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I have been switching my breakers over to AFCI a few at a time. I now have two AFCI breakers running to my garage, which are wired to two 20 amp gfci outlets (dedicated circuits). One of the outlets trips every time I try to run my 15amp miter saw, but runs fine when I have a 6amp drill press and 3amp heater running off it. Here is what I've done so far:

  1. I checked the wiring of the breaker in the panel, and it's fine (neutral and hot from the circuit going to the breaker, and then a separate neutral going from the breaker to the neutral bus bar).
  2. Since the other gcfi outlet works with the miter saw, I swapped the outlets, and it still flips the breaker.
  3. Then I re-ran the 12-2 wiring (not that difficult in my case), which also caused me to re-do the wiring at the breaker anyway, and it still flips when I fire up the miter saw.

The gfci outlets seem to work fine, the miter saw works fine on other gfci outlets, I have new 12-2 lines, the wiring at the breaker is perfect... is it possible I have a bad aFCi breaker? Thoughts and ideas are appreciated.

  • Was the wiring done using backstabs, side screws or clamped-by-screw? – Harper Nov 4 '18 at 22:37
  • At the outlet, its all clamp-by-screw (little metal plate the clamps the wires when the screw is turned). The breaker uses the same system for accepting the circuit wires, but has a backstab for the neutral going to the bus bar (cram the wire in a white hole, and its almost impossible to pull back out). – Phil_T Nov 4 '18 at 23:06
  • Ya know, now that I think about it, the afci breakers I bought didnt have the white curly pigtail neutral on them. I had to add the neutral. I bet the neutral I crammed in the backstab is not situated correctly. Its probably considered a "bad" afci breaker at this point. – Phil_T Nov 4 '18 at 23:16
  • Are you quite sure the AFCI neutral is on the LINE side and not the LOAD side? I am shocked they would use a backstab on a breaker. Is this breaker the same brand as the panel? – Harper Nov 4 '18 at 23:36
  • What make and model are the panel and AFCIs in question? It sounds like you are trying to use Plug-on Neutral breakers in a panel that is not compatible with them. – ThreePhaseEel Nov 4 '18 at 23:44
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AFCI breakers are horrible with motor loads and even heavily loaded circuits with dimmers or ballast, to the point my state exempts circuits that have problems.

Why are they not able to tell the difference? Your miter saw creates tons of little arcs each time the brushes change from 1 commutator pole to the next, on top of pulling 3-5x the current for startup. This looks like arcs that could cause a problem. Dimmers and ballast change the wave shape causing harmonics that if a loaded circuit (well below the 80% standard) they will trip because they cannot tell the difference between the harmonics and an arc.

Using current code, you need GFCIs in the garage, not AFCIs, and if you want to cut with anything that has brush-based based motors or variable speed it will need to be less than 50& of the breaker value on some of the mfg info I have read.

  • That makes sense. I ended up going to 20amp gcfi today, after I tried a new afci and it failed again. If I really, really wanted to do it, I would need to put a "dimmer" on my saw or go up to a 30amp afci? Id probably go all the way to a 50amp just to be safe. This is annoying, thank you for your response. – Phil_T Nov 5 '18 at 1:04
  • The problem would be going to 50 amp the circuit conductors would not be protected, if available I have not seen one that large you could have a fire because the insulation would not hold up at that level, 2nd in a garage GFCI protection is required. You are much more likely to need GFCI protection than AFCI protection in a garage. A dimmer would make the problem worse and would be another direct violation of code you cannot have an outlet that has dimmer controll. – Ed Beal Nov 5 '18 at 2:54
  • I have seen GFCI and AFCI breakers at home depot, but I have not seen something as large as 50amp. Going to a GFCI is probably going to solve the issues just fine. Thanks – Phil_T Nov 6 '18 at 0:35
  • They do have GFCI double pole breakers up to 60 amp for hot tubs. Don't remember if I have seen anything larger. But I haven't seen AFCI that large. – Ed Beal Nov 6 '18 at 13:01
  • Anecdotally, my spark says he's had lots of callbacks from afci breakers that die an early death. Maybe they got rushed out to market to meet a code-forced demand without being as good as they should be. – Aloysius Defenestrate Nov 7 '18 at 14:37

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