Can someone please help me. I am in the US. I have to make a single phase 220v drop from overhead bus bar. I dont know the rating for the bar. To power a rectifier for a test. I am using 12/3 black cord and a strain relief. The black cord has a white, black, and green wire. At the rectifier there is places for 2 lines in and a ground simple. At the bar is where i get confused there are 4 lines inside the top one is for common white wire. The green ground wire grounds to the box. I need to know if Iam doing this right by puttting white wire to top bar, the black one of the other three bars, and the green to the box. This is a circuit breaker protected box. I was going to use 20 amp breaker here is where i get confused again I have options to put in one to three circuit breakers. Should i just use one breaker on the black wire? The white wire /common has no place for a breaker. I dont think it needs one. If i am correct how do i know it is single phase 220v. thanks