modul 4 cable

8/3/2019 Modul 4 Cable

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Distribution Design System

A.Aman 1 BEKP 4783

UNIT 4: CABLE

1.0INTRODUCTION

1.1.

Cable is a mean or tool for which electrical energy is being distributed fromone place to others.

1.2. Cable also means to prevent any person from direct contact with the live

conductors.

1.3. The conductor has to be covered with a layer of insulating material, called

cable insulation.

1.4. To prevent any person from direct contact with the live conductors, a cable can

be definedas a length of insulated single conductor or two or more

conductors with its own insulation which are laid up together. The

insulated conductor or conductors may or may not be provided withoverall covering for mechanical protection.

1.5. Where mechanical protection is required, a second insulating layer known as

sheath is wrapped around the first layer ofinsulation. For stronger protection,

a layer ofsteel wire known as armour is wound around the first layer. A

sheath usually PVC is then placed over the armour.

2.0CONDUCTORS

2.1. The most common conductors used in cables are copper (Cu) and aluminum

(Al).

2.2. The resistance ofcopper at 70oC is 0.017/mm

2/m and aluminum

0.0283/mm2/m.

2.3. Aluminum has become a major alternative to copper as the price is cheaper.

However, it has poorer conductivity than cooper. Thus, the cross sectional

area of aluminum is larger. Example: for the same current rating, 300mm2

Al

cable is approximately equivalent to 185mm2.

2.4. A conductor and its immediate insulation known as core.

2.5. The conductor is identified by colour code:

2.5.1.1. Red, Yellow and Blue for phase conductors.

2.5.1.2. Black for Neutral.

2.5.1.3. Green/Yellow for CPC. Ground wire.


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2.6. Rated Voltage for cable is determined by Vo/V:

2.6.1.1. Vo is voltage between insulated conductor to earth and V is

between phase conductor.

Example: Low voltage power cable 600/1000V.

2.7. Conductors can be solid orstranded. Stranded conductors are more flexibleand easier to handle.

2.7.1.1. Solid conductor- single wire of the required cross-sectional area is

used.

2.7.1.2. Stranded conductor several wires of small cross-sectional are

twisted together to form a larger wire of the required sectional area.

Stranded conductors are more flexible and easier to handle.

Normally 3 to 7 wire twisted to form one conductor.

Figure 1- Cable conductor

2.8. Bare conductors are used in overhead HV transmission cable, earthing system,

lighting protection and busbars. Insulated conductors are used in domestic

and industrial installation, overhead LV (ABC), underground cable system and

submarine cable system.

2.9. Sizes of conductors are denoted by the cross-sectional area of the conductors for example 6mm

2cable means that the total cross-sectional area of all the

strands is 6mm2.

2.10. Copper conductor is widely used for the electrical earthing. The use of

aluminum as earth electrode or earthing conductor is prohibited.

3.0INSULATION MATERIALS

3.1.The selection of insulation materials is based on their physical and electrical

properties. Materials used as insulation for electrical cable must;

3.1.1.1. Provide safe condition of conductors at it rated voltage.

3.1.1.2. Electrical insulating properties should not deteriorate with time.

3.1.1.3. Have high mechanical strength during installation and operation.

3.1.1.4. Have thermal stability- chemical composition not effect by heat.

3.1.1.5. No environment or surrounding effect like weather or water.


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3.2.Types of insulation material used:

a) Butyl rubber can be used in ambient temperature of up to 80oC.

b) Silicone rubber suitable for temperature -60oC to 150

oC.

c) PVC poly-vinyl chloride or thermoplastic. Soften at 80oC.

d) Glass fiber Good heat resistance up to 155oC.

e) Paper paper impregnated with mineral oil mixed resin.

f) Mineral insulating material is magnesium oxide. Withstand very

high temperature up to copper melting point.

g) XLPE cross-linked polyethylene

h) FR fire retardant (specially formulated PVC compound)

4.0CABLE TYPES AND SPECIFICATIONS

4.1.Cables are named in accordance to:

a) Type ofinsulation

b) Mechanical protection

c) External sheath

Example: XLPE/SWA/PVC XLPE- (cross-linked polyethylene) insulation,

SWA steel wire armored (mechanical protection) and PVC external sheath.

