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Voltage Drop Calculations (NEC Chapter 9) — NEC 2020 Edition

Free NEC 2020 practice test on voltage drop: recommended 3%/5% limits, worked single-phase and three-phase voltage drop calculations, conductor sizing to limit voltage drop, and maximum circuit length. Simple English, full explanations.

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The National Electrical Code (NFPA 70) is the benchmark standard for safe electrical installation in the US. It is updated every 3 years by the NFPA.

Study the edition adopted by your state. As of 2025, most states use the 2020 or 2023 NEC. These tests are written to the 2023 edition.

The most tested articles are 100 (Definitions), 200–250 (Wiring & Grounding), 300–314 (Wiring Methods), 400–411 (Conductors), 430 (Motors), and 700–701 (Emergency Systems).

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This Voltage Drop Calculations (NEC Chapter 9) — NEC 2020 Edition has 30 questions with detailed answers and NEC references. Take the full test, review every wrong answer, and look up the NEC article it references before moving on.

Tip: Use the NEC index (back of the book) — find answers by keyword, not by article number.

Topics covered: NEC articles 100, 210, 220, 240, 250, 300, 310, 430, and special occupancies.
Passing score: Aim for 80%+ on NEC code tests before exam day.
Questions: 30 multiple-choice, one correct answer each

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Voltage Drop Calculations (NEC Chapter 9) — NEC 2020 Edition — Question List

Question 1: A 230-volt load is connected to a 240-volt source. Using the NEC's recommended maximum 5% voltage drop, what is the minimum voltage that should reach the load?

228 V
230 V
232 V
225 V

NEC Reference: NEC Informational Note to 210.19(A) / 215.2(A)

Question 2: A 115-volt load is connected to a 120-volt source. What is the minimum voltage the NEC recommends reach the load, using the 5% guideline?

114 V
115 V
120 V
117 V

NEC Reference: NEC Informational Note to 210.19(A) / 215.2(A)

Question 3: Two 10 AWG THHN conductors supply a 16 A, 115 V load located 50 ft from the source. Using R = 1.2 ohm/1000 ft for 10 AWG, what is the voltage drop?

1.92 V
0.96 V
2.4 V
1.8 V

NEC Reference: NEC Chapter 9, Table 8

Question 4: Two 12 AWG THHN conductors connect an 8 A, 120 V load to a source 100 ft away. Using R = 2 ohm/1000 ft for 12 AWG, what is the voltage drop?

3.2 volts
6.4 volts
1.92 volts
2.4 volts

NEC Reference: NEC Chapter 9, Table 8

Question 5: A three-phase, 18 kVA, 208 V load is connected to a panelboard with 100 ft of 2 AWG THHN copper conductor. What is the approximate voltage drop?

1.68 volts
3.2 volts
2.4 volts
3.8 volts

NEC Reference: NEC Chapter 9, Table 8

Question 6: A 12 kVA, three-phase, 480 V load must be connected to a panelboard 220 ft away. What conductor size keeps the voltage drop at or under 3%?

12 AWG
10 AWG
8 AWG
14 AWG

NEC Reference: NEC Chapter 9, Table 8

Question 7: A 4 AWG THHN conductor connects a 480 V, three-phase, 35 kVA transformer to a panelboard. What is the maximum length that keeps voltage drop at or under 3%?

640 ft
320 ft
720 ft
540 ft

NEC Reference: NEC Chapter 9, Table 8

Question 8: What is the maximum combined voltage drop the NEC generally recommends for branch and feeder conductors together?

NEC Reference: NEC Informational Note to 210.19(A) / 215.2(A)

Question 9: What is the largest three-phase load that can be connected over 220 ft of 8 AWG THHN, 480 V conductor without exceeding the NEC-recommended voltage drop?

40 kVA
42 kVA
48 kVA
53 kVA

NEC Reference: NEC Chapter 9, Table 8

Question 10: What is the maximum voltage drop percentage the NEC generally recommends for branch circuit conductors alone?

NEC Reference: NEC Informational Note to 210.19(A)

Question 11: Since the 3%/5% voltage drop values appear in NEC Informational Notes, what does that generally mean?

They are recommendations, not mandatory requirements
They are mandatory and always enforced
They apply only to service conductors
They were removed from the NEC entirely

NEC Reference: NEC Informational Note to 210.19(A) / 215.2(A)

Question 12: Two 8 AWG THHN copper conductors supply a 20 A, 120 V load 75 ft from the source. Using R = 0.78 ohm/1000 ft, what is the approximate voltage drop?

2.34 volts
1.56 volts
3.12 volts
4.68 volts

NEC Reference: NEC Chapter 9, Table 8

Question 13: A calculated voltage drop of 2.34 volts on a 120-volt circuit is approximately what percentage?

