material The methodology taken to the sizing of cables is that of treating the issue as a thermal problem. 

Losses within a cable will create heat.  Depending on the installation conditions this heat will be

dissipated to the surrounding environment at a given rate.  As the cable heats up rate of heat dissipation

will increase. 

At some temperature the rate at which heat is being dissipated to the environment will be the same as the

rate at which it is generated (due to loses).  The cable is then in thermal equilibrium.

The losses (and heat generated) are dependent on the amount of current flowing within the cable.  As the

current increases the losses increase and the thermal equilibrium temperature of the cable will increase. 

At some given current level, the cable temperature at thermal equilibrium will equal the maximum

allowable temperature for the cable insulation.  This is the maximum current carrying capacity of the cable

for the installation conditions depicted by the calculation. 

To illustrate the principle, we can consider a simplistic scenario of a d.c. cable (as shown in the

illustration), surrounded with an insulating material and placed in a homogeneous thermal conducting

material. 

Given:

I - conductor current, A 

R' - d.c. resistance of the conductor per unit length, Ω/m

Θ - maximum conductor operating temperature, °C 

Θa - ambient temperature, °C 

ΔΘ - temperature difference (Θ-Θa), K

T - thermal resistance per unit length between conductor and surrounding, K.m/W

The losses (watts per unit length) generated by the conductor is given by:

The heat flow (watts per unit length) from the conductor is given by:

At thermal equilibrium these will be equal and can be rearranged to give the cable current carrying

capacity (in Ampere):

As an example, consider finding the current carrying capacity of a 50 mm2 conductor, with XPLE

insulation directly buried (with an insulation thermal resistance of 5.88 K.m/W and soil thermal thermal

resistance of 2.5 K.m/W)  and at an ambient temperature  of 25 °C

by using the related resources links given at the end of the posts, we are able to find the

following:

the dc resistance of the cable is 0.387 mΩ/m

the maximum allowable temperature for XLPE insulation is 90 °C

and a total thermal resistance of 5.88+2.5 = 8.38 (insulation, plus soil)

ΔΘ = 90-25 = 65 K, giving

I = √ [65/(0.000387*8.38)] = 142 A

The Standard in More Detail

Applying the IEC 60287 Standard (click to enlarge) The reality of any cable installation is more complex than described above.  Insulating materials have dielectric losses, alternating current introduces skin effect, sheath and eddy current losses, several cables are simultaneously producing heat and the surrounding materials are non-homogeneous and have boundary temperature conditions.

While the standard addresses each of these issues, the resulting equations are more complex do take

some effort to solve.  Anyone attempting to apply this method should be working directly from a copy of

the standard.  As an overview, the standard looks at the following situations:

differences between alternating and direct current systems in calculating cable

capacity

critical temperatures of soil and possible requirements to avoid drying out the soil

cables directly exposed to solar radiation

calculation of the a.c. and d.c. resistance of conductors (including skin effect,

proximity effect and operating temperature)

insulation dielectric losses

conductor I2R losses

losses in sheaths and screens (including flat, trefoil and transposed formations)

circulating current losses (including sheath, armour and pipes)

thermal resistance (and it's calculation)

Each of these areas is discussed in more detail in the following posts (which together form a

comprehensive guide to the standard):

The dc resistance of conductors  - calculation of the d.c. resistance of cables

The ac resistance of conductors  - calculation of the a.c. resistance of cables

Dielectric loss in cables  – calculation of dielectric losses

Cable Sheath and Armour Loss  – calculation of sheath and armour losses

IEC 60287 Current Capacity of Cables  - rated current

Applying the StandardWithin the standard there are a lot of equations and it can be confusing to persons who are new to the

method.  However a step by step working through it approach will enable the current carrying capacity to

be calculated.  The flow chart shows one recommended path for working through a cable sizing exercise

in line with the standard.

Given the number of equations which need to be solved, it is tedious to calculate in accordance with the

standard by using hand or manual methods.  More practically software applications are used, which allow

the sizing of cables to take place quickly. A quick Google search will turn up several software programs

capable of performing the calculation.

Tip:  a cable run can move through different installation environments (for example it may

start in a cable basement, more through ducts in a wall, be buried for some of the route,

suspended under a bridge, buried again, through ducts and into the receiving building).  In

this instance the current capacity should be evaluated for each type of installation condition

and the worse case taken.

Other Related ResourcesCable Sizing Tool  - describes the procedure for the sizing of cable to BS 7671 and IEC 60364

Standard Cable & Wire Sizes  - list of standard IEC 60228  wire sized and AWG conversion table

8   Steps to Low Voltage Power Cable Selection and Sizing  - general guide to selecting/using LV

cables

Cable Insulation Properties - typical properties of various types of cable insulation

SummaryWithin the note the IEC 60287 have been introduces and the problem of finding the current capacity of a

cable boiled down to that of a thermal calculation.  The note has given an overview of the contents of the

standard, ways to navigate and perform the calculation and provided links to more detailed posts.

Hopefully the note has achieved the objective of providing an introduction to the current capacity sizing

methods of IEC 60287.  If you have any comments or something is not clear enough, please post these

below.

- See more at: http://myelectrical.com/notes/entryid/207/iec-60287-current-capacity-of-cables-an-introduction#sthash.AyqqPb27.dpuf

IEC 60287 Current Capacity of Cables - Rated Current By Steven McFadyen on February 18th, 2013 

This note looks at the formulae used to calculate the rated current capacity of a cable in line with IEC

60287 "Calculation of the continuous current rating of cables (100% load factor)".   Before you continue

reading this note, if you have not done so already we would suggest first reading our IEC 60287

introduction note:

      IEC 60287 Current Capacity of Cables - An Introduction

In the previous note we looked at the approach taken by the standard to the sizing of cables and

illustrated this with an example.  We then looked at one method of applying the standard and identified

resources enabling the calculation of all the various parameters involved.   In the note we are going to put

everything together and reveal the necessary equations for actually calculating the cable maximum

current rating.

The image illustrates  the thermal model for a cable. 

 

Heat is generated within the cable by various mechanisms -  conductor I2R loss, dielectric loss, sheath

loss, armour loss and direct solar radiation.  Some or all of this heat is dissipated through the cable

insulation, bedding, serving and into the surrounding medium.  The rate of heat flow is related to the

temperature difference across the cable and affected by the ambient temperature, temperature rises due

to other cables and any critical temperature rise of the soil above ambient.

In thermal equilibrium, when all these factors have balanced and the temperature of the conductor is the

maximum allowable for the insulation; we have the maximum rated current for the cable.  It can probably

be appreciated by now, that in typical real life situations, this can be quite a complicated calculation.

Note: rated current capacity found by the method assumes that the cable is fully loaded for

100% of it's operation time.  For cables which have varying or cyclic loads, the current

rating could possibly be increased.

Tip: the thermal model is worth remembering as enables us to intuitively understand how a

cable is likely to behave in conditions which are not normally encountered.  For example, if

a cable is run along a refrigerated gas pipe, we can hazard that this will reduce the ΔΘ and

hence the cable will be able to carry more current. 

Change in temperature across a material is equal to the heat input multiplied by the thermal resistance of

the material.  In terms of the thermal model and for a simple a.c. cable, the heat balance equation is given

by (a list of symbols is given at the end of the note):

Within the standard this above is used to derive the equations for current rating.  It is simplified for d.c.

cables by eliminating any a.c. only effects and modified for partial drying of soil and solar radiation where

appropriate.

Contents  [hide]

1. Rated Current of Cables

2. List of Symbols

Rated Current of CablesThe standard gives the following equations for the calculation of the cable current rating (for all alternating

current voltages and direct current up to 5 kV):

Buried cables where drying out of the soil does not occur or cables in air

AC cables

DC cables

Buried cables where partial drying-out of the soil occurs

AC cables

I=[Δθ−Wd[0.5T1+n(T2+T3+vT4)]+(v−1)ΔθXR[T1+n(1+λ1)T2+n(1+λ1+λ2)(T3+vT4)]]0.5

 

DC cables

I=[Δθ+(v−1)ΔθXR′[T1+nT2+n(T3+vT4)]]0.5

Buried cables where drying-out of the soil is to be avoided

AC cables

I=[Δθx−nWdT4nRT4(1+λ1+λ2)]0.5

DC cables

Cables directly exposed to solar radiation

AC cables

DC cables

Note: when calculating a cable where some drying of the soil may occur, it is also

necessary to perform the calculation for no drying out of soil and take the worse case

(lower) rating.

The calculation of each element needs some explanation and these have been split across several notes

(with each note dealing with one topic).  For details, please refer to the first note in this series, which

lists the other related notes. 

List of Symbolsn - number of load carrying conductors 

v - ratio of thermal resistivity of dry and moist soils 

I - rated conductor current, A 

R - a.c. resistance of the conductor per unit length,  Ω/m 

R' - d.c. resistance of the conductor per unit length, Ω/m    

T1 - thermal resistance per core between conductor and sheath, K.m/W 

T2 - thermal resistance between sheath and armour, K.m/W 

T3 - thermal resistance of external serving, K.m/W 

T4 - thermal resistance of surrounding medium, K.m/W 

T4* - external thermal resistance (free air) adjusted for solar radiation, K.m/W 

De* - cable diameter over insulation, m 

H - intensity of solar radiation, W/m2 

Wd - dielectric loss per units length, W/m 

λ1 - ratio of losses in metal sheath to total losses in all conductors 

λ2 - ratio of losses in armouring to total losses in all conductors 

σ - absorption coefficient of solar radiation for cable surface 

Θ - maximum conductor operating temperature, °C  

Θa - ambient temperature, °C  

ΔΘ - temperature difference (Θ-Θa), K 

ΔΘx - critical temperature of soil, °C- See more at: http://myelectrical.com/notes/entryid/210/iec-60287-current-capacity-of-cables-capacity-equations#sthash.JdE1IgQN.dpuf

By Steven McFadyen on July 24th, 2011 

A recurring theme on our forums is cable sizing.  Now many installations are unique and require special consideration.  However,  a lot of the time things are just repeated.  When looking at low voltage power cables I generally always start with the same basic strategy. 

1. Default to using XLPE - why bother  with other insulations (PVC, rubber, etc.).  XLPE is

well established, cost competitive and doesn't have any of the degradation or fire

related issues of other insulations.  You will also end up with a smaller cross sectional

area.  Only in special circumstances would you need to look at other installation

types. 

 

2. Use armoured - buried cable mechanical protection is essential.  For indoor cables the

use of armouring is  not essential, however you have the benefit of using the

armouring for the CPC.  On indoor cables, perhaps make the choice of armoured or

not  dependant on local practice. 

 

3. Use LSZH (low smoke zero halogen) sheath - smoke and toxic fumes in a fire situation

are not good.  Why not just avoid  the issue. 

      

4. Calculate the current rating using an acceptable method.  I tend to use the method

given in BS 7671 as this is generally applicable where I work.  Calculate the rating

taking into account both the design current and protective device rating and apply the

necessary derating factors. 

 

5. Calculate the voltage drop - again I tend to use BS 7671 and check it complies with

local regulations.  The voltage drop needs to be the sum of all cables in a circuit (from

source to end load). 

 

6. Ensure the cable can take the fault level - for most larger cables this tends not to be a

problem, but for smaller cables it can be an issue. 

 

7. Use software - if possible use approved software to do items 4 to 5.  It makes life

easier.  Two which come to mind are the myElectrical cable sizing tool and AMTECH. 

 

8. Be practical - make sure your cable size is reasonable.  If you end up with a 120

mm2 cable on a 2 A load due to meeting voltage drops or fault levels start to look a the

system design concept itself.

while you cannot say "once you have selected one cable you have selected all cables',  you

may be able to get away with saying "once you have selected a few cables you have

selected most cables"

Finally we need a disclaimer here.  While the above is good for most situations (low voltage power), it

does not cover every case.  There are situations which are different, unique or require some special

consideration.   To address these situations, one of the best things is to understand fully the

characteristics of the load the cable will be supplying, the environment it is being installed in and be aware

of other overriding issues.  If you can do this,  any necessary adjustments to the eight point plan often

become obvious.

- See more at: http://myelectrical.com/notes/entryid/73/8-steps-to-low-voltage-power-cable-selection-and-sizing#sthash.ulcd8Vk2.dpuf

Calculation of Sheath and Armour LossAny cable sheath (or screen) the loss λ1, consists of two components:

 

λ1=λ1''+λ1''

λ1' - losses caused by circulating currents.  These losses only occur in single core

cables and for any circulating current to be present, it is necessary to the sheaths of

each cable to be bonded at two or more points along its length.

λ1''  - losses caused by eddy currents.  These are small circulating currents setup in

the sheath due to changing magnetic fields.

The loss in armour is considered as only one component, λ2.

Sheath and armour losses are only applicable to alternating current (a.c.) cables.  The actual formula for

calculation of sheath and armour loss depend on the installation and arrangement of cables.  The tables

below presents some of the common installation situations and are based on equations given in IEC

60287:

Calculation of sheath or screen loss in - Single Core CablesFor installations bonded only at one point, circulating currents are not possible and the loss is zero.

Except in the case of large segmental type conductors (see Some Special Cases below), eddy current

loss λ1'', for single core cables can be ignored.

     Sheath Circulating Current Loss, λ1' 

Single core cables 

- trefoil, bonded at both ends  

 

λ1′=RSR11+(RsX)2

Single core cables 

- flat, with transposition,

bonded at both ends 

 

λ1′=RSR11+(RsX)2

Single core cables 

- flat, without transposition,

bonded at both ends

λ11′ - loss factor for the outer cable with the greater losses 

    

λ11′=RsR[0.75P2Rs2+P2+0.25Q2Rs2+Q2+2RsPQXm3√(Rs2+

P2)(Rs2+P2)]

λ12′ - loss factor for the outer cable with the least losses 

 

λ12′=RsR[0.75P2Rs2+P2+0.25Q2Rs2+Q2−2RsPQXm3√(Rs2+

P2)(Rs2+P2)]

 

λ1m′ - loss factor for the middle cable 

 

λ1m′=RsRQ2Rs2+Q2

 

where:  

 

P=X+Xm

 

 

Q=X+Xm3

 

Calculation of sheath or screen loss in - Multi-Core CablesDue to any sheath or screen surrounding all cores, the possibility of circulating current does not exist, and

the λ1' loss can be ignored.  Eddy current loss, λ1'' does need to be considered.

