1

I'll be connecting a #10 cable fed from a 20A breaker to temporarily power a subpanel while we acquire and install the feeder cable. The #10 cable is two-conductor plus ground. Can I safely connect it to a single 20A breaker in the sub-panel? No 240v breakers will be installed.

Obviously this means that only every other bus tab will have power, and that's fine with us.

1

Yeah, that's fine, with the proviso that the backfed breaker needs to be tied down so it can't rock out like most breakers can. It's an option to just feed the main breaker or main lugs. I've never seen tiedown kits for 1-pole breakers so you may need a 2-pole to use one.

The input breaker on the subpanel is redundant, there's no requirement for a breaker there. If the subpanel is in the same building, it doesn't need anything at all and can simply be fed through input main lugs. The wires to the subpanel, and the subpanel itself, are protected by the feed breaker in the main panel. If it's in an outbuilding, it needs a shutoff switch (and a main breaker brings that to the party) but overcurrent ability is not required or useful...

... except in the rare case of the subpanel being too small rating to be protected by the main panel's breaker. Imagine a 100A breaker and #3Cu wire feeding a 60A rated subpanel, in that case it'd need a 60A input breaker. But that is unusual.

Usually it's the reverse, the 100A feed going into a subpanel with a 200A main breaker. That's because panels with small breakers tend to be small panels, and folks want large panels.

If the aim is to synchronize breakers so the local one trips first, that doesn't ever work :)

If you want to feed both input legs of the subpanel with the same pole, feel free. I don't see a code problem with that. Of course 240V loads won't work.

There is no separate set of temorary wiring rules, and I've found no need for one; I've made "permanent" alterations to Code that I rolled back 4 hours later.

  • Thanks. It's a feeder situation. I plan on #1 AL from the lugs of a 200A main panel. I'll need to Google "tied down" and "rock out". – isherwood Nov 10 '18 at 15:48
  • And good to know that I can actually feed both legs. That's convenient when placing breakers. – isherwood Nov 10 '18 at 15:49
  • Ok, I see what "tied down" means. I've noticed those retention screws before. Why is that of particular concern with a backfed breaker? – isherwood Nov 10 '18 at 15:51
2

With the main turned off, I have done this to power tools on a job site, the bonus was that generator had GFCI outputs so temporary power had GFCI protection. Even though the service is not in yet I still lock out the main breaker.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.