About 1.95%
About 5%
About 10%
About 0.5%

NEC Reference: NEC Informational Note to 210.19(A)

Question 14: What does "voltage drop" mean in the context of conductor sizing?

Loss of voltage along a conductor due to its resistance
A type of overcurrent protection device
A required grounding method
A conduit fill calculation

NEC Reference: NEC Chapter 9

Question 15: Which of the following generally increases voltage drop, all else being equal?

Increasing conductor length
Increasing conductor size (larger CM area)
Decreasing load current
None of these affect voltage drop

NEC Reference: NEC Chapter 9

Question 16: Using a larger conductor (above the ampacity-required minimum) generally has what effect on voltage drop?

It decreases voltage drop
It increases voltage drop
It has no effect at all
It always doubles the voltage drop

NEC Reference: NEC Chapter 9

Question 17: What is a common practical consequence of excessive voltage drop on a branch circuit supplying a motor?

The motor runs hot, inefficiently, with reduced starting torque
The motor becomes more efficient
There is no practical consequence
The circuit breaker size must be reduced

NEC Reference: NEC Chapter 9 (general design consideration)

Question 18: In the three-phase voltage drop formula VD = 1.732 x K x I x D / CM, what does 1.732 represent?

The square root of 3, for balanced three-phase systems
The number of conductors in the circuit
The power factor of the load
A safety margin with no mathematical basis

NEC Reference: NEC Chapter 9, Table 8

Question 19: Why is the one-way distance generally doubled in a single-phase voltage drop calculation?

The circuit includes both the outgoing and returning conductor
It is just an arbitrary safety factor
It accounts for three-phase power only
It has nothing to do with circuit length

NEC Reference: NEC Chapter 9, Table 9

Question 20: The copper resistivity constant K is about 12.9. Is the K value for aluminum conductors generally higher or lower?

Higher (aluminum has more resistance)
Lower
Exactly the same
Aluminum has no K value

NEC Reference: NEC Chapter 9, Table 8

Question 21: Two 6 AWG THHN copper conductors supply a 30 A, 240 V load 150 ft away. Using R = 0.49 ohm/1000 ft, what is the approximate voltage drop?

4.41 volts
2.94 volts
5.88 volts
3.5 volts

NEC Reference: NEC Chapter 9, Table 8

Question 22: A three-phase, 480V circuit has a calculated voltage drop of 9.6 volts. What percentage does this represent?

2%
5%
10%
0.2%

NEC Reference: NEC Informational Note to 215.2(A)

Question 23: If a feeder uses 2% voltage drop, what is generally the maximum recommended remaining budget for the branch circuit, to stay within the combined 5%?

NEC Reference: NEC Informational Note to 210.19(A)

Question 24: In VD = 1.732 x K x I x D / CM, what does "CM" stand for?

The conductor's circular mil area
The conduit fill percentage
The number of current-carrying conductors
The ambient temperature

NEC Reference: NEC Chapter 9, Table 8

Question 25: True or false: voltage drop calculations become more important as the distance between source and load increases.

True
False
Only for three-phase circuits
Distance has no effect on voltage drop

NEC Reference: NEC Chapter 9

Question 26: A calculated voltage drop exceeds the recommended 3% for a branch circuit, even though the conductor is large enough for ampacity. What is the recommended remedy?

Use a larger conductor to reduce resistance
Use a smaller conductor
Increase the circuit breaker size
Nothing can be done

NEC Reference: NEC Chapter 9 / Informational Note to 210.19(A)

Question 27: A three-phase, 24 kVA load operates at 208 volts. What is the approximate load current?

About 66.6 amperes
About 115 amperes
About 40 amperes
About 24 amperes

NEC Reference: NEC Chapter 9 (three-phase power formula)

Question 28: Why might a designer target a voltage drop well below the NEC's 3%/5% recommendations for circuits supplying sensitive electronic equipment?

Sensitive equipment can be more susceptible to voltage variation
The NEC requires a stricter limit for electronics
It has no practical benefit
It is required only for motors, not electronics

NEC Reference: NEC Informational Note to 210.19(A)

Question 29: True or false: a conductor sized correctly for ampacity automatically meets the NEC's recommended voltage drop limits.

False -- they are separate checks
True -- ampacity sizing always covers voltage drop
Voltage drop is not a real concern
Ampacity and voltage drop are the same calculation

NEC Reference: NEC Chapter 9 / 310.14

Question 30: A single-phase, 120V, 12A load is supplied by two 10 AWG THHN conductors (R = 1.2 ohm/1000 ft). What is the maximum one-way distance to keep voltage drop at or under 3% (3.6 volts)?

