I replaced a twenty year old window (through the wall) a/c unit with a new one. The new unit is 18000BTU and rated at 12 amps. I replaced the existing outlet with a 125V 20 amp outlet with 12/2 wire going to it’s own breaker (20 amp) in the service box. I only find two bars in the service box for grounds or commons. The existing wires coming in have both white and ground wires attached to the same bars. I did the same. We have 100 amp service. I measure 120 volts at the outlet. The problem is that the a/c fan runs well but the compressor only runs a few minutes and then the breaker trips. I’m wondering if 100 amp service is enough. I’ve calculated the load for the building based on existing appliances and come up with 78 amps. The breaker trips with all other appliances off also. I’m about to return the a/c unit….any ideas before I pull that honker out of the wall?


It is a 15000btu unit!!

It should be on a dedicated line. Nothing but a/c on that line

Yep... it is on a dedicated line.

Amprobe would be nice to see what Amps unit is drawing.
With everything off and AC tripping Breaker. Sounds like AC has a problem.
Check breaker connections in Panel for Main and AC,Might as well tighten all.
Verify Small Prong of outlet is hot. When other probe or lead connected to neutral, and then to ground.

Everything you have for the circuit, and service is fine. I think there is no other choice that the unit does have a problem. But there is a chance the breaker is defective, may be worth swapping the breaker and give that a try before removing the AC.