274 Electrical Power Systems Technology
Alternative Method of Voltage Drop Calculation
In some cases, an easier method to determine the conductor size for
limiting the voltage drop is to use one of the following formulas to find the
cross-sectional (cmil) area of the conductor.
p × I × 2d
cmil = ————— (Single-phase Systems)
VD
or,
p × I × 1.73d
cmil = —————— Three-phase systems
VD
where:
p = the resistivity from Table 8-2
I = the load current in amperes,
VD = the allowable voltage drop, and
d = the distance from source to load, in feet.
The sample problem given for a single-phase system in the preced-
ing section could be set up as follows:
p × I × 2d
cmil = ————
VD
10.4 × 200 × 2 × 400
= —————————
240 × 0.02
1,664,000
= —————
4.8
= 346,666 = 347 MCM
The next largest size is a 350 MCM conductor.
BRANCH CIRCUIT DESIGN CONSIDERATIONS
A branch circuit is defined as a circuit that extends from the last over-
current protective device of the power system. Branch circuits, according