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

Question 1: A 230-volt load is supplied from a 240-volt source. Using the NEC recommendation of 5% maximum total voltage drop (feeder plus branch circuit), what is the minimum recommended voltage at the load?

• 232 V
• 230 V
• 228 V
• 225 V

NEC Reference: NEC 215.2 Informational Note

Question 2: A 115-volt load is supplied from a 120-volt source. Using the 5% total voltage drop recommendation, what is the minimum recommended voltage at the load?

• 110 V
• 114 V
• 115 V
• 117 V

NEC Reference: NEC 215.2 Informational Note

Question 3: Two 10 AWG copper conductors (resistance 1.2 ohms per 1,000 ft) supply a 16 A, 115 V load 50 ft from the source. What is the voltage drop?

• 0.96 V
• 1.80 V
• 1.92 V
• 2.40 V

NEC Reference: NEC Chapter 9, Table 9

Question 4: Two 12 AWG copper conductors (resistance 2.0 ohms per 1,000 ft) supply an 8 A, 120 V load 100 ft from the source. What is the voltage drop?

• 1.6 V
• 2.4 V
• 3.2 V
• 6.4 V

NEC Reference: NEC Chapter 9, Table 9

Question 5: An 18 kVA, 208 V three-phase load is 100 ft from the panelboard on 2 AWG copper (66,360 CM). Using VD = 1.732 x K x I x D / CM with K = 12.9, what is the approximate voltage drop?

• 1.68 V
• 2.40 V
• 3.20 V
• 3.80 V

NEC Reference: NEC Chapter 9, Table 8

Question 6: A 12 kVA, 480 V three-phase load is 220 ft from the panelboard. What is the smallest copper conductor that keeps the voltage drop at or below 3%? (K = 12.9)

• 14 AWG
• 12 AWG
• 10 AWG
• 8 AWG

NEC Reference: NEC Chapter 9, Table 8

Question 7: A 35 kVA, 480 V three-phase load is supplied by 4 AWG copper (41,740 CM). What is the maximum one-way length that keeps the voltage drop at or below 3%? (K = 12.9)

• 320 ft
• 540 ft
• 640 ft
• 720 ft

NEC Reference: NEC Chapter 9, Table 8

Question 8: What is the maximum recommended combined voltage drop for a feeder and branch circuit together?

• 6%
• 5%
• 4%
• 3%

NEC Reference: NEC 215.2 Informational Note

Question 9: A 480 V three-phase circuit uses 8 AWG copper (16,510 CM), 220 ft long. Keeping the voltage drop at 3% (K = 12.9), about how large a load can it carry?

• 53 kVA
• 48 kVA
• 42 kVA
• 40 kVA

NEC Reference: NEC Chapter 9, Table 8

Question 10: What is the maximum recommended voltage drop for a branch circuit alone?

• 2%
• 3%
• 4%
• 5%

NEC Reference: NEC 210.19 Informational Note

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 9

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 than ampacity requires (more circular mils) has what effect on voltage drop?

• It decreases the voltage drop
• It changes only the power factor
• It increases the voltage drop
• It changes only the current

NEC Reference: NEC Chapter 9, Table 8

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

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: In a single-phase voltage drop calculation using Table 9 resistance, why is the one-way distance doubled?

• Because three-phase circuits have two paths
• Because Table 9 is listed per 500 ft
• To add a safety factor for future loads
• The current flows out on one conductor and back on the other

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 9

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 215.2 Informational Note

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%?

• 3%
• 5%
• 1%
• 8%

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: Voltage drop in a conductor is directly proportional to which of the following?

• Its length and the current it carries
• Its insulation temperature rating
• The supply frequency only
• Its circular-mil area

NEC Reference: NEC Chapter 9, Table 8

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 210.19 Informational Note

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

Question 28: Why might a designer keep voltage drop well below the 3%/5% recommendations for sensitive electronic equipment?

• The NEC requires 1% for electronics
• Lower voltage drop increases the breaker rating
• It allows smaller conductors to be used
• Sensitive equipment can malfunction with low or varying voltage

NEC Reference: NEC 210.19 Informational Note

Question 29: A conductor is correctly sized for ampacity on a long run. What must still be checked?

• The color of the insulation
• The number of splices allowed
• The conduit trade name
• The voltage drop at the load

NEC Reference: NEC 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