4.2.The most common type of cables used in distribution systems are:

a) PVC PVC insulated.

b) PVC/PVC PVC insulated, PVC sheath

c) PVC/SWA/PVC PVC insulated, steel wire armored, PVC sheath.

d) FR(Fire Retardant) fire rate equipment e.g. emergency light,KELUAR sign, BOMBA lift.

e) MICC Mineral insulated copper covered.

f) MICS Mineral insulated copper sheath.

g) XLPE/SWA/PVC XLPE insulation, steel wire armored, PVC sheath

underground cables.

h) PILC Paper Insulated Lead Covered used in HV system.

4.3.The current capacity must be sufficient to cater for the maximum sustained

current which normally flows through the cable and the insulation must be

adequate with the system voltage.

4.4.In a single line diagram, the type and size of the cable is written as follows:

Example: 2x120mm2

4C PVC/SWA/PVC (Al) cable means that 2 set of

120mm2, 4 Core, PVC insulated, steel wire armored, PVC sheath and aluminum

conductor.


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4.5.According to BS 6004, all PVC insulated cables are suitable only as long as

the conductor temperature does not exceed 70oC and formineral insulated

cables to BS 6207 can be operated up to 135oC.

4.6.A sample of current carrying capacity (ampacity) is shown in TABLE 4D1A

(IEE Wiring Regulation BS 7671- Requirement for Electrical Installation)

for single core PVC insulated cables, non-armored with or without sheath.

Figure 2- Types of cables

5.0INSTALLATION

5.1. Small wiring PVC or FR cables can be installed in conduits or trunkings. The

conduits and trunkings are made of steel or PVC.

5.2. Schedule ofmethods of installation of cables is shown in TABLE 4A1 ofIEE

Wiring Regulation BS 7671.

5.3.

Table 4A1 in IEE Wiring Regulations BS 7671 shows the methods ofinstallation of cables and conductors.


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5.4. There are seven types of wiring systems and there are twenty methods of

installations in general wiring or in electrical installation. The using of method

of installation is to find the appropriate reference for determining current

carrying capacity of the cable. Below are the lists of seven types of wiring

system.

i. Open and clip directii. Cable embedded direct in building material.iii. In conduitiv. In trunkingv. In the free air, on cleats, brackets or a laddervi. Cable in building voidsvii.Cable in trenches

5.5. Fordomestic wiring installation only method 1, method 3 and method 4

have to consider.

5.6. The method of installation in final circuits wiring installation is importance,

where the methods of installations will influence ofcable size and selection ofrated current of protective devices.

5.7. Common used of LV cables:

a) Non-Armoured PVC cables installed in conduits & trunking for

internal wiring.

b) Non-Armoured PVC/PVC cables general indoor use in domestic &

commercial installation.

c) Armoured PVC cables for mains & sub-mains application.

d) MICS cables areas of extreme temperature or for circuit supplied to

firefighting equipment.

e) FR cables used in severe condition (firefighting / flammable

equipment)

6.0CABLE SIZING AND CURRENT RATING

6.1.The size of the cable to be used in a circuit depends on:

a) The full load current based on design current Ib.

b) Ambient temperature Ca factor.

c) Maximum allowable conductor temperature.

d) Conductor material current ampacity. - Table 4D1A and 4D2A in

BS7671 for copper conductor.

e) The method ofinstallation. Table 4A1 in BS7671

f) Insulation material. - Table 52B (Regulation 523-1) in BS7671

g) The number of circuits installed together Cg.

h) The voltage drop in the cable. - Table 4D1B and 4D2B in BS7671.

