AN ENGINEERING SELECTION FOR SIZE OF LONG CONDUCTORSCONSIDERING FACTORS IN

ADDITION TO NEC

AN ENGINEERING SELECTION FOR SIZE OF LONG CONDUCTORSCONSIDERING FACTORS IN

ADDITION TO NEC

Marcus O. Durham THEWAY Corp / U of TulsaMarcus O. Durham THEWAY Corp / U of Tulsa

Robert A. Durham Radco ConsultingRobert A. Durham Radco Consulting

Marcus O. Durham THEWAY Corp / U of TulsaMarcus O. Durham THEWAY Corp / U of Tulsa

Robert A. Durham Radco ConsultingRobert A. Durham Radco Consulting

Abstract Conductor Sizing– Long conductor – Voltage Drop– Diameter, Length, # Phases– Current, Temperature

Conductor SizingCommon Part of Electrical Design

NEC = insulation temperature

I => I2R losses => heat

For runs < 100 ft

For runs > 100 ft,

voltage drop begins dominate

Conductor SizingImpedance

Zallowed = Vdrop allowed / Irequired

Its the basics

ConductorSmall Size

A mild

d( ) . 22

24

0 7854

A cmil d( ) 2

mil cmil2 0 7854 .

d = 10 mils

A = 100 cmils = 78.54 mils2

Conductor Resistance

Correct resistivity for temperature, mat’l, config.

Base = annealed copper @ 20o C

CU @ 25o C

AL @ 25o C

10 371 102

234 5

254 5. * .

.

.

o

o

C T

C

10 786. cmil / ft

17 35. cmil / ft

Wire DimensionsFirst Pass - Resistance Only

A

lR

DV

IlA

Wire DimensionsResistance - Phase # - Distance

1 phase: ph factor = 1 # cond / ph = 2

3 phase: ph factor = # cond / ph = 1

A

D cond phasefactor I

VD

* # *

3

Voltage DropEffect of Wire Dimensions & Distance

V

D cond phasefactor I

AD * # *

Manipulate area calculation

Wire DimensionsAC -vs- DC

Calculations are DC Sizes < 4/0 DC AC Larger conductors ZAC 1.3 * RDC

Total Z has inductive & capacitive components Z = R + jX

AC ConsiderationsInductive

Wire dimensions reactance

Skin effect negligible @ power frequency

Divide by 2 for coax and transmission

X f L LL 2 377

Ld

r

4 10 1 47* lnl

AC Considerations Capacitive

Parallel, round conductors Divide by 2 for 2 wires of single phase Xc negligible < 2400 V

Yc very large for lines < 10 miles Ignore for cable

Cd r

2 l

ln /

Voltage DropBased on Impedance

Parallel Conductors Neglect Capacitance Calculate VD straightforward

V

IZ R j XD

V

I rf

d

rD

2

2

2

7

2

42 13 816 10

ll * . * ln

LengthFrom Wire Dimensions - Voltage Drop

Contains ‘r’ and ln (r) Can’t solve directly Make assumptions ‘d’ between conductors 3 * radius Equations for VD and A are radicals

ComparisonConventional -vs- Dimensions

For single load, > 100’ Determine size based on I & T, using NEC Calculate size based on V & Z, using technique Select size that is largest for I or V

SummaryWire Size - Conductor Properties

Consider resistance and inductance Correct resistivity for mat’l, config, T Calculate area from resistivity, length,

phase, current, V drop Manipulate for voltage drop and distance

It’s Radical

Questions ?

Questions ?