A subpanel is a smaller distribution panel fed from the main panel, providing additional circuit positions somewhere else in the building. Garages, workshops, finished basements and additions are the usual reasons for one.
The case for a subpanel is usually one of two things. Either the main panel has run out of physical space for breakers, or the new circuits are far enough from the main panel that running each one individually would mean an unreasonable amount of cable. One feeder to a subpanel, with branch circuits distributed locally, is cheaper and tidier.
The important technical distinction is how neutrals and grounds are handled.
In the main panel, the neutral and the equipment grounding conductors are bonded together at a single point. This is the one place in the system where that connection is made.
In a subpanel, they must be kept separate. Neutrals land on an isolated neutral bar, grounds land on a separate ground bar bonded to the enclosure, and the bonding screw or strap that ties the neutral bar to the can must be removed.
The reason matters. If neutral and ground are bonded in both panels, normal return current has two parallel paths back to the source, and part of it will flow on the grounding conductors and on any metal bonded to them. Grounding conductors are not intended to carry current in normal operation. This is one of the most common errors in owner-installed subpanels, and it does not produce any obvious symptom.
The feeder itself needs four conductors: two hots, a neutral, and an equipment ground. Three-conductor feeders were permitted in some circumstances historically and are not acceptable for new work.
Sizing is straightforward arithmetic. The feeder breaker in the main panel protects the feeder conductors, and the subpanel must be rated at least for that breaker. There is no requirement for the sum of the subpanel breakers to be less than the feeder, since not everything runs at once, but the actual expected load should be calculated rather than guessed.
Detached structures bring additional requirements, including a means of disconnect at the structure and a grounding electrode system at that location.
Related guide: home electrical wiring