6.2.It is a normal practice that the design engineer relies on the tabulated current

carrying capacities as shown in Appendix 4 of the IEE Wiring Regulation


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BS 7671. However, it should be noted that the tabulated current ratings are

based upon a given set of condition.

a) Ambient temperature.

b) The heating effect of adjacent cable is not considered.

c) The cable is installed according to the rating table being used.

d) There is no surrounding thermal installation.

6.3.Therefore, any changes on the above said conditions, the cable rating has to be

adjusted according to the appropriate correction factor as follows.

a) Ambient temperature Correction Factor (Ca). - Table 4C1 in

BS7671

b) Grouping Correction Factor (Cg). - Table 4B1 in BS7671

c) Thermal Insulation Correction Factor (Ci).

6.4.For ambient temperature higher than specified temperature of 30oC, the rated of

the heat flow out of the conductor will be lower and thus will increase the

conductors operating temperature above permitted value.

6.5.Correction factor of temperature Ca in determining the current carrying

capacity of cable are provided in Table 4C1 ofIEE Wiring Regulation BS

7671.

6.6.Cables installed in the same enclosure or bunch together will get warm when

all are carrying current. Those cables close to the edges of enclosures will be

able to release the heat outward but will be restricted in losing heat inwards

toward other warm cable. Cable in the centre of the enclosure may find it

difficult to lose heat at all and as a result will increase the conductor

temperature.

6.7.The correction factor for groups Cg is shown in Table 4B1 ofIEE Wiring

Regulation BS 7671.

6.8.Many new building are now provided with better thermal insulating material

for roofs and cavity walls to reduce the heat lose. As thermal insulation is

design to limit heat flow, a cable in contact with it will tend become warmer

than the preferred operation conditions.

6.9.IEE Wiring Regulation 523-04 recommends that for a single cable, which is

likely to be surrounded by thermally insulated material over a length of more

than 0.5m, the thermal insulated Ci is 0.5 times the tabulated current carryingcapacity (method 1). Table 52A ofIEE Wiring RegulationBS 7671 shows

the de-rating factor for cable surrounded by thermal insulation for a length less

than 0.5m.


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7.0DETERMINE THE MINIMUM TABULATED CURRENT RATING

7.1.After considering all the above correction factors, the minimum tabulated

current rating for the cable can be determined as recommended in the IEE

regulation.

7.2.As an example the related table for copper conductor current carrying capacity

7.3.The formula used is as follows:

a) Where overload protection is required:

It min In

Ca x Cg x Ci

b) Where overload protection is not required:

It min Ib

Ca x Cg x Ci

8.0VOLTAGE DROP

8.1.The resistance of the conductor increases as the length increases and/or the

cross sectional area of cable decreases.

8.2.Associated with an increase resistance is a drop in voltage, which means that a

load at the end of long or thin cable will not have the full supply voltage

available.

8.3.To ensure the voltage drop from the supply intake to the terminal of any

appliance does not exceed the appliance operating tolerance, which is normally10%.

8.4.IEE Regulation 525-01 specifies that the voltage drop between the origins of

installation does not exceed 4% of the nominal voltage of the supply.

8.5.The utility supply regulation normally require that the voltage drop in the

circuit does not exceed 2.5% of the nominal voltage, i.e. 6V for single phase

240V supply and 10.4V for a three phase 415V supply. This is also a good

practice for a designer since the cable routing on the paper may differ from the

actual at site installation.

8.6.Values of voltage drop are tabulated for a current of 1 Amp for a 1 meter run,i.e. for a distance of 1 meter along the route taken by the cables. This

Tabulated Voltage Drop constant (TVD) is expressed in m per ampere per

meter run mV/A/m.

8.7.Voltage drop formula is as follows:


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( )1000

llengthITVDV brdrop

= for cable 16mm2

or less

( )1000

sincos lengthITVDTVDV bxrdrop

+

=

8.1.1 VOLTAGE DROP CALCULATION

8.1.1.1 TEMPERATURE CORRECTION ON RESISTIVE VALUE

8.1.1.2In relevant standards and manufacturer specifications, the value of

conductor resistance is usually given at 20oC of conductor

temperature. This conductor resistance value will increase

according to the resistance temperature coefficient when conductor

temperature increases due to load current.

