voltage regulation formulas and reactance of conductors - okonite electrical wire & cable

7/24/2019 Voltage Regulation Formulas and Reactance of Conductors - Okonite Electrical Wire & Cable

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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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