I got a used air compressor rated at 15A (120V). When plugging it in to either of two 15A circuits I have in my basement, it will work for 8-10 minutes, then the breaker will trip. You'd think the most common time to trip might be when the compressor motor cuts in, but it actually trips mid-way through the 2nd or 3rd cycle. To my ear, it sounds absolutely normal and then just quits when the breaker trips.
There is nothing else on the circuits in question. The breakers are clearly 15A. One of the circuits is only about 10'/3m (wire distance) from the panel. My clamp multimeter is terrible (updates very slowly), so I can't see transients very well, but the compressor does appear to draw around 14.5A when the motor is running. The breakers were installed before I bought this house, and have seen minimal use (previous occupant was a renter who had a small fridge plugged in to one circuit, and probably never used the other. The guy sure as hell never used a vacuum cleaner, anyway). Finally, I am not doing anything dumb like using an extension cord–the 6'/2m cord and NEMA 5-15 plug are clearly factory and in good condition. I've cleaned both the outlet and compressor plug, and ensured they make a good connection.
Do I need to run a new circuit and 20A breaker? It'd only be 10'/3m of 12AWG and there's space in the panel. Or do I need to look for a fault in the compressor? Better yet, is this a good time to convince my wife I need a better clamp meter?
Best Answer
I think you need a bit of “headroom” so a new circuit with the 20A breaker should do it.
For both breakers to run the same amount of time seems to suggest both are functioning correctly. The regs I usually consult gives times for breakers to act if the load is close to the rated value so it sounds like those two 15A breakers are just being run close to the edge and tripping due to an extended time.
I would definitely test it with a 20A breaker - safely though...