8.1.1.3The coefficient is approximately equal to 0.004 pero

C at 20o

C forboth copper and aluminium conductor. The value of conductor

resistance is zero mathematically at a conductor temperature of -

230oC.

8.1.1.4The conductor resistance value at other temperature such as 115can be calculated as:

115 = 20 230 + 115 = 20 x 1.38

230 + 20

Fi ure 3: Calculation of resistance value at various


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8.1.1.2CONDUCTOR TEMPERATURE ON VOLTAGE DROP (TVDr)

8.1.2.3 The TVDrvalue is based on the resistance of conductor at rated

temperature (i.e. 70oC for PVC-insulated Cu conductor)

corresponding to the conductor carrying its rated current.

8.1.2.4 If design current is significantly less than the rated current, the actualvalue of TVDrwill be lower due to lower conductor temperature.

Thus, the new value of TVDrat other temperatures is given by:

( TVDr)40o

C = 230 + 40 x ( TVDr)70o

C

230 70

8.1.2.5 The value of reactance (TVDx) is not influenced by temperature,

therefore the temperature correction does not apply to the reactance

values.

9.0PROTECTION AGAINST OVERLOAD

9.1 Overload currents may occur in various circuits in an installation where they are

carrying more current than their rated capacity. The consequences of overload

currents will increase the temperature of conductors and the effectiveness of the

insulation and its expected life may be reduced.

9.1.1REQUIRED CONDITION FOR OVERLOAD PROTECTION

9.1.1.1 To provide an adequate protection for overload, IEE Regulation 43

requires the following conditions to be satisfied:

1) IN IZ

2) I2 1.45 IZ

Where

IN : current rating of the protective device

IZ : current rating of the cable under particular installation methods.

I2 : current magnitude causing an effective operation of the protective

device.

Figure 4: Rated conductor temperature


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9.1.1.2 For MCB, the value I2 is 1.45 IN. For MCCB and ACB, the value I2 is

1.3 IN. For fuse, the value I2 is 1.6 IN.

9.1.1.3 The effective operating time is 2 hours (for MCB, MCCB and ACB),

except for breaker less than 63A where the effective operating time is

reduced to 1 hour. For fuse, the effective operating time is in the rangefrom 1 to 4 hours depending on the current rating.

9.1.1.4 In loading condition (I2 1.45 IZ), the cable is allowed to increase145% of its rated capacity even though conductor temperature is higher

than rated temperature but still below its critical temperature. Critical

temperature refers to the temperature that will cause insulation failure.

9.1.1.5 To assess whether there is an overload protection and how adequate it

is, we define the degree of overload protection as:

OL_P_Yes = 1.45 IZ - I2 x 100%

1.45Z

9.1.1.6 A positive value of (OL_P_Yes) means that the circuit is adequately

protected against overload current and a higher percentage indicate that

the circuit is more unlikely to be overloaded. The condition is vice

versa for negative value of (OL_P_Yes).

9.1.2SMALL OVERLOADS AND CABLE UTILISATION

9.1.2.1 There is possible to have a range of very small overload which will not

be detected by the protective device (i.e. at the range between IZ I I2). In general statement IEE Regulation 433-01-01, it states that every

circuit shall be designed so that a small overload of long duration is

unlikely to occur.

9.1.2.2 Therefore, we define the existence of small overload as:

Sm_OL_No = IZ - I2 x 100%

IZ

9.1.2.4 A positive value of (Sm_OL_No) indicates no undetected small

overload while a negative value means that small overload range

exists. Thus, designer should as far as possible minimize the negative

percentage (i.e. increase the conductor size will totally eliminate the

occurrence of such small overloads).


