I am wiring a workshop in my backyard. I ran 4 gauge wire from the main panel to a pull out fused panel then wire goes inside to a subpanel. The sub panel has 2 - 20 amp circuit breakers for the shop equipment. 1 breaker for each side of the shop. Then I have 1 - 15 amp breaker used for lighting. The wire under ground is 4 gauge - hot/hot/neutral/ground and runs 105 feet. I want to use a GFCI in the main panel and want to know if I should use 40 amp double pole or 60 amp double pole? Thanks for any help.

  • How big is the workshop (in square feet)? Commented Apr 14, 2015 at 3:49
  • As described, you will pull no more than 35 amps on either leg (20A on one leg and 20A+15A on the other). Therefore you have no reason, other than future expansion, to use a master breaker larger than 40A.
    – DoxyLover
    Commented Apr 14, 2015 at 4:38

1 Answer 1


With #4CU you can have up to a 90A feeder breaker. Even at 105' a 90A feeder is FINE.

The sum of the breakers is a completely meaningless number, you need to consider the actual load you will have at any given time. Think about it, a typical 200A panel might be filled with 400, 500, 600, or more amps worth of breakers. Better yet do a load calculation, which in an installation like this is pretty hard to pin down.

IMO there is no reason at all to go with less than a 70A breaker. If it were me the 90 would be in there already. Your wire will handle it, why limit yourself???

Actually, re-reading it You are saying you want to use a GFCI breaker for the feeder? If so, WHY?? That is an expensive breaker that is NOT needed. Simply use GFI receptacles where required at the point of use.

If this is the case then I'd go with the biggest GFCI breaker I could find under 100A, which is most likely 60A.

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