CSLB C-10 Electrical ContractorWiring Methods and MaterialsHard
A 100-ampere circuit breaker protects a copper feeder circuit. According to NEC Table 250.122, what is the minimum size copper equipment grounding conductor required for this circuit?
- A8 AWG
- B10 AWG
- C4 AWG
- D6 AWG
Show answer & explanationAnswer & explanation
Correct answer: A. 8 AWG
NEC Table 250.122 sizes the equipment grounding conductor based on the rating of the overcurrent device protecting the circuit, not the actual load. For overcurrent protection rated up to 100A, the minimum copper EGC size is 8 AWG.
Why the other options are wrong
- B. 10 AWG is only sufficient for OCPD ratings up to 60A.
- C. 4 AWG is required for OCPD ratings up to 300A, unnecessarily large.
- D. 6 AWG is required for OCPD ratings up to 200A, oversized here.
Equipment Grounding Conductor Sizing (Table 250.122)
The EGC size for a circuit is based on the rating of the circuit's overcurrent protective device, per NEC Table 250.122, independent of actual conductor ampacity.
- 60A OCPD → 10 AWG copper
- 100A OCPD → 8 AWG copper
- 200A OCPD → 6 AWG copper
- EGC must be increased proportionally if ungrounded conductors are upsized per 250.122(B)
Memory trick: 'Match the breaker, not the load' when sizing ground wires.