    Sheath Eddy Current Loss, λ1'' 

Two core cable - common sheath,

unarmoured 

  - for round or oval conductors 

 

λ1''=16ω210−14RRs(cd)[1+(cd)2]

 

- for sector shaped conductors 

 

λ1''=10.8ω210−16RRs(1.48r1+td)[12.2+(1.48r1+td)2]

Three core cable - common sheath,

unarmoured

- round or oval conductors, Rs ≤ 100 µΩ.m-1 

 

λ1''=3RsR⎡⎣⎢⎢(2cd)211+(Rsω107)2+(2cd)411+4(Rsω107)2⎤⎦⎥⎥

- round or oval conductors, Rs >100 µΩ.m-1 

 

λ1''=3.2ω2RRs(2cd)210−14

- for sector shaped conductors (any Rs) 

 

λ1''=0.94RsR(2r1+td)211+(Rsω107)2

Two or three core cable - steel tape armour Multiple the unarmoured cable factor by: 

  ⎡⎣⎢1+(ddA)211+dAμδ⎤⎦⎥2

Cables with each core in a separate sheath or

pipe type cables 

 

λ1''=RsR1.51+(RsXSL)

 

where: 

 

XSL=2ω10−7ln(2cd)

 

Calculation of armour lossFor armoured cables, the losses are estimated as shown.

   Armour Loss, λ2  

Non-magnetic armour  Use equation for λ1'', substituting:

parallel combination of sheath and armour resistance for Rs

root mean square of sheath and armour diameter for d

Single core cables - steel

wire armour 

General advice is not to use magnetic armour for single core cables.  

If required, then the guidelines given in IEC 60287 on estimating losses

should be followed. 

Two core cable - steel  

wire armour λ2=0.62ω210−14RRA+3.82Aω10−5R[1.48r1+tdA2+95.7A]2

Three core cable - steel

wire armour

- round conductor  

 

λ2=1.23RAR(2cdA)21(2.77RA106ω)2+1

 

- sector shaped conductor 

 

λ2=0.358RAR(2r1dA)21(2.77RA106ω)2+1

 

Calculating the ParametersSheath (Rs) or armour (RA) resistance  - values used above are calculated at their operating

temperature.  The operating temperature (in °C) and resistance can be determined from:

 

θsc=θ−(I2R+0.5Wd)×T1

    - for any sheath

 

θar=θ−{(I2R+0.5Wd)×T1+[I2R(1+λ1)+Wd]×nT2}

      - for any armour

 

Rs=Rs20[1+α20(θsc−20)]

    - for the cable sheath

 

RA=RA20[1+α20(θar−20)]

    - for the cable armour

 

Note: for calculation of the dielectric loss Wd, refer to our Dielectric loss in cables note.Cable Reactance - for single core cables, where there is significant spacing between conductors, it is necessary to use the reactance in the calculating of circulating current loss.  Accurate values for reactance can be obtained from cable manufacturers or by using software.  Alternatively, the following equations can be used to estimate the reactance (Ω.m-1):

Single core cable reactance estimates (assume bonded at both ends)

 

X=2ω10−7ln(2sd)

    - trefoil or flat without transposition

 

X=2ω10−7ln(232√sd)

  - flat with transposition

   

Xm=2ω10−7ln(2)

-  mutual reactance of flat formation cables

Steel tape armour resistance - depending on how steel tape is wound, the resistance can be estimated as follows:

1. tapes laid longitudinally - calculate the resistance as that of an equivalent cylinder

(same mass and diameter)

2. tapes laid ≈54° to cable axis - use twice the value obtained from (1)

3. tapes with a very short lay - take resistance as infinite (neglect losses)

4. tapes with a very short lay (double layered) - use twice the value obtained form (1)

Cable TranspositionTransposing of cables (see image) is a technique to reduce the circulating currents within cable sheaths

and consequently increase the rating of the cable.

 

Transposed and cross bonded cable

By cross bonding the sheath the induced currents are in opposite directions, cancelling each other out

and significantly improving the current rating of the cable.  Transposing the cables ensures that the

reactance balance out and aids in implementation.

At intermediate transposition points, over voltage devices are installed to protect the cable and personnel

in the event of voltage build up during faults.

In practice, three minor sections (part between the cross bond) would from a major section (three full

transpositions).  It makes sense to do these at each joint point - at each cable drum length.

Transposition and cross bonding are normally carried out in link boxes.

Some Special Situations

Large segmental type conductorsEddy current losses λ1'', are normally small relative to other losses and can be ignored for single core

cables.  This changes for large conductors, which are of a segmented construction.  Under these

conditions, the eddy current loss should be considered.

For this condition, the value of λ1'' is derived from the circulating current loss factor λ1' by:

 

λ1''=λ1′×4M2N2+(M+N)24(M2+1)(N2+1)

where:

 

M=N=RsX

    - for cables in trefoil

 

M=RsX+Xn   and  N=RsX−Xm3

    - for cable in flat formation

Single core cables - variation of route spacingIf the spacing if not maintained the same for the full cable route than the reactance will vary along the

route.  In instances such as these, an equivalent overall reactance can be calculated from:

 

X=laXa+lbXb+…+lnXnla+lb+…+ln

- where la,  lb, ... are the section lengths and Xa, Xb, ... are the reactance of each section

SymbolsA - armour cross sectional area, mm2

R - conductor a.c. resistance, Ω.m-1

RA - armour resistance maximum at operating temperature, Ω.m-1

RA20

- armour resistance at 20 °C, Ω.m-1

Rs - sheath or screen resistance at maximum operating temperature, Ω.m-1

Rs20

- sheath or screen resistance at 20 °C, Ω.m-1

X - sheath or screen reactance, Ω.m-1

Xm - mutual reactance (sheath one cable to conductors of other cables), Ω.m-1

λ1

- ratio of sheath loss to total conductor loss

λ2

- ratio of armour loss to total conductor loss

λ1' - sheath loss caused by circulating currents

λ1''

- sheath loss caused by eddy currents

c - distance between axis of conductors, mm

d - mean diameter of sheath or screen, mm

dA

- mean diameter of armour, mm

r1 - circumscribing radius of sector shaped conductors, mm

s - axial separation of conductors, mm

t - insulation thickness between conductors, mm

T1

- thermal resistance between conductor and sheath, K.m.W-1

T2

- thermal resistance between sheath and armour, K.m.W-1

θ - maximum conductor temperature, °C

θar

- maximum operating temperature of armour, °C

θsc

- maximum operating temperature of screen, °C

ω - angular frequency (2πf)

µ - relative magnetic permeability of armour

δ - equivalent thickness of armour, mm

See Also- See more at: http://myelectrical.com/notes/entryid/235/cable-sheath-and-armour-loss#sthash.f0inMMXr.dpuf

Cable Sizing Tool By Steven McFadyen on May 20th, 2012 

Our cable sizing tool is one of the more popular tools on the site. The tool enables cables to be sized in compliance with BS 7671 (the IEE Wiring Regulations) and by implication IEC 60364.

This post gives some insight into how the tool works, the calculations carried out and how to use it. With

the tool being based on BS 7671, this post will also provide an introduction and explanation of the cable

sizing method given in the standard.

The sizing tool can be found at:

myElectrical Cable Sizing Tool

Contents  [hide]

1. The Procedure

2. The Formulae

1. Current Capacity

2. Voltage Drop

3. Fault Levels

3. Frequently Asked Questions

The ProcedureThe flowchart (click for a larger image) shows the general procedure followed by the tool:

current capacity – a cable size is found which is adequate for the expected current

voltage drop – the voltage drop on the selected cable is checked and if to big the cable

size is increased

fault level – the fault level withstand is checked and if not adequate the cable size is

increase

Any errors or warnings generated during the calculation are passed on the the user. 

In addition to cable size the tool also calculates the cable impedance and fault levels at the load end of

the cable.

The FormulaeThe calculation method follows the procedures given in BS 7671:2008  (the IEE Wiring Regulations), 17th

Edition.

Definitions:

Iz - current carrying capacity of the cable (continuous service under defined installation

conditions)

It - tabulated value of current (for the type of cable, type of installation, and at an ambient of

30oC)

Ib - design current of the circuit (expected in normal service)

In - nominal setting of any protective device

I2 – operating current of the protective device

Ca - correction factor for the ambient temperature

Cc - correction factor for the type of protective device used

Cg - correction factor for grouping of circuits/cables

Ci - correction factor thermal insulation

Ct - correction factor for the operating temperature of the conductor

Np - number of cables in parallel

tp - maximum permitted operating temperature

Current CapacityBy considering any correction factors, the tabled current for a  cable installed in a given situation can be

found.  Once the tabled value of current is found, the cable size is determined by selecting the next

largest cross sectional area in the lookup tables.

For single circuits:

For groups where simultaneous overload is possible:

 

 

 

For groups not liable to simultaneous overload (the maximum of):

 

 

 

 

 

For cables where overload protection is not required:

 

 

Once It is known, this size of cable is then looked up in the current carrying capacity tables:

Tables 4D1A to 4J4A (pages274 to  316) – current carrying capacity

Correction factors are obtained from the following tables (dependant upon cable type and installation):

Table 4B1 – rating factors for cables in air, ambient other than 30 °C

Table 4B2 – rating factors for buried cables, ambient other than 20 °C

Table 4B3 – rating factors for buried cables, thermal resistivities other an 2.5 K.m.W-1

Table 4C1 to  4C3 – rating for grouping of circuits/cables

Table 52.2 – rating factors for cables surrounded by thermal insulation

Voltage DropVoltage drop calculations consider both the power factor of the system and a correction factor for the

cable operating standard.  Tables 4D1B to 4J4B of the standard give voltage drops (resistive and

reactive) in (mV/A/m), which is equivalent to  mΩ/m (or Ω/m if adjusted).

By looking up in these tables and using the cable length, the sizing tool determines the resistance, R and

reactance, X of the cable.  The calculated voltage drop, is then given by:

The temperature correction factor is only applied to the resistance and is given by:

Note: the tool carries out all calculations in complex form.

Fault LevelsThe fault withstand rating of the cable is determined using the adiabatic equation:

Where: 

S is the cable cross sectional area in mm2 

I is the fault current which can flow in A 

t is the operating time of the protective device in S 

k is a factor related to the conductor material and insulation – tables 54.2 to 54.6 of the

standard- See more at: http://myelectrical.com/

By Steven McFadyen on May 15th, 2012 

HDPE Cable Insulation 

Resin

Cable insulation is used to provide electrical separation between conductors of a cable. During the

historical development of cables, numerous types of insulation have been employed. During recent years,

there has been some consolidation in the types of insulation used.

This post provides some information on the more commonly used cable insulation materials.

Contents  [hide]

1. Insulation Materials (PVC)

2. Insulation Materials (PE)

3. Insulation Materials (other)

4. Elastomere Materials

5. High Temperature Materials

6. Halogen Free Compounds

7. See Also

Insulation Materials (PVC)Y Yw Yw Yk

PVC PVC PVC PVC

Polyvinylchlo

ride 

Compounds

Heat-

resistant 

90oC

Heat-

resistant 

105oC

Cold 

Resistant

Density g/cm3 1.35 - 1.5 1.3-1.5 1.3-1.5 1.2-1.

Breakdown Voltage KV/mm

(20oC)

25 25 25 25

Specific Volume Resistivity Ω

cm (20oC)

1013-1015 1012-1015 1012-1015 1012-1015

Dielectric Constant 50 Hz

(20oC)

3.6-6 4-6.5 4.5-6.5 4.5-6.5

Dielectric Loss Factor (tan δ) 4 x 10-2 to 1 x

10-1

4 x 10-2 to 1 x

10-1

4 x 10-2 to 1 x

10-1

4 x 10-2 to 1 x

10-1

Working  Permanent oC -30 +70 -20 +90 -20 +105 -40 +70

Temperature Short Time oC +100 +120 +120 +100

Melt  Temperature +oC >140 >140 >140 >140

Flame Resistance Self

Extinguishing

Self

Extinguishing

Self

Extinguishing

Self

Extinguishing

Oxygen Index LOI (% O2) 23-42 23-42 24-42 24-42

Heating Value H0 MJ.kg-1 17-25 16-22 16-20 17-24

Thermal Conductivity W.K-1.m-

1

0.17 0.17 0.17 0.17

Corrosive Gases In Case Of

Fire

Hydrogen

Chloride

Hydrogen

Chloride

Hydrogen

Chloride

Hydrogen

Chloride

Radiation Resistance max

Mrad

80 80 80 80

Tensile Strength N/mm2 10-25 10-25 10-25 10-25

Elongation At Break % 130-350 130-350 130-350 130-350

Shore Hardness 70-95 (A) 70-95 (A) 70-95 (A) 70-95 (A)

Abrasion Resistance Medium Medium Medium Medium

Water Absorption % 0.4 0.4 0.4 0.4

Halogen Free No No No No

Weather Resistance Medium 

(Black: Good)

Medium 

(Black: Good)

Medium 

(Black: Good)

Medium 

(Black: Good)

Cold Resistance Moderate - Moderate - Moderate - Very  Good

Good Good Good

 

Insulation Materials (PE)2Y 2Y 2X O2Y

LDPE HDPE VPE  

Low

density 

Polyethyl

ene

High

density 

Polyethyl

ene

Cross

Linked 

Polyethyle

ne

Foamed 

Polyethyl

ene

Density g/cm3 0.92-0.94 0.94-0.98 0.92 ≈0.65

Breakdown Voltage KV/mm

(20oC)

70 85 50 30

Specific Volume Resistivity Ω cm

(20oC)

1017 1017 1012-1016 1017

Dielectric Constant 50 Hz (20oC) 2.3 2.3 4-6 ≈1.55

Dielectric Loss Factor (tan δ) 2 x 10-4 3 x 10-4 2x 10-3 5 x 10-4

Working 

Temperature

Permanent oC -50 +70 -50 +00 -35 +90 -40 +70

Short Time oC +100 +120 +100 +100

Melt  Temperature +oC 105-110 130 - 105

Flame Resistance Flammable Flammable Flammable Flammabl

e

Oxygen Index LOI (% O2) ≤22 ≤22 ≤22 18-30

Heating Value H0 MJ.kg-1 42-44 42-44 42-44 42-44

Thermal Conductivity W.K-1.m-1 0.3 0.4 0.3 0.25

Corrosive Gases In Case Of Fire No No No No

Radiation Resistance max Mrad 100 100 100 100

Tensile Strength N/mm2 10-20 20-30 12.5-20 8-12

Elongation At Break % 400-600 500-1000 300-400 350-450

Shore Hardness 43-50 (D) 60-63 (D) 40-45 (D) -

Abrasion Resistance Medium Good Medium -

Water Absorption % 0.1 0.1 0.1 -

Halogen Free Yes Yes Yes Conditiona

l

Weather Resistance Medium 

(Black:

Good)

Medium 

(Black:

Good)

Good -

Cold Resistance Good Good Good Good

 

Insulation Materials (other)3Y 4Y 9Y 11Y TPE-E

(12Y)