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9.1.2.5 If value of IZ is very close to IN, the un-detected small overloads may

occur (under-design) and if the value of IZ is very much higher than IN,

then it implies a larger size of cable has been selected which will

increase the installation cost and not fully utilized (over-design).

9.1.3OMISSION OF OVERLOAD PROTECTION

9.1.3.1 There are several circuits in which a break in current (tripping) by the

operation protective devices can cause danger. Example; breaking the

current of a lifting electromagnet could cause the load to drop, or

breaking the current in CT could induce a very high E.M.F

(electromagnetive force). So, it can be replaced by an overload alarm

instead of overload breaker.

9.1.3.2 For starting current of a motor, the designer has to ensure that the

protective device of the motor circuit will not trip during motor

starting. If starting current is large, the rated current IN may have to

be much higher than the design current IB.

9.1.3.3 Therefore, the current rating of cable, IZ has to be equal to or larger

than IN.

IN >> IB and IZ >> IB

9.1.3.4 However, if overload protection is not required, the designer can select

IZ to be independent of IN, and IZ >> IB.

9.1.3.5 Ifoverload protection is required, the minimum tabulated current

rating for the circuit is obtained by:

It,min = IN

Ca x Cg x Ci

9.1.3.6 Ifoverload protection is not required, the minimum tabulated

current rating for the circuit is obtained by:

It,min = IB

Ca x Cg x Ci

10.0 PROTECTION AGAINST SHORT CIRCUIT

10.1 It is necessary to ensure that the circuits conductor are protected adequately

against short-circuit current by interrupting breaker quickly enough to prevent

thermal damage to the cable.


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10.1 REQUIRED CONDICTIONS FOR SHORT CIRCUIT PROTECTION

10.1.1 During fault conditions up to duration of 5 seconds, the maximum time that

the cable can withstand the fault current can be approximately by the

following formula:

Tcable,max = k2S2

IF2

Where k : maximum thermal capacity of the conductor for the type of

insulation being used

S : cross-sectional area of the conductor in mm2

IF : prospective short-circuit current in amperes

(Note: For more than 5 seconds, more complicated formula has to be used)

10.1.2 (Tcable,max) also known as critical time, which is the time taken for temperature

of conductors to rise from rated value, Q1 to the critical value, QF. If QF is

exceed, the insulation material fails and the whole cable is thermally damage.

10.1.3 To assess whether short-circuit protection is provided and how adequate it is,

let us define:

SC_P_Yes = ( Tcable,max Tbk,3-phase F ) x 100%

Tcable,max

Where Tbk,3-phase F : operating time of the breaker corresponding to a current

during 3-phase fault at the cable.

10.1.4 A positive percentage (SC_P_Yes) indicates that the circuit meets the

requirement for shor-circuit protection and higher percentage implies a higher

margin of short-circuit protection. A negative percentage (SC_P_Yes) is vice

versa.


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10.2 FORMULA FOR SHORT-CIRCUIT CURRENT

10.2.1 Three Phase Fault: the current magnitude in each phase is identical except

that the angle is shifted by 120o

in each phase. Thus the current in each phase

can be calculated as:

IF,3 = VLL / 3

(RS + R1)2

+ (Xs + X1)2

10.2.2 Line-to-Neutral Fault :

IF,LN = VLL / 3

(RS + R1 + RN)2

+ (Xs + X1 + XN)2

10.2.3 Line-to-Line Fault :

IF,LL = VLL

(2RS + 2R1)2

+ (2Xs + 2X1)2

Where RS : Resistance value of supply source (per phase)

XS : Reactance value of supply source (per phase)

R1 : Resistance value of phase conductor (per phase)

Figure 5: Values of k for calculation of the effects of fault current


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X1 : Reactance value of phase conductor (per phase)

RN : Resistance value of the neutral conductor

RN : Reactance value of the neutral conductor

VLL : Line-to-Line voltage.


