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
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 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