TPE-O

PS PA PP PUR    

Polystrol

e

Polyamid

e

Poly-

propylen

e

Poly-

urethan

e

PolYest

er 

Elastom

er

Polyolefi

ne 

Elastome

r

Density g/cm3 1.05 1.02-1.1 0.91 1.15-1.2 1.2-1.4 0.89-1.0

Breakdown Voltage 30 30 75 20 40 30

KV/mm (20oC)

Specific Volume

Resistivity Ω cm (20oC)

1016 1015 1016 1010-1012 >1010 >1014

Dielectric Constant 50 Hz

(20oC)

2.5 4 2.3-2.4 4-7 3.7-5.1 2.7-3.6

Dielectric Loss Factor

(tan δ)

1 x 10-4 2 x 10-2 to

1 x 10-3

4 x 10-4 2.3 x 10-2 1.8 x 10-2 1.8 x 10-2

Working 

Temperatu

re

Permanent oC

-50 +80 -60 +05 -10 +100 -55 +80 -50 +100 -50 +100

Short

Time oC

+100 +125 +140 +100 +140 +130

Melt  Temperature +oC >120 210 160 150 190 150

Flame Resistance Flammabl

e

Flammabl

e

Flammabl

e

Flammabl

e

Flammab

le

Flammabl

e

Oxygen Index LOI (% O2) ≤22 ≤22 ≤22 ≤22 ≤29 ≤25

Heating Value H0 MJ.kg-1 40-43 27-31 42-44 20-26 20-25 23-28

Thermal Conductivity

W.K-1.m-1

0.25 0.23 0.19 0.25 0.5 1.5

Corrosive Gases In Case

Of Fire

No No No No No No

Radiation Resistance

max Mrad

80 10 10 100 10 10

Tensile Strength N/mm2 55-65 50-60 20-35 30-45 30 20

Elongation At Break % 300-400 50-170 300 500-700 >300 >300

Shore Hardness 35-50 (D) - 55-60 (D) 70-100 85 (A), 55 (A), 70

(A) 70 (D) (D)

Abrasion Resistance Good Very Good Medium Very

Good

Good Good

Water Absorption % 0.4 1.0-1.5 0.1 1.5 1.5 1.5

Halogen Free Yes Yes Yes Yes Yes Yes

Weather Resistance Medium -

Good

Good Moderate Very

Good

Very

Good

Very

Good

Cold Resistance Moderate

- Good

Good Good Very

Good

Very

Good

Very

Good

 

Elastomere MaterialsG 2G 3G 4G 5G 6G

NR 

SBR

SiR EPR EVA CR CSM

Natural

Rubber 

Styrol- 

Butadien

e

Rubber 

Compoun

ds

Silicon

e

Rubber

Ethylen- 

Propylen

e 

Rubber 

Compou

nds

Ethylene- 

Vinylacet

ate 

Copolyme

re 

Compoun

ds

Poly- 

Chloropre

ne 

Compoun

ds

Chloro- 

Sulfonate

d 

Polyethyl

ene 

Compoun

ds

Density g/cm3 1.5-1.7 1.2-1.3 1.3-1.55 1.3-1.5 1.4-1.65 1.3-1.6

Breakdown Voltage

KV/mm (20oC)

20 20 20 30 20 25

Specific Volume

Resistivity Ω cm (20oC)

1012-1015 1015 1014 1012 1010 1012

Dielectric Constant 50

Hz (20oC)

3-5 3-4 3-3.8 5-6.5 6-8.5 6-9

Dielectric Loss Factor

(tan δ)

1.9 x 10-2 6x 10-3 3.4 x 10-3 2 x 10-2 5 x 10-2 2.8 x 10-2

Working 

Temperat

ure

Permanent oC

-65 +60 -60

+180

-30 +90 -30 +125 -40 +100 -30 +80

Short

Time oC

+120 +260 +160 +200 +140 +140

Melt  Temperature +oC - - - - - +160

Flame Resistance Flammabl

e

High

Flash

Point

Flammabl

e

Flammable Self 

Extinguishi

ng

Self 

Extinguishi

ng

Oxygen Index LOI (%

O2)

≤22 25-35 ≤22 ≤22 30-35 30-35

Heating Value

H0 MJ.kg-1

21-25 17-19 21-25 19-23 14-19 19-23

Thermal Conductivity

W.K-1.m-1

- 0.22 - - - -

Corrosive Gases In

Case Of Fire

No No No No Hydrogen

Chloride

Hydrogen

Chloride

Radiation Resistance

max Mrad

100 50 200 100 50 50

Tensile Strength

N/mm2

5-10 5-10 5-10 8-12 10-20 10-20

Elongation At Break % 300-600 300-

600

200-400 250-350 400-700 350-600

Shore Hardness 60-70 (A) 40-80

(A)

65-85 (A) 70-80 (A) 55-70 (A) 60-70 (A)

Abrasion Resistance Moderate Modera Moderate Moderate Medium Medium

te

Water Absorption % 1.0 1.0 1.0 1.0 1.0 1.5

Halogen Free No Yes Yes Yes No No

Weather Resistance Moderate Good Very

Good

Good Very Good Very Good

Cold Resistance Very Good Very

Good

Good Good Moderate Moderate

 

High Temperature Materials10Y 7Y 6Y 5YX 5Y

PVDF ETFE FEP PFA PTFE

Polyvinylid

ene 

Fluoride 

Kynar/Dyfl

or

Ethylene- 

Tetrafluoreth

ylene

 

Fluorine

Ethylene 

Propylen

e

Perfluoral- 

Koxypolym

eric

Poly- 

Tetrafluoreth

ylene

Density g/cm3 1.7-1.9 1.6-1.8 2.0-2.3 2.0-2.3 2.0-2.3

Breakdown Voltage

KV/mm (20oC)

25 36 25 25 20

Specific Volume

Resistivity Ω cm

(20oC)

1014 1016 1018 1018 1018

Dielectric Constant

50 Hz (20oC)

9-7 2.6 2.1 2.1 2.1

Dielectric Loss

Factor (tan δ)

1.4 x 10-2 8 x 10-4 3 x 10-4 3 x 10-4 3 x 10-4

Working 

Temperat

Permanen

t oC

-40 +135 -100 +150 -100 +205 -190 +260 -190 +260

ure Short

Time oC

+160 +180 +230 +280 +300

Melt  Temperature

+oC

>170 >265 >225 >290 >325

Flame Resistance Self 

Extinguishin

g

Self 

Extinguishing

Self 

Extinguish

ing

Self 

Extinguishin

g

Self 

Extinguishing

Oxygen Index LOI (%

O2)

40-45 30-35 >95 >95 >95

Heating Value

H0 MJ.kg-1

15 14 5 5 5

Thermal

Conductivity W.K-1.m-

1

0.17 0.24 0.26 0.21 0.26

Corrosive Gases In

Case Of Fire

Hydrofluoric Yes Yes Yes Yes

Radiation Resistance

max Mrad

10 10 1 0.1 0.1

Tensile Strength

N/mm2

50-80 40-50 15-25 25-30 80

Elongation At Break

%

150 150 250 250 50

Shore Hardness 75-80 (D) 70-75 (D) 55-60 (D) 55-60 (D) 55-60 (D)

Abrasion Resistance Very Good Very Good Very Good Very Good Very Good

Water Absorption % 0.01 0.02 0.01 0.01 0.01

Halogen Free No No No No No

Weather Resistance Very Good Very Good Very Good Very Good Very Good

Cold Resistance Very Good Very Good Very Good Very Good Very Good

 

Halogen Free CompoundsH HX

Not Cross

Linked

Cross Linked

Halogen Free 

Polymer

Compounds

Halogen Free 

Polymer

Compounds

Density g/cm3 1.4-1.6 1.4-1.6

Breakdown Voltage KV/mm

(20oC)

25 25

Specific Volume Resistivity Ω cm

(20oC)

1012-1014 1013-1014

Dielectric Constant 50 Hz (20oC) 3.4-5 3.4-5

Dielectric Loss Factor (tan δ) ~10-3 10-2-10-3

Working 

Temperature

Permanent oC -30 +70 -30 +90

Short Time oC +100 +150

Melt  Temperature +oC >130 -

Flame Resistance Self Extinguishing Self Extinguishing

Oxygen Index LOI (% O2) ≤40 ≤40

Heating Value H0 MJ.kg-1 17-22 16-25

Thermal Conductivity W.K-1.m-1 0.17 0.20

Corrosive Gases In Case Of Fire No No

Radiation Resistance max Mrad 100 200

Tensile Strength N/mm2 8-13 8-13

Elongation At Break % 150-250 150-250

Shore Hardness 65-95 (A) 65-95 (A)

Abrasion Resistance Medium Medium

Water Absorption % 0.2-1.5 0.2-1.5

Halogen Free Yes Yes

Weather Resistance Medium in Black: 

Good

Medium in Black: 

Good

Cold Resistance Average Average

- See more at: http://myelectrical.com/notes/entryid/178/cable-insulation-properties#sthash.uUKfqjdn.dpufnotes/entryid/182/cable-sizing-tool#sthash.AV

AtAKwn.dpuf

IEC 60287 Current Capacity of Cables - Rated Current By Steven McFadyen on February 18th, 2013 

This note looks at the formulae used to calculate the rated current capacity of a cable in line with IEC

60287 "Calculation of the continuous current rating of cables (100% load factor)".   Before you continue

reading this note, if you have not done so already we would suggest first reading our IEC 60287

introduction note:

      IEC 60287 Current Capacity of Cables - An Introduction

In the previous note we looked at the approach taken by the standard to the sizing of cables and

illustrated this with an example.  We then looked at one method of applying the standard and identified

resources enabling the calculation of all the various parameters involved.   In the note we are going to put

everything together and reveal the necessary equations for actually calculating the cable maximum

current rating.

The image illustrates  the thermal model for a cable. 

 

Heat is generated within the cable by various mechanisms -  conductor I2R loss, dielectric loss, sheath

loss, armour loss and direct solar radiation.  Some or all of this heat is dissipated through the cable

insulation, bedding, serving and into the surrounding medium.  The rate of heat flow is related to the

temperature difference across the cable and affected by the ambient temperature, temperature rises due

to other cables and any critical temperature rise of the soil above ambient.

In thermal equilibrium, when all these factors have balanced and the temperature of the conductor is the

maximum allowable for the insulation; we have the maximum rated current for the cable.  It can probably

be appreciated by now, that in typical real life situations, this can be quite a complicated calculation.

Note: rated current capacity found by the method assumes that the cable is fully loaded for

100% of it's operation time.  For cables which have varying or cyclic loads, the current

rating could possibly be increased.

Tip: the thermal model is worth remembering as enables us to intuitively understand how a

cable is likely to behave in conditions which are not normally encountered.  For example, if

a cable is run along a refrigerated gas pipe, we can hazard that this will reduce the ΔΘ and

hence the cable will be able to carry more current. 

Change in temperature across a material is equal to the heat input multiplied by the thermal resistance of

the material.  In terms of the thermal model and for a simple a.c. cable, the heat balance equation is given

by (a list of symbols is given at the end of the note):

Within the standard this above is used to derive the equations for current rating.  It is simplified for d.c.

cables by eliminating any a.c. only effects and modified for partial drying of soil and solar radiation where

appropriate.

Contents  [hide]

1. Rated Current of Cables

2. List of Symbols

Rated Current of CablesThe standard gives the following equations for the calculation of the cable current rating (for all alternating

current voltages and direct current up to 5 kV):

Buried cables where drying out of the soil does not occur or cables in air

AC cables

DC cables

Buried cables where partial drying-out of the soil occurs

AC cables

I=[Δθ−Wd[0.5T1+n(T2+T3+vT4)]+(v−1)ΔθXR[T1+n(1+λ1)T2+n(1+λ1+λ2)(T3+vT4)]]0.5

 

DC cables

I=[Δθ+(v−1)ΔθXR′[T1+nT2+n(T3+vT4)]]0.5

Buried cables where drying-out of the soil is to be avoided

AC cables

I=[Δθx−nWdT4nRT4(1+λ1+λ2)]0.5

DC cables

Cables directly exposed to solar radiation

AC cables

DC cables

Note: when calculating a cable where some drying of the soil may occur, it is also

necessary to perform the calculation for no drying out of soil and take the worse case

(lower) rating.

The calculation of each element needs some explanation and these have been split across several notes

(with each note dealing with one topic).  For details, please refer to the first note in this series, which

lists the other related notes. 

List of Symbolsn - number of load carrying conductors 

v - ratio of thermal resistivity of dry and moist soils 

I - rated conductor current, A 

R - a.c. resistance of the conductor per unit length,  Ω/m 

R' - d.c. resistance of the conductor per unit length, Ω/m    

T1 - thermal resistance per core between conductor and sheath, K.m/W 

T2 - thermal resistance between sheath and armour, K.m/W 

T3 - thermal resistance of external serving, K.m/W 

T4 - thermal resistance of surrounding medium, K.m/W 

T4* - external thermal resistance (free air) adjusted for solar radiation, K.m/W 

De* - cable diameter over insulation, m 

H - intensity of solar radiation, W/m2 

Wd - dielectric loss per units length, W/m 

λ1 - ratio of losses in metal sheath to total losses in all conductors 

λ2 - ratio of losses in armouring to total losses in all conductors 

σ - absorption coefficient of solar radiation for cable surface 

Θ - maximum conductor operating temperature, °C  

Θa - ambient temperature, °C  

ΔΘ - temperature difference (Θ-Θa), K 

ΔΘx - critical temperature of soil, °C- See more at: http://myelectrical.com/notes/entryid/210/iec-60287-current-capacity-of-cables-capacity-equations#sthash.MOCOY4Hw.dpuf

By Jeson Pitt on August 12th, 2013 

Power cables can basically be classified into earthed and unearthed cables, where earthed and

unearthed refer to the application for which the cable is used. Earthed system refers to a three phase

system whose star point is grounded directly and the voltage between the healthy phases and the ground

will be - 11kV/1.732 or 6.6/1.732. In the case of unearthed cable, ground voltage is equal to phase to

phase voltage.

Earthed Cables   Unearthed Cables

 

 

Medium Voltage (MV) voltage power distribution system cables can be both earthed and unearthed. If the

system is earthed, then we use earthed rated cable for manufacturing; and if the system is unearthed, we

use an unearthed rated cable for manufacturing.

Compared with the earthed cable as per the manufacturer's specifications, the unearthed cable needs

higher insulation levels.

