I am installing a water heater. I know that the supply requires 240V across both wires (red & black). However, when I measure one wire (red or black) to ground, I get 120V. But, if I measure across the 2 wires, I get 0 volts. What would cause this?
It sounds like you're talking about an American centre tapped 120/240V single phase system, but it would be a good idea to say if that's what it is; electrical systems vary considerably in different places. If this is the case, since both red and black wires are reading 120V to ground they must be connected to something. Since there is no Voltage between them it sounds like they have both been connected to the same side of the supply, i.e. both wires are going to where the black one should be connected, or where the red wire should be connected, rather than one wire to each side.
Two possibilities. You either have both hots on the same phase or one of them is open, effectively the same thing since the other phase is reflected through the load. Check it at the breaker, If that is bad too, be sure the breaker is hitting both phase rails. Some small GE (and perhaps others) panels will let you plug in a double breaker on the same bus
It sounds like you installed a "twin" breaker, rather than the required
2-pole breaker, so both terminals on the breaker are being fed from the same phase.What you need is a real 2-pole breaker, which will clip onto 2 adjacent busbars, providing 240V between the 2 phases.
Bob Weiss N2IXK
Thank you all. This greatly helps my understanding of what is going on. I'll have someone look at the breaker panel and fix the issue.
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