Master Electrician Exam (NEC)Services, Feeders, and Branch CircuitsMedium
A commercial office space is being renovated, and a new 40A, 208V, 3-phase, 3-wire feeder is being installed to supply a small subpanel. The feeder conductors are 8 AWG copper with 75°C insulation. What is the minimum size copper equipment grounding conductor (EGC) required for this feeder?
- A10 AWG
- B8 AWG
- C6 AWG
- D12 AWG
Show answer & explanationAnswer & explanation
Correct answer: A. 10 AWG
According to NEC Table 250.122, for a feeder protected by a 40A overcurrent device, the minimum size copper equipment grounding conductor is 10 AWG.
Why the other options are wrong
- B. 8 AWG copper is the feeder conductor size, but not the minimum EGC size required for a 40A OCPD.
- C. 6 AWG copper is larger than required for a 40A circuit.
- D. 12 AWG copper is for 20A circuits, not 40A.
Feeder Equipment Grounding Conductor Sizing
The equipment grounding conductor (EGC) for a feeder provides a low-impedance path for ground-fault current from the subpanel and connected equipment back to the service, ensuring proper operation of overcurrent protective devices.
- Sized based on the feeder's overcurrent protective device rating.
- Referenced in NEC Table 250.122.
- Must be copper, aluminum, or copper-clad aluminum.
Memory trick: Feeder's EGC: The protective path, sized by the feeder's fuse.