IEC Conductors and Branch Circuits 5 — Questions and Answers
Question 1: A branch circuit overcurrent device is rated 20 amperes. The connected load includes a single motor with a full-load current of 16 amperes. Does this comply with NEC branch circuit rules?
- Yes, because 16A is within 80% of 20A for continuous loads
- No, motors require a separate feeder, not a branch circuit
- Yes, but only if the motor is not a continuous duty motor
- No, motor branch circuits must be sized at 125% of FLC per NEC 430.22 (Correct answer)
Correct answer: No, motor branch circuits must be sized at 125% of FLC per NEC 430.22
NEC Section 430.22 requires motor branch circuit conductors to be sized at not less than 125% of the motor's full-load current, so 16A × 1.25 = 20A minimum conductor ampacity.
Question 2: When is it permissible to use 10 AWG copper on a 30-ampere branch circuit per NEC Article 210?
- Never; 10 AWG copper is only rated 30A on a feeder
- When the circuit supplies a single receptacle rated 30 amperes (Correct answer)
- When the 10 AWG conductor has a 90°C insulation rating
- When the run is less than 50 feet to limit voltage drop
Correct answer: When the circuit supplies a single receptacle rated 30 amperes
NEC Section 210.21(B) permits a single 30-ampere receptacle outlet to be supplied by 10 AWG copper (30A at 75°C) on a 30-ampere branch circuit.
Question 3: Which insulation type is listed for use in wet locations as well as dry locations, making it suitable for outdoor branch circuit wiring in conduit?
- THHN
- TW
- THWN (Correct answer)
- RHH
Correct answer: THWN
THWN (Thermoplastic, Heat- and Moisture-Resistant, Nylon-jacketed) insulation is listed for both wet and dry locations and is commonly used in outdoor conduit runs.
Question 4: Where two or more branch circuits supply devices in the same enclosure, what color coding must the grounded (neutral) conductors follow per NEC 200.6?
- All neutrals must be white regardless of voltage system
- Neutrals must be identified by white or gray color, or three white stripes on another color (Correct answer)
- Neutrals can be any color as long as they are labeled at each end
- Neutrals must be green or bare
Correct answer: Neutrals must be identified by white or gray color, or three white stripes on another color
NEC Section 200.6 requires grounded conductors to be identified by a continuous white or gray outer finish, or three continuous white or gray stripes on other than green insulation.
Question 5: In a dwelling unit, how many small-appliance branch circuits are required by NEC 210.11(C)(1) for kitchen, pantry, breakfast room, and dining room receptacles?
- One 15-ampere circuit
- At least two 20-ampere small-appliance branch circuits (Correct answer)
- Three 20-ampere circuits
- One 20-ampere circuit per countertop space
Correct answer: At least two 20-ampere small-appliance branch circuits
NEC Section 210.11(C)(1) mandates a minimum of two 20-ampere small-appliance branch circuits for receptacles in the kitchen, pantry, breakfast room, and dining room.
Question 6: What is the effect of raising the ambient temperature above 30°C (86°F) on a conductor's allowable ampacity?
- Ampacity increases because heat dissipates faster
- Ampacity is unchanged; temperature ratings are absolute limits
- Ampacity must be derated using the correction factors in NEC Table 310.15(B)(1) (Correct answer)
- Ampacity is limited only by conduit fill, not ambient temperature
Correct answer: Ampacity must be derated using the correction factors in NEC Table 310.15(B)(1)
Higher ambient temperatures reduce a conductor's ability to dissipate heat, requiring a correction factor from NEC Table 310.15(B)(1) to derate the allowable ampacity.
Question 7: A branch circuit runs 150 feet from the panelboard to a 120V, 10-ampere load. Which NEC guideline addresses keeping voltage drop within acceptable limits?
- NEC 210.19(A) Informational Note recommends voltage drop not exceed 3% on branch circuits (Correct answer)
- NEC 310.15 mandates a maximum 5% voltage drop for all circuits
- NEC 220.12 sets a hard 2% limit for branch circuits
- Voltage drop is not addressed anywhere in the NEC
Correct answer: NEC 210.19(A) Informational Note recommends voltage drop not exceed 3% on branch circuits
NEC Section 210.19(A) contains an Informational Note recommending that voltage drop on branch circuits not exceed 3% to maintain efficiency and equipment performance.
IEC Conductors and Branch Circuits 5
A branch circuit overcurrent device is rated 20 amperes.
The connected load includes a single motor with a full-load current of 16 amperes.
Does this comply with NEC branch circuit rules?