I'm working on a small finished outbuilding that has a sub panel for lights, outlets, etc. From the main he sent (2) separate 70amp lines to the building sub panel into (2) 40 amp breakers, each 70 runs to a 40. Is that correct? There are 4 other varied 15/20 breakers feeding off that.

Thanks David

  • Is that the royal "he"? Jun 3, 2015 at 2:58

2 Answers 2


If the wire is sized for a 70 amp run, then the breaker in the subpanel could be any size that size or lower. A lower rating will protect that circuit better if you don't anticipate the need for that qty of current, but can be swapped out quickly if increasing size is needed. However, depending on the distance traveled, the difference between the breaker sizes might be preferred to prevent nuisance tripping (if both were 40 amp breakers and you were getting close to the overall limit, then the inside breaker would likely trip before the outside due to the wire line load to feed the building itself.


I am not sure what you mean by a "70 amp line". As long as each wire is fat enough to handle 40 amps that is all that is required as far as load is concerned. If an electrician used fatter lines, so much the safer. In long runs you have to upsize the wire anyway. For a 30-foot run supplying 40 amps you would use AWG 4 or fatter.

  • What I mean is that there are 2 separate 70amp breakers in the main that are feeding the sub panel. Each line is run to a 40amp breaker in the sub panel box which also feeds (2) 15A and (2) 20A breakers. I'll be adding an additional (2) 15A and one 20A.
    – David
    Jun 3, 2015 at 23:36
  • Well, if there are 70 amp breakers on one end and 40 amp breakers on the other, then it is a 40 amp line. How long is the run? Jun 4, 2015 at 13:16

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