I had a single phase 115v ac compressor connected to a 20A circuit. It worked in that location for many years.
Recently, I moved the compressor to another outlet on the same circuit. Now it trips the breaker whenever the compressor comes on - but only if I'm not watching it... if I turn the compressor on and off manually the breaker never trips.
The breaker itself is fairly new. All wiring is 12 ga nonmetallic cable. The new outlet location is the first one on the circuit. In other words the previous compressor's location was downstream of that outlet, in fact it was the last one on the circuit so it should have suffered from more voltage drop than the new location.
If I move the compressor back to where it used to be, it works fine and the breaker never trips.
I couldn't find anything wrong with the receptacle, but I replaced it with a brand new 20A commercial grade duplex receptacle anyway. I couldn't find anything wrong with the wire nuts but I replaced them too. All wires are wrapped nice and tight.
The compressor motor nameplate indicates 15A, with a locked motor current of 98A. Obviously its starting current is a lot more than 20A and I know I'm pushing the limits of a 20A circuit. Starting current might be around 60 to 80A, possibly more, but that misses the point that the compressor worked and continues to work in its old location.
There is nothing else on that circuit except for a very small battery maintainer that doesn't draw much current. And remember, all I have to do to make things work again is move the compressor to its old location, all the way at the end of that same circuit. Which by the way used a residential grade 15A duplex receptacle.
I don't have any way of actually measuring current that big, but have no reason to doubt it's really drawing too much for that breaker.
What the heck is going on?