So I was checking a friends hot water heater and tested the AC going to it. The breaker was tripped (they had no hot water). The hot water heater is 240v but think that it only has 120v going to it. I told them they need to get an electrician out there.

Anyways my testing (breaker turned back on) has me confused and that's what I am trying to figure out. There where two wire (black and white going in) and a ground. The incoming wires tested (not hooked up to the heater) black to white =120v, black to ground =120v, white to ground=0v. Now with the heater hooked up the black and white wires =240v. This was when I told them they needed an electrician because I don't know how you can go from 120v not hooked to 240v hooked up. Anyone have any insight on this? Not looking to fix this just the "why" will bug me. Thanks!

 

Did you test (power off) continuity of the heater?
With the wire disconnected from the heater, use a different load, e.g. incandescent lamp.

 

I didn't use load but did ohm the wires to the heater (looking to see if I could find what tripped breaker). The two wires (red and black) and ground all were open. No resistance detected between them.