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I have three circuits in my shop, each dedicated to powering 2 duplex 110V receptacles. The circuits originate from a 100A subpanel inside the shop.

Two of the circuits are 15A, and one is 20A. Everything is wired with 12/2 MC cable with ground, and metal boxes. Each circuit is about the same length. Each circuit has a GFCI receptacle first, with a regular duplex receptacle downstream. the 15A circuits have 15A rated receptacles, the 20A circuit has 20A rated receptacles.

Funny thing is - the 20A circuit will sometimes trip using tools that might draw only 8 or 9 amps max, while I hardly ever trip the 15A circuits with the same tools, or even when using a tool that will draw 15A max (briefly, like a table saw).

What could cause this? I have a multimeter - Can it help me infer something here?

  • Are you sure there are no other loads on the 20A circuit? – mikes Mar 20 '18 at 11:42
  • absolutely sure – paul Mar 20 '18 at 11:42
  • sometimes breakers get weak. How old is the 20A breaker? – Nicknamednick Mar 20 '18 at 11:53
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    Do the 15A circuits have more than one 15A socket on them? What make are the breakers? What make is the panel? – Harper Mar 20 '18 at 16:00
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    Check for a loose connection... especially the wire connected to the breaker itself. A loose connection there will generate heat which will cause nuisance trips. – Tyson Mar 20 '18 at 18:20
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Get another 20A breaker and replace one of the 15A breakers with it. There is a special exception in Code which allows 20A breakers to be used on circuits where

  • all the wiring is 12AWG Cu or larger
  • There is more than one 15A socket. That's easy; most come with two.

A 20A circuit can't feed only this.enter image description here enter image description here One of these is fine.

FYI, there's also a UL rule that 15A receptacles must be good for 20A passthru. That applies to most GFCIs too.


Now with two 20A breakers in the panel, you can experiment more and get back to us.

Also make sure the breakers actually are the same make/model as the panel -- the simple way here is to make sure your panel brand equals your breaker brand. Wrong brands/types will seem to snap in, but don't properly mate and will arc and burn up the buses.

This isn't scamming you, we're not name-brand-obsessed, if you want to use a GM/Delco brand brake pad made for an F150 on your F150, have at. That's just not very common with breakers.

  • Harper, what am I trying to accomplish by doing this? Experiment with what? Yes, the breakers are the correct ones for the panel. – paul Mar 23 '18 at 1:23
  • @paul to find out if another 20A will do the same thing, to determine whether it's a defect in that breaker or congenital to all that maker's breakers. Breakers are like $5, so it's not a big expense. – Harper Mar 23 '18 at 2:06
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It's possible that you have a bad connection somewhere. A loose or bad connection is a resistance and creates a voltage drop, so as soon as you put a load on it, the current flow makes the VD worse. This increases the current drawn by that load until the breaker trips. That first GFCI receptacle would be the first place I'd look.

It's also (albeit more remotely) possible that when installing your wiring you smashed down too hard on a staple, or when doing other work right after that, a screw or nail has pierced the insulation and is causing a high resistance short. So not enough current flow to trip the breaker by itself, but combined with your load, it is. This is what "Arc Fault Circuit Breakers" are intended to prevent, because it happens more than most people realize. If you don't have a way of checking to see if current is flowing in that wire when nothing is plugged into it, you might want to call an electrician. An "arcing fault" like that can smolder for a long time and then suddenly catch fire inside of the wall somewhere.

  • Good info J. Raefield, thanks. I will examine the circuit more closely. The whole thing is surface mounted and visible. – paul Mar 23 '18 at 1:25
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Buy a new 20A breaker. Swap it in. If the problem goes away, it's the breaker. Take the bad one back, explain that it is defective, tell them when you bought it, and that you would expect breakers to last longer than that. and that you have already bought the replacement. If it doesn't go away swap again, and take the new one back.

Verify the load part 1. Use a clip around amp meter and see if there is a load on the circuit when you aren't doing anything.

Verify the load part 2. Make a short (1 foot) extension cord with separate wires so that you can use your clip around amp meter. See how much your device draws on startup.

My suspicion is that you have a fast trip breaker, and that the start up current of one of your devices is enough to set it off. Check the part number on the breaker then look up the specs. Edit: I'm informed that such don't exist in the North American Market. So the breaker is either defective, or something else is wrong.

If you do have a fast trip breaker, then you can attempt to return it as 'wrong item for task' Depending on store policy they may or may not accept the exchange.

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    Hold on there Jesse James. Don't buy a product solely to use it for testing with full intent to return it if unneeded. That is stealing, that is fraud, unless it's with a store (typically big box stores) who has chosen to have that return policy willfully. If you return a 2-year-old one claiming it's the new one you just bought, that is always theft/fraud. (now if you explain the situation honestly to the clerk and they say "that's fine", that's just conspiracy to fraud; no reasonable juror would believe that's actually HD/Lowes' policy). – Harper Mar 20 '18 at 15:56
  • Also, North American (UL489) breakers do not come in variant trip curves save for the occasional High Magnetic breaker. There is nothing in UL489 that's akin to the various trip curve letters you can get an IEC breaker in... – ThreePhaseEel Mar 21 '18 at 0:21
  • Thanks for the enlightenment. I never looked for fast/slow breakers. I've used fast blow or slow blow fuses in electronics, and assumed that you could get breakers that did the same. Hell hath no fury like an unjustified assumption. Indeed, I wish I could get a slow trip breaker. I have a table saw that I have to slack the belt to turn it on, if I have a heavy blade on it. Draws too much current while starting up. – Sherwood Botsford Mar 22 '18 at 14:13

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