20amp afci breaker with gfci outlet tripping under 15amp load, but not lower loads

Question

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.

Answer 1

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.