The greatest difference arises in the voltage grade (Uo/U), which is:

Earthed System Unearthed System

1.9/3.3 kV, 3.8/6.6 kV, 6.35/11 kV, 12.7/22

kV and 19/33 kV

3.3/3.3 kV and 11/11 kV

 

In an exception to the abover, the cables of 6.35/11kV for an earthed system can also be used in the

place of 6.6/6.6 kV for an unearthed system. This is because each core of the cable has the insulation

level to withstand 6.6kV due to which between core to core insulation level will be 6.6kV+6.6kV = 11kV.

Contents  [hide]

1. The Difference in Origin

2. Insulation Strength

3. Cable Requirements

4. Preferable Cable for MV Transmission

The Difference in OriginThe first generators and transformers had small capacities in which the fault current was less and the star

point was solidly grounded due to which they were called earthed system. Generators that are now

available have 500MVA capacity and higher fault levels. So, if there is an earth fault, a heavy current

flows into the fault, which leads to the damage of the generators and transformers. In such a scenario, to

reduce the fault current, the star point is connected to the earth through a resistance. In case of an earth

fault in one phase, the voltage of the faulty phase with respect to the earth appears across the resistance.

Due to this, the voltage of the remaining two healthy phases with respect to the earth rises by 1.7 times. If

the insulation system is not designed to sustain these increased voltages, they are likely to develop earth

faults.

In case of earthed cable, three phase cables are earthed to a ground and each of the phase system is

grounded to the earth. While the unearthed system (if system neutral is not grounded) phase to ground

voltage can be equal to phase to phase voltage; in such situations the insulation level of the conductor to

the armor should be equal to the insulation level of conductor to conductor. In the three phase earthed

systems, phase to earth voltage is 1.732 times less than phase to phase voltage. Thus, the voltage stress

on the cable to armor is 1.732 times less than the voltage stress between conductor to conductor.

Insulation StrengthUnearthed cable requires more insulation strength than earthed cable. If a fault occurs in the phase to

ground voltage is √3 time the normal phase to ground voltage. So, if an earthed rated cable is used in an

unearthed system, it may result in an insulation puncture. Hence, it is essential to use unearthed rated

cable in such situations, especially in the case of 6.6kV systems where resistance type earthing is used.

Cable RequirementsCarrying forward the above point, 11kV earthed cable can be used in place of 6.6kV unearthed system

since the cable manufacturing process is the same. The size of the cable will depend on the current rating

and voltage level. So,

Voltage grade (Uo/U) where Uo is phase to earth voltage and U is phase to phase

voltage

Earthed system has an insulation grade of kV/1.75x kV

For earthed system (Uo/U): 1.9/3.3 kV, 3.8/6.6 kV, 6.35/11 kV, 12.7/22 kV and 19/33

kV

Unearthed system has insulation grade kV/kV

3 phase 3 wires system generally comes with unearthed grade cable and 3 phase 4

wire systems can be used as earthed grade cables

Preferable Cable for MV TransmissionFor MV transmission, earthed cable will be more economical, but unearthed cable offers more insulation.

This is because, if an earth fault occurs in the underground system, the voltage between the healthy

phases and the ground will be equal to phase to phase voltage - 11kV or 6.6kV and higher insulation level

is required. The voltage of the healthy phases rises by nearly 1.7 times resulting in an earth fault since the

insulation of these phases is not designed for increases voltage. It is advisable to opt for an unearthed

cable so that the core insulation has enough strength.

- See more at: http://myelectrical.com/notes/entryid/226/cables-for-mv-power-distribution-earthed-versus-unearthed-systems#sthash.BtDZJa5J.dpuf

By Steven McFadyen on November 12th, 2013 

Cable cross section showing 

insulation 

Dielectrics (insulating materials for example) when subjected to a varying electric field, will have some energy loss.   The varying electric field causes small realignment of weakly bonded molecules, which lead to the production of heat.  The amount of loss increases as the voltage level is increased.  For low voltage cables, the loss is usually insignificant and is generally ignored.  For higher voltage cables, the loss and heat generated can become important and needs to be taken into consideration.

Dielectrics (insulating materials for example) when subjected to a varying electric field, will have some

energy loss.   The varying electric field causes small realignment of weakly bonded molecules, which lead

to the production of heat.  The amount of loss increases as the voltage level is increased.  For low voltage

cables, the loss is usually insignificant and is generally ignored.  For higher voltage cables, the loss and

heat generated can become important and needs to be taken into consideration.

Dielectric loss is measured using what is known as the loss tangent or tan delta (tan δ).  In simple terms,

tan delta is the tangent of the angle between the alternating field vector and the loss component of the

material.  The higher the value of tan δ the greater the dielectric loss will be.  For a list of tan δ values

for different insulating material, please see the Cable Insulation Properties note.  

Note: in d.c. cables with a static electric field, there is no dielectric loss.  Hence the

consideration of dielectric loss only applies to a.c. cables.

Contents  [hide]

1. Cable Voltage

2. Cable Dielectric Loss

3. Symbols

4. See Also

Cable VoltageDielectric loss only really become significant and needs to be taken into account at higher voltages.  IEC

60287 "Electric Cables - Calculation of the current rating", suggests that dielectric loss need only be

considered for cables above the following voltage levels:

  Cable Type   U0,

kV

Butyl Rubber 18

EDR 63.5

Impregnated Paper (oil or gas filled) 63.5

Impregnated Paper (solid) 38

PE (high and low density) 127

PVC 6

XLPE (filled) 63.5

XLPE (unfilled) 27

 

Cable Dielectric LossCable Capacitance

Cable capacitance can be obtained from manufacturers or for circular conductors calculated

using the following:

 

C=ε18ln(Didc)10−9F.m−1

Given the tan δ and capacitance of the cable, the dielectric loss is easily calculated:

 

Wd=ωCU02tanδ

It is possible to use the above for other conductor shapes if the geometric mean is substituted

for Di and dc.

Symbolsdc - diameter of conductor, mm 

Di - external diameter of insulation, mm 

C - cable capacitance per unit length, F.m-1 

U0 - cable rated voltage to earth,  V 

Wd - dielectric loss per unit length, W.m-1 

tan δ - loss factor for insulation 

ε - insulation relative permitivity 

ω - angular frequency (2πf)

See Also- See more at: http://myelectrical.com/notes/entryid/241/dielectric-loss-in-cables#sthash.Nw6gAiWM.dpuf

Power Transformers - An Introduction By Steven McFadyen on November 29th, 2012 

One of the fundamental requirements of an alternating current distribution systems it to have the ability to

change the magnitude of voltages.  It is more efficient to transmit power at high voltages over long

distances, whereas it is safer and more practical to use a low voltage to drive appliances and equipment.  

Transformers are used to achieve this. 

Contents  [hide]

1. General Theory

1. Three Phase Transformers

2. Practical Aspects

1. Vector Group

2. Transformer Tapping

3. Temperature Derating

4. Altitude Derating

3. References

General TheoryA transformer is a device consisting of two (or more) windings coupled together magnetically.  A changing

current in one winding (normally called the primary), will generate a magnetic field.  This magnetic field

links with the second winding (normally called the secondary) and will induce a current into this winding.

Transformer operation principal

The illustration shows how a voltage V1 applied to the primary winding of N1 turns, creates a current

I1 which causes the generation of the magnetic flux in the core.  The  flux in the core with generates

voltage V2 in the secondary winding of N2 turns, giving a current I2 in the load.

Core - to facilitate distribution of the magnet field, transformer cores are usually made of

steel laminations.  Laminations are preferable to a solid steel cores as they reduce losses.

The relationship between voltage, number of turns and current is given by:

      and   

Tip - transformer efficiencies are high and by assuming the input power equals the output

power the above voltage and current relationships are easily derived: input power (in VA) =

V1 x I1 with equals the output power = V2 x I2, which rearranged give the above.  

Transformers are not perfect and there are losses.  This can be divided into two types:

1. I2R losses - in the windings occur due to resistive losses in both the primary and

secondary windings.  Resistive losses increase with load magnitude.

2. Core losses -  result from eddy current and hysteresis losses in within the transformer

core.  Losses in the core are fairly constant regardless of loading.

The total loss is the sum of the core losses Pc and the resistive losses due to primary current I1, primary

winding resistance R1, secondary current I2 and secondary winding resistance R2:

The efficiency of the transformer can be expressed as:

Three Phase Transformers

Transformer operation principal Three phase transformers can be made by combining single phase transformers.  One of the more common implementations is to construct a core of three limbs closed at the top and bottom.  Each individual cores contains the primary and secondary winding of a single phase. 

Windings may be connected either in star or delta depending on requirements.  The image shows one

start connected and one delta connected winding.

Practical Aspects

Vector GroupTransformers can be wound in various configurations (delta-star, star-star, etc.).  Depending on the

configuration there will be a phase shift between the primary and secondary of the transformer.  The

transformer configuration and phase shift is termed the vector group.

The vector group is represented by a capital letter for the primary winding, a lower case letter for the

secondary winding and followed by a number (1 to 11).  The letter indicated the winding arrangement -

D=Delta, Y=Star and Z=Zigzag.  The number is the phase shift in multiples of 30 degrees.  For example:

Dy11 - delta connected primary, star secondary, 330° (-30°) phase shift 

Dd0 - delta connected primary, delta secondary, no phase lag

Sometimes a third letter is added to indicated the neutral is brought out, for example Dyn11 (delta-star

transformer with the secondary neutral brought out).

Transformer Tapping

Three phase transformer (core type winding) The nominal voltage of a transformer is related to the turns ration between the primary and secondary.  In use the primary voltage can vary and the secondary current can vary.  Both these will affect the output voltage of the transformer.

To cater for varying primary and secondary conditions, transformers are often fitted with taps on one of

the windings; so that the turns ratio can be adjusted somewhat.  These are often expressed as numbers.

For example, a low voltage transformer may have -5%, -2.5%, 0%, +2.5% and +5% taps.  At 0% tap the

transformer will be operating at its designed turns.  At +2.5% the transformer secondary voltage will be

2.5% larger than what it would be if set at 0% (for the same primary voltage and secondary current).

An example of use would be setting a +2.5% or +5% tap on a transformer which is heavily loaded to help

compensate for the voltage drop in the cables.

Temperature DeratingAccording to the standards[1][ 2][3], transformers are designed for:

a maximum temperature - 40 °C

30 °C monthly average of the hottest month

20 °C yearly average

outdoor transformer minimum temperature -25 °C

indoor transformer minimum temperature -5 °C

When the transformer is designed for service where the temperature of the cooling air exceeds the

maximum allowable, the temperature rise limits shall be reduced by the amount of the excess.

Alternatively the temperature differences can be taken into account by adjusting the transformer capacity:

A

m

b

i

e

n

t

t

e

m

p

e

r

a

t

u

r

e

(

a

n

n

u

a

l

a

v

e

r

a

g

e

) 

C

a

p

a

c

i

t

y

-

2

0

°

C

1

2

4

%

-

1

0

°

C

1

1

8

%

0

°

C

1

1

2

%

+

1

0

°

C

1

0

6

%

+

2

0

°

C

1

0

0

%

+

3

0

°

C

9

3

%

 

Altitude Derating

Transformers are designed for an altitude of 100 m above sea level.  For other altitudes, the limit of the

average winding temperature rise shall be reduced by[2][3]:

oil immersed, air naturally cooled - 1 K for every 400 m above design altitude

oil immersed force cooled - 1 K for every 200 m above design altitude

oil immersed water cooled - no correction for altitude

dry-type air naturally cooled - 2.5% for every 500 m above design altitude

dry-type  air force cooled - 5% for every 500 m above design altitude

References- See more at: http://myelectrical.com/notes/entryid/199/power-transformers-an-introduction#sthash.QDcO9HVU.dpuf

By Steven McFadyen on July 5th, 2011 

While there are a vast array of cable insulation materials, these are often divided into two general  types;  Thermoplastic or  Thermosetting.   For example the current capacity determination of a cable in accordance with  the UK Wiring Regulations is categorized into thermoplastic and thermosetting cables.

Thermoplastic materials are composed of chains of  molecules (polyethylene for

example).  When heat is applied the energy will allow the bonds to separate and the

material can flow (melt) and be reformed.

Thermosetting materials are formed when materials such as polyethylene undergo

specific heating or chemical processes. During this process the individual chains become

cross linked by smaller molecules making a rigid structure.    Thermosetting materials

cannot reheated, melted and remolded.

While thermoplastic materials have the advantage of being able to be reformed, thermosetting materials

are generally more heat resistance and have greater strength.

The operating temperature of any cable is an important parameter in determining the maximum allowable

current.   While the actual temperature varies depending on the material used, the UK Wiring Regulations

limits the choice in calculating the current rating to two temperatures only:

Thermplastic 70 0C

Thermosetting 90 oC

Note: more specific calculations based on actual material properties are allowed.  However, for ease of

use most practical application will use 70 oC or 90 oC and the methods outlined in the regulations.

See Also

- See more at: http://myelectrical.com/notes/entryid/72/thermoplastic-and-thermosetting#sthash.5RvpSHhI.dpuf\

Cable Sizing Tool  

By Steven McFadyen on May 20th, 2012 

Our cable sizing tool is one of the more popular tools on the site. The tool enables cables to be sized in compliance with BS 7671 (the IEE Wiring Regulations) and by implication IEC 60364.

This post gives some insight into how the tool works, the

calculations carried out and how to use it. With the tool being based

on BS 7671, this post will also provide an introduction and

explanation of the cable sizing method given in the standard.

The sizing tool can be found at:

myElectrical Cable Sizing Tool

Contents  [hide]

1. The Procedure

2. The Formulae

1. Current Capacity

2. Voltage Drop

3. Fault Levels

3. Frequently Asked Questions

The ProcedureThe flowchart (click for a larger image) shows the general procedure followed by the tool:

current capacity – a cable size is found which is adequate for the expected current

voltage drop – the voltage drop on the selected cable is checked and if to big the cable

size is increased

fault level – the fault level withstand is checked and if not adequate the cable size is

increase

Any errors or warnings generated during the calculation are passed on the the user. 

In addition to cable size the tool also calculates the cable impedance and fault levels at the load end of

the cable.

The FormulaeThe calculation method follows the procedures given in BS 7671:2008  (the IEE Wiring Regulations), 17th

Edition.

Definitions:

Iz - current carrying capacity of the cable (continuous service under defined installation

conditions)

It - tabulated value of current (for the type of cable, type of installation, and at an ambient of

30oC)

Ib - design current of the circuit (expected in normal service)

In - nominal setting of any protective device

I2 – operating current of the protective device

Ca - correction factor for the ambient temperature

Cc - correction factor for the type of protective device used

Cg - correction factor for grouping of circuits/cables

Ci - correction factor thermal insulation

Ct - correction factor for the operating temperature of the conductor

Np - number of cables in parallel

tp - maximum permitted operating temperature

Current CapacityBy considering any correction factors, the tabled current for a  cable installed in a given situation can be

found.  Once the tabled value of current is found, the cable size is determined by selecting the next

largest cross sectional area in the lookup tables.

