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Voltage regulation is often the limiting factor in the choice of either conductor or type of insulation.
While the heat loss in the cable determines the maximum current it can safely carry without
excessive deterioration, many circuits will be limited to currents lower than this in order to keep the
voltage drop within permissible values. In this connection it should be remembered that the high
voltage circuit should be carried as far as possible so that the secondary runs, where most of the
voltage drop occurs, will be small.
The voltage drop of a feeder may be calculated from the following formulae:
V = 100 (VSVL) / VL
V = Voltage regulation in percent
VL= Voltage across loadVS= Voltage at source
VS=
= is the angle by which the load current lags the voltage across the
load
Cos =Power factor of load
R = Total a-c resistance of feeder
X = Total reactance of feeder
I= Load current
Approximate formula for voltage drop:
(VSVL) = RI cos + XI sin
This above formula is satisfactory where the power factor angle is nearly the same as the
impedance angle. It is exact when they are equal.
That is: tan = X / R
Above values apply directly for single phase lines when resistance and reactance are loop values
and voltage is voltage between lines.
For 3-phase circuits, use voltage to neutral and resistance and reactance of each conductor to
neutral. This gives voltage drop to neutral. To obtain voltage drop line-to-line, multiply voltage drop by
. (The percent voltage drop is of course the same between conductors as from conductor to
ground and should not be multiplied by ).
Example: 3 s ingle coated copper conductors 600 volt cables in non-metallic conduit.
Size conductor =4/0, Awg Copper .080 insulation, .045 jacket.
O.D. = .810
Voltage = VS= 440 volts 3 phase
Current = I= 250 amperes
Power Factor = cos = 0.8
Length = 750 ft.
Resistance
Per conductor = R
= .0525 ohms 1000 feet at 25C
= .047 ohms for 750 feet at 75C
Reactance
Per conductor = X
= .031 ohms 1000 feet (see table)
= .028 ohms for 750 feet
(including 20% for random lay)
VS=
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Solving for VL; VL= 240.4
Line-to-line voltage = 240.4 = 417
Voltage drop = 440 417 = 23 volts
Approximate Formula:
Voltage drop = line to neutral
= Rl cos + Xl sin
= 0.047 X 250 X .08 + 0.028 X 250 X 0.6
= 9.4 + 4.2 = 13.6Line-to-line voltage drop = 13.6 = 23.5 volts
Conductor ReactanceThe following table shows a nomogram for determining the reactance of any solid or concentric
stranded conductor. This covers spacings encountered for conduit wiring as well as for open wire
circuits. Various modifications necessary for use under special conditions are covered in notes on
the nomogram. The reactances shown are for 60-Hertz operation.
Where regulation is an important consideration several factors should be kept in mind in order to
obtain the best operating conditions.
Open wire lines have a high reactance. This may be improved by using parallel circuits but is much
further reduced by using insulated cable. Three conductors in the same conduit have a lower
reactance than conductors in separate conduits.
Single conductors should not be installed in individual magnetic conduit because of the excessive
reactance.
Three conductors in magnetic conduit will have a somewhat higher reactance than cables in non-
magnetic conduit.
Reactance of conductors at 60Hz(Series inductive reactance to neutral)
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