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?

I am trying to install a new GFI. Key word is "trying" :) When I took old one (dated 1979) out it had two white, two black and a green ground. The part that confused me is that when I took them off and marked them on the wall, I listed the "top" black and white were to be kept together. Okay, but with the new outlet it had neat little holes to put each black and white wire in. Hmmmm. Little green light comes on but no power to this outlet or the other one in the other bathroom that it controls? Do I use a black and white jumper to group all the "top" wires together? Should I switch the "top" two wires around so the black and white are on each side?