For single circuits:

For groups where simultaneous overload is possible:

 

 

 

For groups not liable to simultaneous overload (the maximum of):

 

 

 

 

 

For cables where overload protection is not required:

 

 

Once It is known, this size of cable is then looked up in the current carrying capacity tables:

Tables 4D1A to 4J4A (pages274 to  316) – current carrying capacity

Correction factors are obtained from the following tables (dependant upon cable type and installation):

Table 4B1 – rating factors for cables in air, ambient other than 30 °C

Table 4B2 – rating factors for buried cables, ambient other than 20 °C

Table 4B3 – rating factors for buried cables, thermal resistivities other an 2.5 K.m.W-1

Table 4C1 to  4C3 – rating for grouping of circuits/cables

Table 52.2 – rating factors for cables surrounded by thermal insulation

Voltage DropVoltage drop calculations consider both the power factor of the system and a correction factor for the

cable operating standard.  Tables 4D1B to 4J4B of the standard give voltage drops (resistive and

reactive) in (mV/A/m), which is equivalent to  mΩ/m (or Ω/m if adjusted).

By looking up in these tables and using the cable length, the sizing tool determines the resistance, R and

reactance, X of the cable.  The calculated voltage drop, is then given by:

The temperature correction factor is only applied to the resistance and is given by:

Note: the tool carries out all calculations in complex form.

Fault LevelsThe fault withstand rating of the cable is determined using the adiabatic equation:

Where: 

S is the cable cross sectional area in mm2 

I is the fault current which can flow in A 

t is the operating time of the protective device in S 

k is a factor related to the conductor material and insulation – tables 54.2 to 54.6 of the

standard

Frequently Asked QuestionsHow does the tool taking into account parallel conductors?

Additional parallel cables are automatically added to group derating (you don't need to increase this

manually). 

You can click the ignore checkbox to change this (for example if your cables are more than two diameters

apart).: code changes have been made. The updated version of the calculator is now live.

What exactly is the No.Circuits/Cables?

Group derating takes into account heat generated by adjacent cables.  Each circuit is usually supplied by

one cable and the number of circuits will equal the number of cables.  Sometimes several cables are run

a parallel for a single circuit and this will increase the total number of cables (and group derating).

Tip: if cables are spaced more than twice their overall diameter apart, then no group derating is required.- See more at: http://myelectrical.com/notes/entryid/182/cable-sizing-tool#sthash.kNNQ5pcU.dpuf

Harmonised Cable Codes and Colours By Steven McFadyen on August 28th, 2013 

Within Europe the European Committee for Electrotechnical Standardization (CENELEC) has

standardised the both the designation and colour of cables.   These are published in CENELEC document

HD 361 S3:1999 "System for cable designation" and HD 308 S2: 2001

"Identification of cores in cables and flexible cords".  This note provides a general overview to the

harmonised system and gives some examples.

Contents  [hide]

1. Cable Designation

1. Designation Codes

2. Cable Code Examples

2. Cable Colour Codes

Cable DesignationThe HD 261 document, classifies the construction of the cable by allocating codes (letters or numbers) to

represent the cable voltage, insulation material\, structural elements, sheath, special features and

conductor type.

For a full list of codes and their meanings, it is best to refer to the standard.  A typical cable specification

would take the form of:

Laying out the Harmonised Cable Code

The layout of the cable code can be split into three parts.  Fist the standard and nominal voltage are

given.  This is followed by the insulation material, construction features and sheath.  Finally the cores and

cross section are specified. 

Each cable element is designated with a alpha numeric code as detailed in the tables below. Typically

these are connected together to form the final cable designation.  Optional codes or codes that have no

relevance to the cable under consideration are simply omitted.

Note: some codes are preceded by a '-' sign, for example -A (aluminium).

Designation Codes

Identification of Designation 

A    authorised national standard 

H    harmonised standard 

Nominal Voltage 

01   100 V 

03   300/300 V 

05   300/500 V 

07   450/750 V 

Structural Elements 

- Concentric Conductors  

A       Concentric aluminium conductor 

A6     Concentric aluminium conductor,

meander-shaped 

Insulation & Sheath Materials 

B        Ethylene-propylene rubber (EDR)

+90°C  

B2      Ethylene-propylene rubber (EDR),

hardened 

B3      Butyl rubber 

E        Polyethylene 

E2      Polyethylene, high density 

E4      Poly-tetrafluorethylene 

E5      Eethylene propylene rubber 

E6      Ethylene tetrafluorethylene 

E7      Polypropylene 

G       Ethylene-vinylacetate (EVA) 

J        Glass fibre braiding 

J2      Glass fibre wrapping 

C      Concentric copper-conductor 

C6    Concentric copper-conductor, meander-

shaped 

C9     Divided concentric copper conductor 

- Screen 

A7     Aluminium screen 

A8     Aluminium screen, individual

conductors 

C4     Copper braid screen 

C5     Copper braid screen, individual

conductors 

C7     Copper tape screen 

C8     Copper tape screen, individual

conductors 

D      Screen of one or more thin steel tapes 

- Armouring 

Z2     Armouring of round steel wires 

Z3     Armouring of flat steel wires 

Z4     Armouring of steel tape  

Z5     Braiding of steel wires     

Z6     Supporting braid of steel wires 

Z7     Armouring of sectional steel wires 

Y2     Armouring of round aluminium wires 

Y3     Armouring of flat aluminium wires 

Y5     Armouring of special materials 

Y6     Armouring of steel wires and/or tape

and copper wires 

Conductor Material 

w/o designation  Copper 

- A      Aluminium 

- Z     Special material and/or special shape 

Special Design Features 

- Supporting Structures 

D2     Textile or steel wires over cable

conductor 

D3     Textile elements stranded in conductor

cable 

D4     Self-supporting cables and wires 

D5    Central conductor element 

M       Mineral insulation 

N        Chloroprene rubber (CR) 

N2      Chloroprene-rubber (CR), welding

cable 

N4      Chlorinated polyethylene 

N5      Nitril-rubber 

N6      Fluorinated rubber 

N7      PVC nitril rubber compound 

N8      Polychloroprene rubber, water

resistant 

P        Impregnated paper insulation 

Q       Polyurethane (PUR) 

Q2     Polyethyleneterephthalate 

Q3     Polystyrole 

Q4     Polyamide 

Q5     Polyamide 

Q6     Polyvinylidene fluoride 

R      (NR, SR) natural or synthetic rubber 

S      (SIR) silicone rubber 

T      Textile braiding 

T2    Textile braiding with flame retardant 

T3    Textile conductor wrapping or tape 

T4    Textile conductor wrapping or tape,

flame retardant 

T5    Corrosion protection 

T6    Textile braiding over individual

conductor or cable 

V       PVC 

V2     PVC soft, resistant to increased

temperature, +90°C 

V3     PVC soft, for low temperatures 

V4     PVC soft, cross-linked 

V5     PVC soft, oil resistant 

X      Cross-linked polyethylene 

Z      Cross-linked compound, LSZH 

Z1    Thermoplastic compound, LSZH 

Note: for details in insulation properties,

please refer to: 

- Cable Insulation Properties

- Special Versions 

w/o designation round cable construction 

H       Flat type as separable cables with or

without jacket  

H2     Flat type of cables not separable 

H3     Building Cable, flat webbed  

H4     Multi conductor flat cable with one plain

conductor 

H5     Two or more single conductor stranded,

non-jacket 

H6     Flat cables with 3 or more conductors  

H7     Cable with two-jacket extruded

insulation 

H8     Coiled conductor 

Conductor Type 

- D      fine wire stranded for welding cables  

- E      extra fine wire stranded for welding 

cables  

- F      fine wire stranded for flexible cables  

- H      extra fine wire stranded for flexible

cables 

- K      fine wire stranded conductor for fixed

installation  

- M     Milliken conductor  

- R     conductor of multi stranded wires  

- S     sector-shaped conductor of multi

stranded wires  

- U     round conductor of single wire  

- W    sector-shaped conductor of single wire  

- Y     tinsel conductor 

- Z     conductor of special material 

Protective Core 

G     with green/yellow earth conductor 

X     without earth conductor

Cable Code ExamplesDifferent manufacturers vary the way in which they present the harmonized designation for their cables. 

Here are a few examples of varying cable designations:

H05VV5-F 2G075 is 00/500 V, PVC insulated, PVC sheathed, stranded flexible

conductor, 2 core 2.5 mm2 with protective conductor

H05V-K 1X1 is 300/500 V, PVC insulated, fine wire stranded, single core 1 mm2 with

no protective conductor

S03VV-F 3G0.75 is national standard (VDE in this case), 300/300 V, PVC insulated, PVC

sheathed, fine wire stranded flexible cable, 3 core 0.75 mm2 with protective conductor

H07RV-F 3X10 is 450/750 V, natural rubber insulation, PVC sheath, fine wire stranded,

three core 10 mm2 with no protective conductor

H05Z-K 1X2.5 is 300/500 V, XLPE LSZH, fine wire stranded, single core 2.5 mm2

without protective conductor

Cable Colour CodesCENELEC (including BS 7671 - IEE Wiring Regulations)

Function Alpha- 

numeri

c

Colour

Protective conductors     Green and yellow

Functional earthing conductor   Cream

a.c. power circuit

Phase of single-phase circuit L Brown

Neutral of single- or three-phase circuit N Blue

Phase 1 of three-phase a.c. circuit  L1 Brown

Phase 2 of three-phase a.c. circuit L2 Black

Phase 3 of three-phase a.c. circuit L3 Grey

Two-wire unearthed d.c. power circuit

Positive of two-wire circuit L+ Brown

Negative of two-wire circuit L- Grey

Two-wire earthed d.c. power circuit

Positive (of negative earthed) circuit L+ Brown

Negative (of negative earthed) circuit M Blue

Positive (of positive earthed) circuit M Blue

Negative (of positive earthed) circuit L- Grey

Three-wire d.c. power circuit

Outer positive of two-wire circuit derived from

three-wire system

L+ Brown

Outer negative of two-wire circuit derived

from three-wire system

L-  Grey

Positive of three-wire circuit L+ Brown

Mid-wire of three-wire circuit M Blue

Negative of three-wire circuit L- Grey

Control circuits, ELV and other

applications

Phase conductor L Brown, Black, Red, Orange, 

Yellow, Violet, Grey, White, Pink  or

Turquoise

Neutral or mid-wire N or M Blue

 

If anyone notices any mistakes or corrections which are needed, please leave a comment below.

- See more at: http://myelectrical.com/notes/entryid/228/harmonised-cable-codes-and-colours#sthash.TVhNjX5H.dpuf

By Steven McFadyen on May 10th, 2012 

Everything physical in electrical engineering from insulations to conductors revolves around materials.  Here we are listing common materials along with their most useful properties in relation to electrical engineering. 

Contents  [hide]

1. The Table of Resistivities

2. The Properties Tables

1. Properties of Non Metallic Solids (at 293 k)

2. Properties of Metallic Solids (at 293 K)

3. Properties of Gasses

4. Properties of Liquids (at 293 K)

5. Properties of Semiconductors

6. Properties of Commercial Permanent Magnetic Materials

The Table of ResistivitiesThis table shows the resistivity and temperature coefficient of various materials at 20°C (68 °F)

Material Resistivity (Ω-m) at 20 °C Coefficient*

Silver 1.59×10-8 0.0038

Copper 1.72×10-8 0.0039

Gold 2.44×10-8 0.0034

Aluminium 2.82×10-8 0.0039

Calcium 3.3x10-8 ?

Tungsten 5.60×10-8 0.0045

Nickel 6.99×10-8 ?

Iron 1.0×10-7 0.005

Tin 1.09×10-7 0.0045

Platinum 1.1×10-7 0.00392

Lead 2.2×10-7 0.0039

Manganin 4.82×10-7 0.000002

Constantan 4.9×10-7 0.00001

Mercury 9.8×10-7 0.0009

Nichrome 1.10×10-6 0.0004

Carbon 3.5×10-5 -0.0005

Germanium 4.6×10-1 -0.048

Silicon 6.40×102 -0.075

Glass 1010 to 1014 ?

Hard rubber approx. 1013 ?

Sulfur 1015 ?

Paraffin 1017 ?

Quartz (fused) 7.5×1017 ?

PET 1020 ?

Teflon 1022 to 1024 ?

* the numbers in this column increase or decrease the significant portion of the resistivity. For example, at

30°C (303.15 K), the resistivity of silver is 1.65×10-8. This is calculated as Δρ = α ΔT ρo where ρo is the

resistivity at 20°C and α is the temperature coefficient .

