Electrical Engineering
Electricity and Magnetism
Level
2
Calculate the maximum percentage voltage drop in a single-phase 120 Volt, 20 Amp circuit with #12 uncoated copper wires in steel conduit, which feeds a 750 VA load operating at 95% power factor, if the length of the circuit is 75′, and there are no other items connected to that circuit.
3.49%
4.5%
2.2%
1.0%
1.5%
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1 solution
Apply the formula for voltage drop:
D = (2 x (W – 1)-1 x (R x F / 100 + X x SIN(ACOS(F / 100))) x L x S) / (10 x V2)
Where:
D = Voltage drop as a percentage of the circuit voltage (V)
W= Number of wires in the circuit = 2 (for single phase)
R= Resistance of conductor per 1,000ft (Ω) – See NEC Table 9 = 1.98
X= Reactance of conductor per 1,000ft (Ω) – See NEC Table 9 = 0.068
F= Power factor of load (%) = 95
L= Length of conductor (ft) = 75
S= Apparent power of load (VA) = 750
V= circuit voltage (V) = 120
I = Line current (A)
So now we have:
D = (2 x (2 – 1)-1 x (1.98 x 95 / 100 + 0.068 x SIN(ACOS(95 / 100))) x 75 x 750) / (10 x 1202)
D = (2 x 1 x (1.88 + 0.068 x 0.312) x 56,250) / (144,000)
D = 1.5 %