Voltage Drop Calculations1 ( NEC Code Questions)
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Question 1 of 10
1. Question
1 pointsWhat is the minimum NEC recommended voltage for a 230 volt load connected to 240 v source?
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NEC recommended maximum voltage drop for both feeder and branch conductor is 5%.
Maximum voltage drop = 240 * 0.05 = 12 V.
Minimum load voltage = 240 – 12 = 228 V

Question 2 of 10
2. Question
1 pointsIf a 115 volt load is connected to 120 V source. What is minimum voltage that is supplied to the load as recommended by NEC?
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The maximum voltage drop for branch conductor and feeder conductor recommended by NEC is 5%.
120 * 0.05 = 6V.
Minimum voltage = 120 – 6 = 114 V.

Question 3 of 10
3. Question
1 pointsWhat is the voltage drop of two 10 AWH THHN conductors that supply 16 A, 115 V load located 50 ft from the source?
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Voltage drop = I * R
I = load current = 16 A
R = 1.2 V / 1000 ft ( NEC chapter 9 and Table 9 for 10 AWG)
Voltage drop = 16 * 1.2 * 100 / 1000 ( for two conductors)
= 1.92 V

Question 4 of 10
4. Question
1 pointsTwo 12 AWG THHN conductors are placed to connect 8A, 120 V load to the source which is 100 ft distance. What is maximum voltage drop in conductor?
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Answer:
Voltage Drop (VD) = I * R
I = load current = 8A
R = Resistance of conductor = 2V / 1000 ft ( Chapter 9 Table 9 for 12 AWG)
VD = 8 * 2/1000 * 200 ( for 2 conductors)
= 3.2 Volts 
Question 5 of 10
5. Question
1 pointsA 3 phase 18 KVA, 208 volts load is connected to panel board with 100 ft length of 2AWG THHN copper conductor. What is the approximate voltage drop in the conductor?
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Answer:
Approximate Voltage Drop (VD) = 1.732 * K * I * D / CM
K = 12.9 ( Copper)
I ( Load current ) = power / ( 1.732 * Voltage) = 18000 / (1.732 * 208) = 50 A
D = 100 ft
CM = 66360 ( NEC Chapter 9 Table 8 for 2 AWG THHN)
VD = 1.732 * 12.9 * 50 * 100 / 66360 = 1.68 volts

Question 6 of 10
6. Question
1 pointsA 12 KVA , 3 phase 480 Volts load should be connected to panel board with a distance of 220 ft. What size conductor is required to prevent the voltage drop from exceeding 3 %?
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Answer:
VD = 1.732 * K * I * D / CM
CM = 1.732 * K * I D / VD
K = 12.9 ( Copper)
I (load current) = Power / ( 1.732 * Voltage) = 12000 / (1.732 * 480) = 14.4 A
D = 220 ft
VD = 480 * 0.03 ( NEC recommended maximum voltage drop for branch conductor is 3%)
CM = 1.732 * 12.9 * 14.4 * 220 / 14.4 = 4915 ( 12 AWG NEC Chapter 9 Table 8)

Question 7 of 10
7. Question
1 pointsA 4 AWG THHN is used to connect a 480 V 3 phase 35 kVA transformer to panel board. what is the maximum length of conductor that can be used in which the voltage drop doesn’t cross 3% ?
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Answer:
VD = 1.732 * K * I * D / CM
D = (VD * CM ) / ( 1.732 * I * K)
VD = 480 * 0.03 = 14.4 Volts
I ( load current) = Power / ( 1.732 * Voltage) = 35000 / (1.732 * 480) = 42 A
CM = 41740 ( NEC Chapter 9 Table 8 for 4 AWG)
D = ( 14.4 * 41740) / ( 1.732 * 12.9 * 42) = 640 ft.

Question 8 of 10
8. Question
1 pointsWhat is the maximum voltage drop that is recommended by NEC for both branch and feeder conductors?
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Question 9 of 10
9. Question
1 pointsWhat is 3 phase load that can be connected to a 220 ft, 8 THHN, 480 V conductor with out exceeding the NEC recommended voltage drop?
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VD = 1.732 * I * K * D / CM
I = ( VD * CM ) / ( 1.732 * K * D)
D =220 ft
K = 12.9 ( Copper)
VD = 480 * 0.03 = 14.4 ( NEC recommended maximum voltage drop is 3%)
CM = 16510 ( NEC chapter 9 Table 8 for 8 THHN)
I = ( VD * CM) / (1.732 * K * D)
= ( 14.4 * 16510 ) / (1.732 * 12.9 * 220) = 48.36 A
Power = 1.732 * Voltage * Current = 1.732 * 480 * 48.36 = 40.2 KVA ( 40 KVA)

Question 10 of 10
10. Question
1 pointsWhat is maximum voltage drop in percentage recommended by NEC for branch circuit conductors?
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