Can a GFCI from main breaker panel feed a subpanel with multiple breakers

Question

Can a GFCI be connected in series with a regular breaker? If yes, is the child breaker panel GFCI protected?

I have a 20 Amp GFCI, and two 15 AMP breakers. I think then that the subpanel can draw a max of 20 amps, and each circuit on the subpanel can draw up to 15.

Neutral for the subpanel comes from the GFCI neutral output.

Answer 1

Yes, a GFCI breaker can protect the feeder going to a subpanel. The GFCI protects whatever is downstream of it; when that happens to be a subpanel then all the circuits in that panel are protected.

I used this technique in my detached shop building. There are two panels in the building. The first holds a 20 amp GFCI breaker to service the lights inside the building (they're mounted to the ceiling, but they're cord-and-plug connected, so GFCI is required) and it also holds a 50 amp two-pole GFCI breaker. This latter breaker feeds the second panel. The second panel holds a variety of circuits: one 30 A/240 V each for the welder/plasma cutter, air compressor, and metal brake; a few 20 A/120 V for receptacles in various places. I was able to use normal inexpensive breakers for all those circuits because they're receiving GFCI protection from upstream.

There are some drawbacks:

• 50 A is the largest GFCI breaker I could find. It seems a bit low but the reality is the max simultaneous draw on it will never really approach 50 A. If that ever changes there's room in the upstream panel to move any high-draw circuit to a new breaker in that panel.
• When there is a GFCI trip due to an extension cord left lying outdoors when a rain storm comes along all my receptacles go out. That's been an annoyance a few times now; "one of these days" I'll do something about it. The solution will probably be a separate GFCI circuit from the upstream panel to feed those receptacles most likely to have a GFCI trip (the ones on the exterior of the building).

Answer 2

To add a bit to Greg Hill's excellent answer, there are 3 reasons to not go "too big" on a GFCI breaker feeding other breakers:

• "All out" - As already noted, if you use this setup, a trip on any connected branch circuit will make everything go out. This is a particular problem if a problem with a power tool results in the lights going out, as the very problem that caused the GFCI trip might require light to deal with safely. In Greg's example, the lights are on a separate GFCI so that problem is avoided.
• Cumulative Effect - GFCI is not, as much as it seems, an absolute. A few milliamps of current is allowed, with trip starting at around 5ma. If you have some very small current leaks in multiple branch circuits, they could combine to a GFCI trip even when there is no true safety hazard. And nuisance trips lead to getting rid of GFCI which leads to real problems. RCD used in some other countries is essentially a "whole house GFCI" but triggers at a higher threshold than US GFCI breakers, which results in RCD not having the cumulative effect nuisance trip problem but also not providing the same level of human safety that US GFCI provides.
• Code Requirements - Since NEC requires GFCI (and similarly, AFCI) on lots of branch circuits but does not require it on really big circuits (e.g., feeds to subpanels), industry does not have the economic incentive to produce large breakers with GFCI protection built in. If code ever mandates it, they will build it. As it is, for 120V branch circuits many people will opt for GFCI protection at the receptacle instead of the breaker to save money. (For 240V circuits, the breaker is normally the only option.)