A master electrician is installing a new subpanel in a detached garage, fed from the main service panel of a dwelling. The subpanel is supplied by a 240V, 3-wire feeder. According to the NEC, how must the neutral conductor be handled at the detached garage subpanel?
- AThe neutral conductor must be bonded to the subpanel enclosure and isolated from the equipment grounding conductor.
- BThe neutral conductor must be isolated from the subpanel enclosure and connected to its own grounding electrode.
- CThe neutral conductor must be bonded to the subpanel enclosure and a grounding electrode.
- DThe neutral conductor must be isolated from the subpanel enclosure and connected to the equipment grounding conductor.
Show answer & explanationAnswer & explanation
Correct answer: D. The neutral conductor must be isolated from the subpanel enclosure and connected to the equipment grounding conductor.
For a separately derived system or a feeder to a separate building, the neutral must be isolated from the panel enclosure and connected only to the equipment grounding conductor, which is then connected to the grounding electrode system at the detached structure. This prevents objectionable current on the grounding path.
Why the other options are wrong
- A. The neutral must be isolated from the enclosure but connected to the equipment grounding conductor for the fault path.
- B. The neutral should not be connected to its 'own' grounding electrode separate from the equipment grounding conductor.
- C. Bonding the neutral to the enclosure at a subpanel creates objectionable current on the grounding path.
Separately Derived System Grounding
A wiring system whose power is derived from a source other than a service, having no direct connection to the service conductors except through the earth, and whose neutral is grounded at the source of the separately derived system.
- Neutral must be bonded to ground at the source of the separately derived system.
- Neutral must be isolated from equipment grounding conductors downstream.
- Often applies to transformers, generators, or feeders to separate buildings.
Memory trick: Subpanel neutral: separate the white from the green, but connect to the ground bar.