125 ft
250 ft
62.5 ft
100 ft

NEC Reference: NEC Chapter 9, Table 9

Key NEC References for This Test

Article	What It Covers
Ch. 1–4	General rules — apply to all installations
Ch. 5–7	Special rules — override Ch. 1–4 where they conflict
Ch. 8	Communications — completely independent of Ch. 1–7
Ch. 9	Tables — referenced by articles, not standalone enforceable rules
Art. 210.12	AFCI — major expansion in NEC 2026
Art. 230.67	Surge protection at service equipment — new NEC 2026

Study Tip: Learn the NEC index — use keywords, not article numbers. Practice finding 5 different topics in under 2 minutes each.

Voltage Drop Calculations (NEC Chapter 9) — NEC 2020 Edition

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Key NEC References for This Test

Voltage Drop Calculations (NEC Chapter 9) — NEC 2020 Edition

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Voltage Drop Calculations (NEC Chapter 9) — NEC 2020 Edition — Question List

Question 1: A 230-volt load is connected to a 240-volt source. Using the NEC's recommended maximum 5% voltage drop, what is the minimum voltage that should reach the load?

Question 2: A 115-volt load is connected to a 120-volt source. What is the minimum voltage the NEC recommends reach the load, using the 5% guideline?

Question 3: Two 10 AWG THHN conductors supply a 16 A, 115 V load located 50 ft from the source. Using R = 1.2 ohm/1000 ft for 10 AWG, what is the voltage drop?

Question 4: Two 12 AWG THHN conductors connect an 8 A, 120 V load to a source 100 ft away. Using R = 2 ohm/1000 ft for 12 AWG, what is the voltage drop?

Question 5: A three-phase, 18 kVA, 208 V load is connected to a panelboard with 100 ft of 2 AWG THHN copper conductor. What is the approximate voltage drop?

Question 6: A 12 kVA, three-phase, 480 V load must be connected to a panelboard 220 ft away. What conductor size keeps the voltage drop at or under 3%?

Question 7: A 4 AWG THHN conductor connects a 480 V, three-phase, 35 kVA transformer to a panelboard. What is the maximum length that keeps voltage drop at or under 3%?

Question 8: What is the maximum combined voltage drop the NEC generally recommends for branch and feeder conductors together?

Question 9: What is the largest three-phase load that can be connected over 220 ft of 8 AWG THHN, 480 V conductor without exceeding the NEC-recommended voltage drop?

Question 10: What is the maximum voltage drop percentage the NEC generally recommends for branch circuit conductors alone?

Question 11: Since the 3%/5% voltage drop values appear in NEC Informational Notes, what does that generally mean?

Question 12: Two 8 AWG THHN copper conductors supply a 20 A, 120 V load 75 ft from the source. Using R = 0.78 ohm/1000 ft, what is the approximate voltage drop?

Question 13: A calculated voltage drop of 2.34 volts on a 120-volt circuit is approximately what percentage?

Question 14: What does "voltage drop" mean in the context of conductor sizing?

Question 15: Which of the following generally increases voltage drop, all else being equal?

Question 16: Using a larger conductor (above the ampacity-required minimum) generally has what effect on voltage drop?

Question 17: What is a common practical consequence of excessive voltage drop on a branch circuit supplying a motor?

Question 18: In the three-phase voltage drop formula VD = 1.732 x K x I x D / CM, what does 1.732 represent?

Question 19: Why is the one-way distance generally doubled in a single-phase voltage drop calculation?

Question 20: The copper resistivity constant K is about 12.9. Is the K value for aluminum conductors generally higher or lower?

Question 21: Two 6 AWG THHN copper conductors supply a 30 A, 240 V load 150 ft away. Using R = 0.49 ohm/1000 ft, what is the approximate voltage drop?

Question 22: A three-phase, 480V circuit has a calculated voltage drop of 9.6 volts. What percentage does this represent?

Question 23: If a feeder uses 2% voltage drop, what is generally the maximum recommended remaining budget for the branch circuit, to stay within the combined 5%?

Question 24: In VD = 1.732 x K x I x D / CM, what does "CM" stand for?

Question 25: True or false: voltage drop calculations become more important as the distance between source and load increases.

Question 26: A calculated voltage drop exceeds the recommended 3% for a branch circuit, even though the conductor is large enough for ampacity. What is the recommended remedy?

Question 27: A three-phase, 24 kVA load operates at 208 volts. What is the approximate load current?

Question 28: Why might a designer target a voltage drop well below the NEC's 3%/5% recommendations for circuits supplying sensitive electronic equipment?

Question 29: True or false: a conductor sized correctly for ampacity automatically meets the NEC's recommended voltage drop limits.

Question 30: A single-phase, 120V, 12A load is supplied by two 10 AWG THHN conductors (R = 1.2 ohm/1000 ft). What is the maximum one-way distance to keep voltage drop at or under 3% (3.6 volts)?

Key NEC References for This Test

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