The Properties Tables

Properties of Non Metallic Solids (at 293 k)

ρ' Tm λ εr

Legend: ρ' = Density, kg m-3 Tm = Melting point, K λ = Thermal conductivity, W m-1K-1 εr = Relative permittivities at 293 K

Alumina,ceramic 38002300

29 -

Amber - - - 2.8

Bone 1850- - -

Brick,building 2300- 0.6 -

Brick,fireclay 2100- 0.8 -

Brick,paving 2500- - -

Brick,silica 1750- 0.8 -

Carbon,graphite 23003800

5 -

Carbon,diamond 3300- 900 -

Concrete 2400- 0.1 -

Cork 240 - 0.05-

Cotton 1500- - -

Ebonite - - - 2.7-2.9

Epoxy resin 1120- - -

Fluon (PTFE) 2200- 0.25-

Glass,crown 26001400

1 5-10

Glass,flint 42001500

0.8 5-10

Glass wool 501400

0.045-10

Ice 920 273 2 75

Kapok 50 - 0.03-

Magnesium oxide 36003200

- -

Marble 2600- 2.9 8.5

Melamine formaldehyde 1500- 0.3 -

Mica - - - 5.7-6.7

Naphthalene 1150350 0.4 -

Nylon 1150470 0.25-

Paraffin wax 900 330 0.252-2.3

Perspex 1190350 0.2 3.5

Phenol formaldehyde 1300- 0.2 -

Polyethylene,low density 920 410 - -

Polyethylene,high density 955 410 - -

Polypropylene 900 450 - -

Polystyrene 1050510 0.082.55

Polyvinylchloride (PVC),non-rigid1250485 - 4.5

Polyvinylchloride (PVC),rigid 1700485 - 4.5

Polyvinylidine chloride - 470 - -

Quartz fibre 26602020

9.2 -

Rubber (polyisoprene) 910 300 0.15-

Silicon carbide 3170- - -

Sulphur 2070386 0.263.6-4.3

Teflon - - - 2.1

Titanium carbide 4500- 28 -

Wood,oak (with grain) 650 - 0.15-

Wood,Spruce (with grain) 600 - - -

Wood,Spruce (across grain) - - - -

 

 

Properties of Metallic Solids (at 293 K)

ρ' Tm λρ x10-

8 α20 x10-

4

Legend: ρ = Density, kg m-3 Tm = Melting point, K λ = Thermal conductivity, W m-1 K-1 ρ = Electrical Resistivity, Ω m α20 = Temperature coefficient of resistance, K-

1

Aluminium 2710 932 2012.65 40

Aluminium, strong alloy 2800 800 1805 16

Antimony 6680 904 18 40 50

Bismuth 9800 544 8 115 45

Brass (70Cu/30Zn) 8500 13001108 15

Bronze (90Cu/10Sn) 8800 130018030 -

Cobalt 8900 176569 6 66

Constantan 8880 136023 47 0.4

Copper 8930 13563851.7 39

German silver (60Cu/25Zn/15Ni)8700 130029 33 4

Gold19300

13402962.4 34

Invar (64Fe/36Ni) 8000 180016 81 20

Iron, pure 8780 181080 10 65

Iron, cast grey 7150 150075 10 -

Iron, cast white 7700 142075 10 -

Iron, wrought 7850 181060 14 60

Lead11340

600 35 21 43

Magnesium 1740 924 1504 43

Manganin 8500 - 22 45 0.1

Monel (70Ni/30Cu) 8800 160021042 20

Nickel 8900 172659 59 60

Nickel, strong alloy 8500 1320- - -

Phosphor bronze - - - 7 60

Platinum21450

204269 11 38

Silver10500

12304191.6 40

Sodium 970 371 1344.5 44

Solder, soft (50Pb/50Sn) 9000 490 - - -

Stainless Steel (18Cr/8Ni) 7930 180015096 6

Steel, mild 7860 170063 15 50

Steel, piano wire 7800 170050 - -

Tin 7300 505 65 11 50

Titanium 4540 195023 53 38

Zinc 7140 693 1115.9 40

 

Properties of Gasses

ρ' Tb λ εr

Legend: ρ' = Density, kg m-3 Tb = Boiling point, K λ = Thermal conductivity, W m-1K-

1 εr = Relative permittivities at 293K

Acetylene (C2H2)1.173

189 0.0184-

Air1.293

83 0.02411.000536

Ammonia (NH3)0.771

240 0.0218-

Argon (Ar)1.784

87 0.01621.000545

Carbon dioxide (CO2)1.977

195 0.01451.000986

Carbon monoxide (CO) 1.25 81 0.02321.0007

Chlorine (Cl2)3.124

238 0.0072-

Cyanogen (C2N2)2.337

252 - -

Deuterium - - - 1.00027

Ethylene (C2H4) 1.26 170 0.0164-

Helium (He)0.179

4.25 0.14151.00007

Hydrogen (H2) 0.09 20.350.16841.00027

Hydrogen Chloride (HCl) 1.64 189 - -

Hydrogen sulphide (H2S)1.538

211 0.012 -

Methane (CH4)0.717

109 0.0302-

Neon - - - 1.000127

Nitric oxide (NO) 1.34 121 0.0238-

Nitrogen (N2) 1.25 77 0.02431.00058

Nitrous oxide (N2O)1.978

183 0.0151-

Oxygen (O2)1.428

90 0.02441.00053

Sulphur dioxide (SO2)2.927

263 0.00771.00082

Water vapour (273 K H2O)0.8 - 0.01581.0006

 

Properties of Liquids (at 293 K)

ρ' Tm Tb λ εr

Legend: ρ’ = Density, kg m-3 Tm = Melting point, K Tb = Boiling point, K λ = Thermal conductivity, W m-1K-

1 εr = Relative permittivities at 293K

Acetic acid (C2H4O2) 1049 2903910.18 -

Acetone (C3H6O) 780 1783300.16121.3

Benzene (C6H6) 879 2793530.14 2.28

Bromine (Br) 3100 266332- -

Carbon disulphide (CS2) 1293 1623190.144-

Carbon tetrachloride (CCl4)1632 2503500.1032.17

Chloroform (CHCL3) 1490 2103340.121-

Caster oil - - - - 4.5

Ether,diethyl (C4H10O) 714 1573080.1274.34

Ethyl alcohol (C2H6O) 489 1563520.17724.7

Glycerine - - - - 43

Glycerol (C3H8O3) 1262 2935630.27 -

Mercury (Hg)13546

2346307.96 -

Methyl alcohol (CH4O) 791 1793370.201-

Nitrobenzene (C6H5NO2) 1175 2794840.16 35.7

Olive oil 920 - 5700.17 -

Paraffin,medical - - - - 2.2

Paraffin oil 800 - - 0.15 -

Pentane - - - - 1.83

Phenol (C6H60) 1073 314455- -

Silicon oil - - - - 2.2

Toluene (C7H8) 867 1783840.134-

Turpentine 870 2634290.1362.23

Water (H2O) 998 2733730.59180.37

Water,sea 1025 264377- -

 

 

Properties of Semiconductors Germanium Silicon

Crystal Structure diamond diamond

Bonding covalent covalent

Lattice constant 5.6575 5.4307

Atomic volume, m3 kg-1 mol-1 13.5x10-3 12.0x10-3

Density, kg m-3 5.32x103 2.33x103

Cohesive energy, J kg-1 mol-1 3.72x108 4.39x108

Melting point, oC 958.5 1412

Mobility, m2 V-1 s-1electrons 0.38 holes 0.18

electrons 0.19 holes 0.05

Energy gap, eV (at 20 oC) 0.67 1.107

Density of states effective masselectrons 0.35 me holes 0.56 me

electrons 0.58 me holes 1.06 me

a, mm mK-1 5.75 7.6

 

 

Properties of Commercial Permanent Magnetic Materials Composition RemananceCoercivityBxH

Al Ni CoCuNbBr, TBHC A m-3

(BH)max J m-

3 Remarks

Alnico IV H 1226 8 2 - 0.6 63000 10x103 isotropic

Ticonal C 8 13.5243 0.61.26 52000 430 isotropic

Column 8 13.5243 0.51.35 64000 64 columnar

Pt-Co alloy - - 23- - 0.45 210000 300 ductile

Barium Ferrite - - - - - 0.2 135000 7550 isotropic

(BaO6Fe2O3Co5Sm) - - - - - 0.85 600000 140000

Elongated single domain magnet (Fe50%Co50%)

- - - - - 0.905 80000 40 mechanically weak

If you notice any errors or omissions in the tables, please add a comment below.

- See more at: http://myelectrical.com/notes/entryid/177/material-properties#sthash.28vhxKzP.dpuf

Introduction to Cathodic Protection By Steven McFadyen on April 25th, 2013 

Sacrificial aluminium anodes on a ship Image Source: Cathodic Marine Engineering If two dissimilar metals are touching and an external conducting path exists, corrosion of one the metals can take place.  Moisture or other materials acting as an electrolyte between the metals create an electrochemical cells  (similar to that of a battery).  Depending on the metals, one will act as a cathode and one as an anode of the cell. 

Under this arrangement, stray d.c. currents will flow. In the same was a a normal cell, an electrochemical

reaction takes place and there is a resulting corrosion of the anode. 

In practice, while it could be two dissimilar metals (steel aluminium for example),  corrosion

could also be a result of microscopic differences in composition of the surface of a single

metal.

Cathodic protection works be converting all anodes which are likely to corrode the cathodes.  There are

two principle methods of doing this:

1. attaching a more active metal to form a new anode (making the existing anode the

cathode) - resulting in the new material (sacrificial anode) being corroded rather than

the protected material

2. injection of a a d.c. current (impressed current) uses an anode connected to an

external d.c. source to provide the protection

History:  it was the English Chemist, Sir Humphrey Davis (1778-1829), who first proposed

the concepts of cathodic protection. The first application was to protect the copper plating

of British naval ships in 1824[1].

Contents  [hide]

1. Sacrificial Anode

2. Impressed DC Current

3. Cathode and Anodes

4. References:

Sacrificial AnodeThis is the practice of using a more active metal (sacrificial anode) connected to a structure to be

protected, knowing that this metal will be corroded.  One example of this would be the use of aluminium

sacrificial anodes to protect steel structures in seawater.

Sacrificial anodes need to be electrically connected to the structure being protected.

Note: galvanised steel cable trays and trunking are commonly used. Here a sacrificial

coating of zinc is applied which acts acts the anode, preventing corrosion.

Impressed DC Current

Principle of ICCP Impressed current cathodic protection (ICCP) forces the structure to be protected to become the cathode by connection to an anode and injection of a  direct current.  The d.c. power supplies typically vary the current to achieve a required protection potential.

In ICCP systems, anodes can range from low end consumable metals to more exotic materials which will

exhibit little or no corrosion. 

Cathode and AnodesWhen two metals are connected, determination of which will be the cathode and anode is made by

looking at the relative galvanic potentials of each material.  Of the two materials, the metal with the lowest

potential will be the anode.

When measuring metals to find their galvanic potential each needs to be measured against a common

common cathode (hence the term "Anodic Index" is often used).  The following table shows typical

galvanic potential of several metals as measured using a gold anode.

If measured against a different cathode, while the values of the galvanic potentials would be

different, it is the relative difference in potential between the two metals under

consideration in any situation which is important.

Metal Potential

Gold 0.00 (most cathodic)

Rhodium -0.05

Silver -0.15

Nickel -0.30

Copper -0.35

Brass & bronzes -0.40 to -0.45

Stainless Steels -0.50

Chromium Plated -0.60

Tin -0.65

Lead -0.70

Aluminium (wrought) -0.75 to -0.90

Iron, wrought -0.85

Aluminium (cast) -0.95

Zinc -1.20 to -1.25

Magnesium -1.75

Beryllium -1.85 (most anodic)

The amount of potential difference required between metals for corrosion to occur varies and is defendant

on the environment. As a rule of thumb,  many people take an 0.25 V difference of normal environments,

0.5 V where the humidity (and temperature) are controlled and 0.15 V for more harsh industrial

environments[1].

As an example of using the table, we can see the potential difference between copper and aluminium is of

the order 0.6 V, giving a combination which is to be particularly avoided.  In practice special bi-metalic

connections need to be employed whenever aluminium conductors are to be connected to copper

conductors.

References: [1] Cathodic Protection, Wikipedia 

http://en.wikipedia.org/wiki/Cathodic_protection - accessed February 2013- See more at: http://myelectrical.com/notes/entryid/219/introduction-to-cathodic-protection#sthash.tansAAUQ.dpuf

Motor Efficiency Classification By Steven McFadyen on March 28th, 2012 

Electric motors are one of the most widely used items of electrical equipment.  Improving motor efficiency

benefits include, reduced power demand, lower operating costs and reduced environment impact. 

In recognising the impact of motors on both power generation requirements and environmental issues,

regulation in many countries now dictate efficiency limits.  When specifying motors, both designers and

purchasers should be concerned with efficiency performance.

Within the note, we look at both the European Efficiency Classification and IEC 60034 Efficiency Limits. 

At the end we give some guidance on how to calculate the cost savings associated with the user of higher

efficiency motors.

Contents  [hide]

1. European Efficiency Classification

1. How it works

2. Efficiency values

2. IEC 60034 Efficiency Limits

3. Calculation of cost savings

European Efficiency Classification

European Efficiency Classification The European Scheme to designate energy efficiency classes for low voltage AC motors has been in operation since 1999. The scheme established through co-opera ton between CEMEP and the European Commission is an important element of the European efforts to improve energy efficiency and thus reduce CO2 emissions.

How it worksMotors are defined by levels of efficiency per kW rating and the number of poles. The efficiency is

expressed at both full load and 3/4 load and labels must appear on the motor.

Motors included in the scheme are defined as totally enclosed fan ventilated (normally IP 54 or IP 55),

three phase AC, squirrel cage [[induction motor|induction motors]] in the range of 1.1 to 90 kW, rated for

400 V, 50 Hz, S1 duty class standard design. 

 

Efficiency valuesFor motors designed 380 to 400 V with efficiency values based on 400 V.

2 pole motor specified efficiencies (%)

kW EFF1 EFF2 EFF3

1.1 >= 82.8 >= 76.2 < 76.2

1.5 >= 84.1 >= 78.5 < 78.5

2.2 >= 85.6 >= 81 < 81

3 >= 86.7 >= 82.6 < 82.6

4 pole motor specified efficiencies (%)

kW EFF1 EFF2 EFF3

>= 1.1 83.8 >= 76.2 < 76.2

>= 1.5 85 >= 78.5 < 78.5

>= 2.2 86.4 >= 81 < 81

>= 3 87.4 >= 82.6 < 82.6

4 >= 87.6 >= 84.2 < 84.2

5.5 >= 88.6 >= 85.7 < 85.7

7.5 >= 89.5 >= 87 < 87

11 >= 90.5 >= 88.4 < 88.4

15 >= 91.3 >= 89.4 < 89.4

18.5 >= 91.8 >= 90 < 90

22 >= 92.2 >= 90.5 < 90.5

30 >= 92.9 >= 91.4 < 91.4

37 >= 93.3 >= 92 < 92

45 >= 93.7 >= 92.5 < 92.5

55 >= 94 >= 93 < 93

75 >= 94.6 >= 93.6 < 93.6

90 >= 95 >= 93.9 < 93.9

>= 4 88.3 >= 84.2 < 84.2

>= 5.5 89.2 >= 85.7 < 85.7

>= 7.5 90.1 >= 87 < 87

>= 11 91 >= 88.4 <8 8.4

>= 15 91.8 >= 89.4 < 89.4

>= 18.5 92.2 >= 90 < 90

>= 22 92.6 >= 90.5 < 90.5

>= 30 93.2 >= 91.4 < 91.4

>= 37 93.6 >= 92 < 92

>= 45 93.9 >= 92.5 < 92.5

>= 55 94.2 >= 93 < 93

>= 75 94.7 >= 93.6 < 93.6

>= 90 95 >=9 3.9 < 93.9

IEC 60034 Efficiency Limits

IEC 60034 Efficiency Limits IEC 60034-30 defines three efficiency classes for of single speed, three phase, cage induction motors.

IE1 - Standard efficiency (efficiency levels roughly equivalent to EFF2)

IE2 - High efficiency (efficiency levels roughly equivalent to EFF1, identical to EPAct in USA)

IE3 - Premium efficiency (identical to "NEMA 

Premium" in the USA)

IEC 60034-30 covers almost all motors, with the notable exceptions of motors made solely for converter

operation and motors completely integrated into a machine (and which cannot be tested separately) .

