282 Electrical Power Systems Technology
Solution:
- Find the maximum voltage drop of the circuit.
VD = % × Load
= 0.02 × 240
= 4.8 volts
- Find the current drawn by the load.
Power
I = ————
Voltage
85,000
= ———
240
= 354.2 amperes
- Find the minimum circular-mil conductor area required. Use the for-
mula given for finding the cross-sectional area of a conductor in sin-
gle-phase systems, which was previously given in the “Alternative
Method of Voltage Drop Calculation” section.
cmil = p × I × 2d
—————
VD
10.4 × 354.2 × 2 × 260
= ——————————
4.8
= 399,065.33 cmil
- Determine the feeder conductor size. The next larger size conductor in
Table 10-2 is also 400 MCM. Check Table 10-3, and you will see that
a 400 MCM RHW copper conductor will carry 335 amperes. This
is less than the required 354.2 amperes, so use the next larger size,
which is a 500 MCM conductor.
The conductor size for a three-phase feeder circuit is determined in a
similar way. In this problem, the feeder size will be determined on the ba-
sis of the circuit voltage drop.