272 Electrical Power Systems Technology
of voltage drop that a system can have. This means that long runs of con-
ductors must ordinarily be avoided. Remember that a conductor with a
large cross-sectional area will cause a smaller voltage drop, since its resis-
tance is smaller.
To better understand how to determine the size of conductor re-
quired to limit the voltage drop in a system, we will look at a sample
problem.
Sample Problem:
Given: a 200-ampere load located 400 feet (121.92 meters) from a
240-volt single-phase source. Limit the voltage drop to 2 percent of the
source.
Find: the size of an RH copper conductor needed to limit the voltage
drop of the system.
Solution:
- The allowable voltage drop equals 240 volts times 0.02 (2%). This
equals 4.8 volts. - Determine the maximum resistance for 800 feet (243.84 meters). This
is the equivalent of 400 feet (121.92 meters) × 2, since there are two
current-carrying conductors for a single-phase system.
VD
R = —
I
4.8 V
= ———
200 A
= 0.024 ohm, resistance for 800 feet
- Determine the maximum resistance for 1000 feet (304.8 meters) of
conductor.
800 feet 0.024 ohm
———— = —————
1000 feet R
800 R = (1000) (0.024)
R = 0.030 ohm