IEC 60034 Efficiency Limits 

Efficiency limit values IEC 60034-30; 2008

Output 

kw

IE1 - Standard Efficiency IE2 - High  Efficiency IE3 - Premium  Efficiency

2 pole 4 pole 6 pole 2 pole 4 pole 6 pole 2 pole 4 pole 6 pole

0.75 72.1 72.1 70.0 77.4 79.6 75.9 80.7 82.5 78.9

1.1 75.0 75.0 72.9 79.6 81.4 78.1 82.7 84.1 81.0

1.5 77.2 77.2 75.2 81.3 82.8 79.8 84.2 85.3 82.5

2.2 79.7 79.7 77.7 83.2 84.3 81.8 85.9 86.7 84.3

3 81.5 81.5 79.7 84.6 85.5 83.3 87.1 87.7 85.6

4 83.1 83.1 81.4 85.8 86.6 84.6 88.1 88.6 86.8

5.5 84.7 84.7 83.1 87.0 87.7 86.0 89.2 89.6 88.0

7.5 86.0 86.0 84.7 88.1 88.7 87.2 90.1 90.4 89.1

11 87.6 87.6 86.4 89.4 89.8 88.7 91.2 91.4 90.3

15 88.7 88.7 87.7 90.3 90.6 89.7 91.9 92.1 91.2

18.5 89.3 89.3 88.6 90.9 91.2 90.4 92.4 92.6 91.7

22 89.9 89.9 89.2 91.3 91.6 90.9 92.7 93.0 92.2

30 90.7 90.7 90.2 92.0 92.3 91.7 93.3 93.6 92.9

37 91.2 91.2 90.8 92.5 92.7 92.2 93.7 93.9 93.3

45 91.7 91.7 91.4 92.9 93.1 92.7 94.0 94.2 93.7

55 92.1 92.1 91.9 93.2 93.5 93.1 94.3 94.6 94.1

75 92.7 92.7 92.6 93.8 94.0 93.7 94.7 95.0 94.6

90 93.0 93.0 92.9 94.1 94.2 94.0 95.0 95.2 94.9

110 93.3 93.3 93.3 94.3 94.5 94.3 95.2 95.4 95.1

132 93.5 93.5 93.5 94.6 94.7 94.6 95.4 95.6 95.4

160 93.7 93.8 93.8 94.8 94.9 94.8 95.6 95.8 95.6

200 94.0 94.0 94.0 95.0 95.1 95.0 95.8 96.0 95.8

250 94.0 94.0 94.0 95.0 95.1 95.0 95.8 96.0 95.8

315 94.0 94.0 94.0 95.0 95.1 95.0 95.8 96.0 95.8

355 94.0 94.0 94.0 95.0 95.1 95.0 95.8 96.0 95.8

375 94.0 94.0 94.0 95.0 95.1 95.0 95.8 96.0 95.8

 

From June 16, 2011 machine builders are only permitted to use high-efficiency motors with

a minimum efficiency class of IE2 (IEC 60034:2008). The new EU Directive 2005/32/EC is

applicable to low-voltage asynchronous motors of 0.75 to 375 kW.

The aim of the change is that by reducing losses, carbon-dioxide emissions and operating

costs are reduced.

Calculation of cost savingsA quick calculation of annual savings is given by:

where:

hrs        = annual running time (hours)

kW        = motor rating in kW

%FL       = fraction of full load power motor is running at

Rate      = electricity cost per kWh

ηstd       = efficiency of standard motor

ηeff        = efficiency of better motor- See more at: http://myelectrical.com/notes/entryid/155/european-motor-efficiency-classification#sthash.gcfQpCQI.dpuf

Power Factor By Steven McFadyen on July 10th, 2012 

Power Factor Power factor is the ratio between the real power (P in kW) and apparent power (S in kVA) drawn by an electrical load. 

The reactive power (Q in kVAr) causes the real and apparent power to be displaced from each other. 

Reactive power provides the necessity for electric and magnetic fields to enable the power system to

work.

In addition to being the ratio of real power to apparent power, the power factor can also be express as the

cosine of the angle between the two.

If the reactive power of the load is inductive, the real power will lag the apparent power and the power

factor will be lagging.  If the reactive power is capacitive the power factor will be leading.

In it's simplest form power factor, can also be considered a measure of the useful work obtained from a

power system

Three phase power factor and single phase power factor follow the same concepts

For a theoretical discussion of power factor, see the section on Complex Power on the Alternating

Current Circuits note.

Contents  [hide]

1. Typical Power Factors

1. Harmonic Distorted Waveforms

2. Power Factor Correction

1. Power Factor Correction Tool

Typical Power FactorsAverage Power Factor values for the most commonly used plant, equipment and appearances:

plant and appliances cos φ tan φ

induction motor - loaded at 0% 0.17 2.80

induction motor - loaded at 25% 0.55 1.52

induction motor - loaded at 50% 0.73 0.94

induction motor - loaded at 75% 0.80 0.75

induction motor - loaded at 100% 0.85 0.62

lamps incandescent 1.0 0

lamps fluorescent (uncompensated) 0.5 1.73

lamps fluorescent (compensated) 0.93 0.39

lamps discharge 0.4 to 0.6 2.29 to 1.33

oven resistance elements 1.0 0

oven induction heating (compensated) 0.85 0.62

oven dielectric heating 0.85 0.62

resistance type soldering machines 0.8 to 0.9 0.75 to 0.48

arc-welding fixed 1-phase 0.5 1.73

arc-welding motor-generator set 0.7 to 0.9 1.02 to 0.48

arc-welding transformer-rectifier set 0.7 to 0.8 1.02 to 0.75

arc furnace 0.8 0.75

Source: Groupe Schneider - Electrical installation guide 

(According to IEC International Standards), 1996

Harmonic Distorted WaveformsExample - true power factor

A load is operating with a displacement power factor of 0.875 and THD of 13.4%.  What is

the true power factor?

The distortion power factor is given by:

 

Resulting in a true power factor of:

Power factor as set up above assumes a sinusoidal wave form.  In a modern power system with the

growth of power electronic devices, the waveform is generally not sinusoidal.  In this instance the

definition of power factor becomes a little more complicated. 

displacement power factor - is the power factor of the 50 Hz fundamental for a

harmonic distorted waveform

distortion power factor - is the amount the displacement power is reduced due to

harmonic content

true power factor - is the actual power factor, taking into account the harmonic

distortion

In a non-sinusoidal waveform the harmonic content reduces the power delivered to the load.  True power

factor will always be less than the displacement power factor. The ratio of the true power factor to the

displacement power factor is the distortion power factor.  For purely sinusoidal waveforms the distortion

power factor is always 1. 

If the total harmonic distortion is know, then the distortion power factor can be found from:

Power Factor CorrectionCalculation of Required kVAr

Existing Situation is defined as load P in kW with power factor pf1  (assume lagging)

Leading  to, complex power S1 in kVA 

Current Phase angle φ1 is given by:  

The desired situation is defined as new power factor pf2 

New Phase angle φ2 is give by:  

Required compensation Q2 in kVAr:  

  and

 

Numerical Example

Existing Situation: P = 450 kW pf1 = 0.83 

S1 = 450/0.83 = 542 kVA

φ1 = cos-1(0.83) = 33.9 degrees

Desired Situation: pf2 = 0.95  

φ2 = cos-1(0.95) = 18.2 degrees

Calculation Results: 

Q2 = 450 * (tan(33.9)-tan(18.2)) = 154 kVAr

S2 = 450/0.95 = 473 kVA (12.7% reduction)

By improving the power factor, power supply authorities need to generate less reactive power and power

distribution systems become more efficient.  Power supply authorities often charge a penalty for power

factor and it can be financially beneficial for the owner of equipment to provide systems to improve their

power factor.

Power factor correction is typically carried out by the addition of capacitors – creating reactive power

180o out of phase with that created by the loads (typically inductive).

Power factor correction may be applied as bulk correction at the main plant switchboard or installed

locally at each load.

 

 

 

If the kW is to remain constant then:

 

and

giving:

Power Factor Correction Tool

Power Factor Correction Unit

The calculation of the amount of reactive power required to achieve a given improvement is relatively

easy to calculate.

To assist in the calculation of required power factor compensation we have added a power factor

correction calculation tool to our site. See the links below for details.

myElectrical's Power Factor Calculator  - easy to use online calculator- See more at: http://myelectrical.com/notes/entryid/197/power-factor#sthash.rrRDwsUf.dpuf

By Steven McFadyen on April 2nd, 2012 

Anyone specifying or using electric motors should have a basic understanding how the insulation is related to temperature. Three classes of insulation are in common use (with 'F' being the most common):

class B - with a maximum operating temperature of 130 oC

class F - with a maximum operating temperature of 155 oC

class H - with a maximum operating temperature of 180 oC

The image (which is form an ABB catalogue for their low voltage performance motors), shows how

temperature rise is distributed across the insulation.

Typically motors are designed for a maximum ambient temperature of 40 oC.

The difference between the average winding temperature and any hot spot is limited and it is usual to

allow a 10 oC margin for class 'B' and 'F' insulation and a 15 oC margin for class 'H'.

Considering the ambient temperature and hot spot allowance gives the maximum temperature rise within

which the motor must be designed to operate (105 oC for class 'F' for example).

When specifying (buying) a motor there are a couple of options. An insulation class could be specified

and the motor specified as designed to run within that class. Alternatively the motor could be specified for

an insulation class, but be design to run at a low class (for example insulation class 'F', temperature rise

'B').

The advantage of the second method is that there is an inherent 25 oC safety margin - useful if you are in

a region with high ambient temperatures or need to date the motor for some other reason. Running

motors at a reduced temperature will also significantly extend the useful life.- See more at: http://myelectrical.com/notes/entryid/122/understanding-electric-motor-insulation-temperature#sthash.Isv7Ieas.dpuf

By Steven McFadyen on March 28th, 2012 

Danfoss Variable Frequency Drives

Variable frequency drives are widely used to control the speed of a.c. motors. This note

looks at the mechanisms which enable drive units to control the speed. In addition to speed,

other advantages offered by variable speed drives are investigated.

Contents  [hide]

1. Speed Control of AC Motors

1. General Theory

2. Frequency Control of Speed

3. Circuit Operation

4. Motor Cooling & Derating

2. Variable Speed Drive Units

1. Feature & Considerations

2. Multiple motors on one unit

3. Galvanic Isolation

4. Power Flow and Quadrants

5. See Also

Speed Control of AC MotorsGeneral TheoryThe speed of an a.c. motor is given by:

Where:

f     = frequency, Hz 

p    = number of pole pairs 

S    = slip

It is possible to vary the speed of a motor by changing any of the above variables (frequency, pole pairs

or slip). However, many of these techniques have some problems associated with them and the most

popular method is that of varying frequency.

Frequency Control of Speed

Pulse Width Modulation

In frequency control, the variable frequency drive (inverter) supplies a series of dc voltage pulses to the

motor terminals at a high frequency (typically 1000 to 10000 Hz). The width of the pulses is varied so that

the average voltage seen by the motor is a sine wave (of a given frequency). Changing the width and

frequency of the dc pulses varies the frequency of the voltage applied to the motor (and hence the motor

speed). This method of frequency control is called pulse width modulation (PWM).

The frequency controller can never give a perfect sinusoidal voltage and hence harmonics will be present.

In addition, the motor will generate more noise and high losses.

As the frequency is changed the applied voltage to the motor, is also changed (such that the ratio of

voltage to frequency is held constant). This is necessary to provide a constant torque over the operating

range.

Circuit OperationThe circuit shows the basic power operation of a variable frequency drive.

Simple Frequency Drive Power Circuit

Diodes (or silicon controlled rectifiers, SCR) on the left side form a three phase rectifier whose job is to

convert the incoming a.c. supply to a d.c.  The arrangement shown is that of a six pulse rectifier.  More

advance twelve and eighteen pulse rectifiers using phase shifting transformers, which while more

expensive can provide a smooth d.c. and generate less harmonics.

The d.c. capacitor acts as an energy store to provide a more constant d.c. voltage to the inverter.

Insulated-gate bipolar transistor (IGBT) on the right side for the inverter and generate the PWM signal. 

The IGBT act as switches which are turned on by applying a signal to the transistor get connection.  By

turning on the top IGBT, positive d.c. pulses are generated, whereas the bottom transistors generate

negative pulses.  Control circuits with the drive control the width of each pulse to generate the PWM

signal.

Motor Cooling & DeratingAs the PWM waveform is not truly sinusoidal, increase losses will be developed within the motor and will

be shown by an increase in the running temperature of the motor. In addition, when the speed of the

motor is reduced, the cooling effect will also be reduced and additional external cooling may be required.

The characteristics of the motor need to be considering in deciding if there is a need for additional

external cooling.

Due to the increase in losses associated with using a variable speed drive and in the absence of specific

motor data, as a rule of thumb it is recommended that motor output be derated by 10%.

 Note: he higher the switching frequency the closer the output will match that of a sine

wave.  However, due to increased switching the IGBT will incur more losses.

Variable Speed Drive UnitsFeature & ConsiderationsIn addition to basic speed control, modern variable speed drive units have many additional features and

benefits. Some of these features are listed below.

Acceleration & Deceleration 

Drive units allow adjustable acceleration and deceleration rates allowing the speed of

motors to be gradually increased or decreased. 

Drive units can also monitor the acceleration (by current) and deceleration (by dc bus

voltage) and hold the acceleration /deceleration ramp if the motor is about to stall.

This can prevent nuisance tripping during the starting of high inertia loads.

Motor Protection 

Drive units provide full electrical protection for the motor, including over current, over

voltage, under voltage, over temperature, and earth leakage.

Boost 

Normally the output voltage is changed so that the voltage / frequency ratio is kept

constant, thereby giving a constant torque. In some situations, it is beneficial to alter

this pattern and most modern drive units allow several different patterns to be

selected. By altering the voltage frequency ratio, different torque characteristics can

be obtained (i.e. higher or lower starting torque, etc.).

Braking 

Breaking of a motor can be achieved by allowing it to naturally coast to a stop,

injecting a dc current or by regenerative breaking (supplying power back into the

mains grid or dissipating it across a resistor). Modern drive units can allow all these

types of breaking. 

Injecting a dc current into the stator of a motor causes the rotating magnetic field to

collapse and the rotor to stall. Normally drive units allow both the length of breaking

and magnitude of breaking to be controlled by varying the dc current injected. Care

should be taken to ensure that both the time and length of braking are set correctly

for the connected load. 

Regenerative breaking takes the inertia of the load and converts this in the electrical

energy, which is fed back into the mains grid (or dissipated across a resistor).

Reversing Operation 

Variable speed drive units allow the motor to be operated in reverse (without any

changing of the phase of the motor supply cables).

