Electric Power Generation, Transmission, and Distribution

Ampacity

Iamp¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

TcTa

RacRth

r

,

whereTc ¼conductor temperature
Ta ¼ambient temperature
Rac¼AC resistance at the operating temperature
Rth¼thermal resistance of surrounding environment

Voltage drop

Voltage drop¼IcableðÞRcablecosðÞþf XcablesinðÞf ,

whereIcable¼current in conductor
Rcable¼total ac resistance of the cable
Xcable¼total ac reactance of the cable
f ¼phase angle between supply voltage and current

For single-phase calculations the values of the main and the return conductors must be used.
Pulling tension single cable in straight conduit

T¼mWL,

wherem¼coefficient of dynamic friction (0.2–0.7 dependent on cable exterior and type of conduit)
W¼cable weight per unit length
L¼length

Pulling tension single cable through conduit bend

Tout¼Tinemf(pounds),

whereTin¼the tension entering the bend
m ¼coefficient of dynamic friction (0.2–0.7 dependent on cable exterior and type of conduit)
f ¼bend angle in radians

The pulling tensions of each segment of the conduit path are added together to determine the total
pulling tension.
When multiple single cables are installed in a conduit, a multiplier must be applied to the cable
weight, accounting for configuration as follows:
For three cables with a triangular configuration the weight multiplier is

Wmultiplier triangular¼

2

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1 

d

Dd

s  2 :

For three cables with a cradled configuration

Wmultiplier cradled¼ 1 þ

4

3

d

Dd

 2

,

whered¼single cable outside diameter
D¼conduit inside diameter.