Journeyman Electrician Exam (NEC)Wiring Methods and MaterialsHard
A journeyman is installing a new 240-volt, 50-amp circuit for an electric range in a dwelling unit using nonmetallic-sheathed cable (NM-B). The cable contains three 6 AWG copper conductors and one 10 AWG bare copper grounding conductor. What is the minimum size of the equipment grounding conductor required by the NEC for this circuit?
- A10 AWG
- B6 AWG
- C12 AWG
- D8 AWG
Show answer & explanationAnswer & explanation
Correct answer: A. 10 AWG
According to NEC Table 250.122, for an overcurrent protective device rated at 50 amperes, the minimum size copper equipment grounding conductor required is 10 AWG. The question specifies a 50-amp circuit.
Why the other options are wrong
- B. Incorrect. 6 AWG is the size of the circuit conductors, but not the minimum required grounding conductor, which can be smaller per 250.122.
- C. Incorrect. 12 AWG is too small for a 50-amp circuit's grounding conductor.
- D. Incorrect. While 8 AWG would be acceptable, it is larger than the minimum required.
Equipment Grounding Conductor Sizing
The size of the equipment grounding conductor (EGC) is determined by the rating of the overcurrent protective device protecting the circuit, not by the size of the circuit conductors, according to NEC Table 250.122.
- NEC Table 250.122 is the primary reference for sizing EGCs.
- The EGC must be capable of safely carrying fault current until the overcurrent device trips.
- It's generally smaller than the ungrounded circuit conductors, but sized relative to the breaker/fuse rating.
Memory trick: Breaker size rules the ground wire's might!