Maximum & Minimum Frequency 

Variable speed units allow the maximum and minimum frequency of use to be set. The

means that regardless of setting of the speed control input, the load can always be run

under safe conditions.

Skip Frequencies 

Sometimes resonance occurs within a motor at certain frequencies. Drive units allow

bands of skip frequencies to be programmed which will allow the controller to pass

quickly through these bands during acceleration and deceleration.

Speed Control Input 

The speed control input to a variable speed drive unit can take many forms. Most

typically this would be a potentiometer on the front of the unit or an external analogue

signal (4-20 mA or 0-10V).

Multiple motors on one unitIt is possible to connect multiple motors to one variable frequency drive.  This has the advantages of cost

saving and a simpler installation.   On the other hand all motors will need to be run in an identical

operating mode.

If this technique is used, it should be noted that with several motors the variable frequency drive will be

unable to detect any overload in an individual motor. Each motor will need it's own overload device. 

Galvanic IsolationGalvanic isolation is applied to the control terminals of the variable speed drive and provides a barrier

between these terminals and the rest of the drive. If any of the control inputs or outputs is accidentally

earthed, the galvanic isolation should protect the sensitive electronics.

Power Flow and Quadrants

VFD Quadrants Power flow within a variable frequency drive can be defined by four quadrants (see image):

Quadrant 1 - the motor is rotating clockwise, with the torque  the same direction

(motor accelerating)

Quadrant 2 - the motor is rotating clockwise, but the torque in the reverse direction

(motor accelerating)

Quadrant 3 - motor and torque are rotating in the opposite direction (motor

accelerating)

Quadrant 4 - motor rotating in the

Understanding Motor Duty Rating By Steven McFadyen on December 12th, 2011 

One of the comments on my Motor Starting Series was asking for something on duty cycles.  Here it

is. 

As a purchaser of a motor, you have responsibility to let the manufacturer know the anticipated duty of the

motor.  To assist in the communication of this information, the standard IEC 60034-1 (Rotating electrical

machines) defines several duty characteristics, denoted S1 to S10:

http://myelectrical.com/Portals/0/Images/PostImages/IEC60034DutyCycle.gif

S1  Continuous duty The motor operates at a continuous load for sufficient

time to enable machine to reach thermal equilibrium. 

S2 Short Time duty Operation at a load for a time not sufficient to reach

thermal equilibrium, followed by enough time for the

motor to cool down.

S3  Intermittent periodic

duty

Series of identical duty cycles each a constant load for a

period, followed by a rest period. Thermal equilibrium is

not reached during the cycle.

S4 Intermittent periodic

duty with starting

Similar to S3, but there is a significant starting time within

the periodic operation.

S5 Intermittent periodic

duty with electric

braking

Sequence of identical duty cycles - starting, operation,

braking and rest.  Again thermal equilibrium is not

reached.

S6 Continuous operation

periodic duty

Identical duty cycles with a period at load followed by a

period at no load.  Difference between S1 is that the

motor runs at no-load, without actual stopping.

S7 Continuous operation

periodic duty with

electric braking

As per S6, but with a significant starting and electric

breaking periods.  Again motor operates at no-load for 

period instead of stopped. 

S8 Continuous operation

periodic duty with

related load/speed

changes

Series of identical repeating duty cycles, where within

each cycle the motor operates at several different load

levels and speed. There is not stopped time and thermal

equilibrium is not reached.

S9 Duty with non-periodic

load and speed

variations    

Load and speed vary periodically within the permissible

operating range. Frequent overloading may occur.

S1

0

Duty with discrete

constant loads and

speeds

Duty with discrete number of load/speed combinations,

with these maintained long enough to reach thermal

equilibrium.

     

 

Thermal Equilibrium is the state reached when the temperature rise of the machine does not vary by more

than 2K per hour.   If you don't specify the duty cycle, the manufacturer will likely assume S1. Click on the

image to see a larger version, illustrating the duty cycles.

If anyone has anything to add, please do so below. 

Estimating Power Demand Using IEC Methods By Steven McFadyen on July 27th, 2011 

Estimating power demand is combination of science and art. It is an area of electrical engineering where there is no correct answer. Plug the figures in your preferred method of calculation and then as an engineer you need to relay on instincts to say if the answer feels right or not. This is a look at one method inline with what could be considered IEC practice.

  reproduced from Schneider's 'Electrical Installation Guide - According to IEC International Standards' Estimating power demand is combination of science and art. It is an area of electrical engineering where there is no correct answer. Plug the figures in your preferred method of calculation and then as an engineer you need to relay on instincts to say if the answer feels right or not.

Individual loads do not necessarily operate at full rated nominal power nor at the same time. Estimating

power demand involves both looking at the total connected load and the maximum expected demand on

the system. As we will see these are not the same.

Contents  [hide]

1. IEC Method

2. Typical Utilisation & Simultaneity Factors

1. Utilization Factor (ku)

2. Simultaneity Factor (ks)

3. Basic Demand Data and Preliminary Planning

4. Related Links

IEC MethodDepending where you are, different methods, figures and procedures are used to estimate the power

demand of an installation. This is a look at one method inline with what could be considered IEC practice.

To get going it is useful to understand some basic definitions:

voltage V - the voltage of the electrical system

load current Ib - the current required to operate an item of equipment

apparent power kVA - the product of the voltage V and load current Ib

real power kW - the actual power consumed by the load or equipment

power factor - the ratio of the real power to apparent power (kW/kVA)

utilisation factor ku - see below

simultaneity factor ks - see below

Utilisation factor ku - name plate ratings invariably list higher values of current than will

be seen in use, motors rarely run at full load, etc. A utilisation factor can be applied to these

ratings to establish a more realistic load current, thereby not overestimating the demand.

Simultaneity factor ks - not all equipment runs a the same time; for example one motor

may be duty and the other standby. The same applies to installations; for example a group

of houses or apartments will not all consume the full design current at the same time.

Applying a simultaneity factor takes care of this. Often the term diversity is used and has

the same meaning.

The diagram illustrates how the utilisation and simultaneity factors are used to estimate the power

demand of an installation. Click on the image for a larger version.

Following the diagram, the apparent power of the load or equipment is multiplied by the utilisation factor

to give a realistic power demand to be supplied by a distribution board. Summing these power demand

figures gives the total connected apparent demand (at that board). The distribution board would normally

be sized for this demand.

An appropriate simultaneity factor is applied to the connected apparent demand at the distribution board

and this [diversified] load is carried upstream to higher levels boards. Repeating this procedure will lead to

an expected total demand for the full installation.

In a nutshell, that’s all there is to it - in principal at least. There are often problems in deciding what

simultaneity factor to use and here experience can be really useful.

Tip: estimating power demand this is normally carried out using either apparent or real

power. I prefer real power as it gives me the actual kW required and is an algebraic sum.

Many people will use apparent power, which strictly speaking is a vector sum.   As we are

dealing with estimates (ball park figures even), using either real or apparent power will yield

usable results.

Typical Utilisation & Simultaneity FactorsIdeally utilisation and simultaneity factors should be developed specifically for each application and based

on a knowledge of how that particular system will operate.  For certain situations it may be necessary to

use factors given by supply authorities or some other industry adopted factors.

The factors below are based on those given in the Schneider Electrical Installation Guide and can be

used in the absence of other sources or to provide reality checks on figures being used.

Utilization Factor (ku)Actual power used in equipment is often less than the rated power.  A utilization factor (ku) is used to give

a more realistic estimation of maximum power.

Typical values of Utilization Factor ku:

Type of load ku

Motors (Typical 0.75

Lighting

Circuits

1

Socket Outlets 0.1 to 0.2

Simultaneity Factor (ks)If is rare in practice that all loads operate simultaneously. The simultaneity factor ks is

applied to each group of loads (e.g. being supplied from a distribution or sub-distribution

board). Simultaneity factor is sometimes called diversity factor.

Typical values of Simultaneity Factor ks  by circuit function:

Type of load ks

Lighting 1

General Heating 1

Space Heating 0.8

Air Conditioning 1

Socket Outlets 0.1 to 0.2

Building Installations ks

Escalator 0.5

Elevator 0.3

Sanitary systems 0.5

Sprinklers 0.1

Heating 0.8

Apartment Blocks

ks

2 to 4 1

5 to 9 0.78

10 to 14 0.63

15 to 19 0.53

20 to 24 0.49

Type of load ks

Lifts/Hoists Most powerful motor

1

Second most powerful motor

0.75

For all motors 0.6

Assemblies -Number of Circuits ks

2 and 3 0.9

4 and 5 0.8

6 to 9 0.7

10 and more 0.6

Air conditioning 0.8

Cooling water system 0.7

Refrigeration 0.7

25 to 29 0.46

30 to 34 0.44

35 to 39 0.42

40 to 49 0.41

50 and more 0.40

 

We've produced an Excel spreadsheet for estimating building total connected load and

maximum demand. 

If your interested in obtaining a copy, you can get it here.

Basic Demand Data and Preliminary PlanningSiemens produce a series of publications providing typical demand figures for various building functions

[see Steven's Technical List, Buildings Technology for a list of these].  The following tables are based

on values given in these publications:

Buildings according to their type of use:

Building Use Average Power Demand

Simultaneity Factor

Different functional and building areas

Functional Area/ Building Area

Average Power Demand

Simultaneity Factor

Bank 40-70 w/m2 0.6

Library 20-40 w/m2 0.6

Office 30-50 w/m2 0.6

Shopping centre 30-60 w/m2 0.6

Hotel 30-60 w/m2 0.6

Department store 30-60 w/m2 0.6

Small hospital (40-80 beds)

250-400 w/m2

0.6

Hospital (200-500 beds)

50-80 w/m2 0.6

Warehouse (no cooling) 2-20 w/m2 0.6

Cold store 500- 1,500 w/m2

0.6

Apartment complex (without night storage or continuous-flow water heater)

10-30 w/m2 0.6

Museum 60-80 w/m2 0.6

Parking garage 3-10 w/m2 0.6

Production plant 30-80 w/m2 0.6

Data centre 500-2,000 w/m2

1 .0

School 10-30 w/m2 0.6

Gym hall 15-30 w/m2 0.6

Stadium (40,000-80,000 seats)

70-120 w/seat

0.6

Old people’s home 15-30 w/m2 0.6

Greenhouse (artificial 250-500

Hallway, anteroom or lobby

5-15 w/m2 0.3

Staircase 5-15 w/m2 0.3

General utilities 5-15 w/m2 0.3

Foyer 10-30 w/m2 1.0

Access ways (e.g. tunnel)

10-20 w/m2 1 .0

Recreation room/kitchenette

20-50 w/m2 0.3

Toilet areas 5-15 w/m2 1 .0

Travel centre 60-80 w/m2 0.8

Office areas 20-40 w/m2 0.8

Bookstore 80-120 w/m2 0.8

Flower shop 80-120 w/m2 0.8

Bakery/butcher 250-350 w/m2

0.8

Groceries 80-120 w/m2 0.8

Bistro/ice cream parlour 150-250 w/m2

0.8

Cafe 180-220 w/m2

0.8

Diner/restaurant 180-400 w/m2

0.8

Tobacco shop 80-120 w/m2 0.8

Hairdresser 220-280 w/m2

0.8

Dry-cleaner’s or laundry 700-950 w/m2

0.7

Storage area 5-15 w1/m2 0.3

lighting) w/m2

 

Office Equipment Demand

Recommendations

Equipment Average Power Demand

Data Source

All in one Printer/ Fax/Scanner

75 w CIBSE

Ceiling Projector Lift 50 w Estimated

Ceiling Projector Screen

80 w Estimated

Colour Printer/Copier 200 w CIBSE

Colour Scanner 50 w CIBSE

Computer Peripherals 400 w Estimated

Convenience Sockets 200 w/socket DEWA

Cost Recovery Devices 3,000 w Estimated

Desktop 100 w CIBSE

DVD Player 70 w Estimated

Fixed Camera 30 w Estimated

Laptop 100 w CIBSE

Large Smart Board 300 w Estimated

Monitor 200 to 400 W CIBSE

Paper Shredder 50 w CIBSE

Personal Printer/Fax 50 w CIBSE

Portable Wireless Controller 20 w Estimated

Projector 300 w CIBSE

Other areas:

Area Average power demand

Electric floor heating bedrooms

65-1 00 w/m2

Electric floor heating bathroom

130-150 w/m2

Night storage heating: Low-energy house

60-70 w/m2

Night storage heating: house with “standard” insulation

100-110 w/m2

Small air conditioning unit 60 w/m2

Photovoltaic (maximum output of the modules)

100-130 w/m2

Rack Equipment in Credenza

400 W Estimated

Shredder 190 w Estimated

Technology Wells in Table Top

200 w Estimated

Teleconference Module 50 w Estimated

Wall Mounted Controller 20 w Estimated

Wall Mounted LCD 200 w Estimated

Related Links - See more at: http://myelectrical.com/notes/entryid/74/estimating-power-demand-using-iec-

methods#sthash.UGG8ltey.dpuf

v- See more at: http://myelectrical.com/notes/entryid/106/understanding-motor-duty-

rating#sthash.9oAZkHer.dpuf opposite direction, with clockwise torque (motor

decelerating)

Differing types of equipment will operate in different quadrants, for example pumps typically operate only

in quadrant 1, while an electrical car would use all four quadrants.

- See more at: http://myelectrical.com/notes/entryid/152/variable-frequency-drive#sthash.YpZKLRbL.dpuf

By Steven McFadyen on March 1st, 2010 

ABB has produced a range of technical guides that offer concise explanations of the major technologies and technical issues in low voltage AC drives.  The technical guides cover subjects such as the basics of variable-speed drives, dimensioning a drive system, electrical braking, harmonics and AC drives, and bearing currents.

 Currently the ABB site lists the following guides:

Application Guide, Guide to extruders in AC drives

Sustainability guide, Driving energy efficiency worldwide, ABB motors and drives

Technical Guide, Direct Torque Control

Technical guide, Functional safety

Application guide, ABB drives, Using variable speed drives (VSDs) in pump applications

Technical Guide, EU Council Directives and Adjustable Speed Electrical Power Drive

Systems

Technical guide, EMC compliant installation and configuration for a power drive system

Technical Guide, Guide to Variable Speed Drive

Technical Guide, Bearing Currents in Modern AC Drive Systems

Technical Guide, Guide to Harmonics with AC Drives

Technical Guide, Dimensioning of a Drive system

Technical Guide, Electrical Braking

Technical guide, Guide to motion control drives

The guides can be accessed at:

ABB Technical Guides - Motor Operation- See more at: http://myelectrical.com/notes/entryid/34/abb-technical-guides-motor-operation#sthash.VU2L2KBU.dpuf