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Over the past few days I've been doing some cutting with a miter saw rated at 15 amps. And afterwards, when I would be cleaning up with my shop vac (11 amp) the circuit breaker would trip. Both times I just reset the breaker and continued vacuuming with no issues. The circuit is labeled 20 amp GFI on the panel and as far as I see it controls 3 outlets in my garage. The receiver from my cable company is plugged into one, garage door opener in another. And the third outlet is the one I use for the saw and the vacuum cleaner.

At first I thought it was the shop vac and the filter did look dirty and clogged, however I would think that it would trip the breaker every time I'd use it, which isn't the case at all. Plus it doesn't sound like it's overheating or struggling to pull in the dust and it would be the only thing working at that time. I'm not sure how it can be the saw either, since it doesn't trip WHILE I use it. I actually remembered another time the same circuit breaker tripped, which was again a few minutes after I made a cut with the saw and nothing else was on. I again just reset the breaker and didn't think much of it.

The outlets don't feel warm to the touch, no buzzing noises or smells, and I used an outlet tester to confirm that all are properly wired. It would make sense to me if the breaker tripped while I was making a cut with the saw, vacuuming and opening the garage door all at the same time, but this just seems very strange that it would happen a few minute afterwards. Does the circuit stay "warm" for some time after the load is removed? The circuit also never trips unless I'm doing something in the garage, so it again tells me it can't be anything other than the saw or the shop vac, however I just can't see how either one of them could be doing it by themselves. I'm going to call an electrician to have a look at the outlets as I'm not comfortable doing any sort of electrical troubleshooting myself, but does anyone have an explanation of what might be happening here? Thank you.

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    Have you considered that your shop vac may be faulty, leaking some current to ground, and the GFI breaker is just doing its job ... ? – brhans Mar 26 '18 at 14:07
  • To answer a side question of yours, breakers do trip when they get too hot, so using just over the rated amperage for a long time can cause it to trip. The breaker would take time to warm up, and would hold that heat so tripping would be easier the longer you use it. In other words, a 20A breaker doesn't trip as soon as you use 21A. Anything over 20A will eventually cause it to warm up and trip - more current makes it happen faster. For a short circuit, a different mechanism trips the breaker immediately. – JPhi1618 Mar 26 '18 at 14:16
  • If the lug where the circuit connects to the breaker is even slightly loose it can generate enough extra heat that the breaker trips when not quite fully loaded. – Tyson Mar 26 '18 at 14:19
  • Your breaker is a combination device. It detects (and trips) on two separate conditions: 1) overcurrent - too many amps. 2) Ground Faults (GF) - a defective appliance leaking current to earth. As such I have 2 questions. #1 does the vacuum have a 3-prong plug? #2 Read the manual for the breaker online, and see if it is able to display the reason for the trip (overcurrent vs ground fault) - if so, which is displaying? – Harper Mar 26 '18 at 15:45
  • Have you plugged your vac and or saw into another circuit and used like you normally would in the garage? This could help you determine if its the devices or a wiring issue. – Jeff Cates Mar 27 '18 at 13:11
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A large inductive load, such a motor starting, often causes a ground-fault circuit interrupter to trip even when there is no fault in the device. A snubber, a small capacitor and resistor across the line, can help mitigate the issue. A voltage surge protector can also help, but avoid protectors with large varistors from line to ground, which can also trigger the GFCI.

You might also need a Class C or other type of GFCI which allows greater fault current to flow for a longer time, depending on shop voltage, appliance grounding and insulation.

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If it's a RCD, there may be a current leak to the ground, or the 'apparent' power may be over 15A. It may mean that your saw power factor [a.k.a. cos(fi)] is lower than 0,82 (check on the label).

15 < P*cos(fi)+P*sin(fi) where fi is between 0 and 90 (in your case cos(fi) is lower than 0,82). In your case P*sin(fi)=10 so we can go to fi < 63°.

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