What is a subpanel, and what is a key difference when wiring a subpanel versus a main panel?

• A. A subpanel is a weatherproof panel; it uses only conduit wiring.
• B. A subpanel is a secondary electrical panel; the neutral must be isolated from the grounding conductor in a subpanel.
• C. A subpanel is a main service disconnect; it bonds neutral to ground at the subpanel.
• D. A subpanel is a branch panel; it requires no grounding conductor.

Answer: (B)

A subpanel is a secondary distribution panel fed from the main service panel to extend power to a portion of a building. The important difference when wiring a subpanel is keeping the neutral and grounding conductors separate inside the subpanel. The neutral bus must be isolated from the panel enclosure, meaning it is not bonded to ground there. A separate grounding bus is bonded to the enclosure and connected back to the main panel with a grounding conductor. In practice, that means a four-wire feeder—two hot conductors, a neutral, and a grounding conductor—from the main panel to the subpanel, and neutral-ground bonding is only at the main service disconnect. This setup reduces stray current on the grounding path and is required for proper fault protection and interference control.