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11.0 TABLES AND FIGURES (FROM IEE WIRING REGULATIONS)

11.1. Conductors Temperature

11.2. Cable Ampacity


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Table 4D1A Single-core Copper Conductor Current Capacity


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Table 4D2A Multi-core Copper Conductor Current Capacity


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11.3. Voltage Drop


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Table 4D1B Single-core Copper Conductor Voltage Drop

Table 4D2B Multi-core Copper Conductor Voltage Drop


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11.4. Ambient Temperature Correction Factor, Ca

Table 4C1 Correction factor for ambient temperature.

11.5. Grouping Correction Factor, Cg

Table 4B1 Correction factor for group


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11.6. Schedule of Methods of Installation

Table 4A1 An example of methods of insallation


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Example 1:

If four cores PVC insulated cable enclosed in trunking has tabulated current

capacity 80A. If the ambient temperature 50C, the current rating of cable

reduced 80A x 0.71 = 56.8A. The ambient temperature is refers to the

temperature of the cable surrounding media and does not temperature of

equipment.

* 0.71 refers to Table 4C1 temperature correction factor IEE Wiring Regulation.

* Tabulated current refers to Table 4D2A

Example 2:

A heater rated at 230V, 3kW is to be installed using twin-with-earth PVC

insulated and sheathed cable clip direct in a roof space which has an ambient

temperature of 40C. The circuit is protected by 15A MCB. The cable is

bunched with four other-twin-earth cables for a short distance as shown in

Figure 1. Determine the minimum tabulated current rating of the circuit and the

size of the conductor.

Figure Example1

Solution

The design current is

AIB

13230

3000==

and the current rating f the protective device is IN=15A.

From table 4C1 the temperature correction factor at 40C is:

Ca = 0.87

From table 4B1 the grouping correction factor of five circuits is:

Cg = 0.6

The minimum tabulated current rating Iz,min for the circuit is

ACC

II

ga

Nz 74.28

6.087.0

15min, =

=

=

Referring Table 4D2A of IEE Wiring Regulation, column 6, a 4mm2

which

tabulated current rating of 36A is selected. Although the design current is only

13A, a 4mm2cable rated 36A is selected. Without correction factor ofCa and


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Cg under same 13A operating conditions a 1mm2

cable rated at 15A is

sufficient.

Example 3:

The voltage drop on a circuit supplied from a 240V source by 16mm2

two-corecopper cable 23m long, clipped direct and carrying a design current 33A, will

be;

Solution:

Using formula( )

1000

llengthITVDV br

drop

= due to less than 16mm2

TVDr = 2.8mV - From Column 3 Table 4D2B, cable size 16mm2

two core

cable single phase.

Ib = 33A

Length = 23m

( )1000

llengthImVV bdrop

=

V125.21000

23338.2=

=

The maximum voltage drop for 240V installation less than 4% from the IEE

Regulation is 9.6V, so the voltage drop fulfills the IEE Regulation requirement.The maximum cable length of installation with these parameters could be

determined.

b

drop

ImV

VMaxLengh

=

1000

m149238.2

10006.9=

=

Example 4:

A 3 phase motor with full load current of 102A, and PF of 0.8 is to be fed

by single core, PVC insulated, copper conductor, non-armoured cables,

clipped direct on non metallic surface at 75 meter rub shown in Figure

Example 2. Determine the size of conductor if the permissible voltage

drop is 2%.


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Figure Example 2

Solution:

Let the design current Ib is the motor full load current. From table 4D1A,

column 7, a 25mm2

cable with rated current of 104A is initially selected

and the voltage drop is calculated as:

( )1000

sincos lengthITVDTVDV bxrdrop

+

=

( )

.983.9

1000

751026.0175.08.05.1

V=

+=

9.983 are 2.40% of 415V which exceeds 2% of 415V. Thus the next

higher size of 35mm2

is selected and Vdrop is re-calculated:

( )

.512.7

1000

751026.017.08.01.1

V=

+=

7.512V are 1.81% of 415V, thus in order to fulfill the requirement of Vdrop

less than 2% a 35mm2

cable is selected.

modul 4 cable

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