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I have setup a new water heater where I used 10-2 wire to a disconnect box. Then, connected a square D 30 Amp dbl pole in my breaker box. Water heater connections were black to black white to red and grounded to plate. My breaker was Square D QO230 wired with Black to one pole and white to the other. I then grounded it. As soon as I turned it on the circuit tripped. I thought maybe I had my connections switched on the breaker so I switched them and the same thing happened. I am starting to think I am supposed to use two 30 amp single pole breakers but I thought that was the same thing as a 30amp double pole breaker does that sound right? Could it be I have a different issue?

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    Does the breaker still trip when you open the disconnect, then try to turn the breaker back on? – ThreePhaseEel Dec 16 '19 at 23:52
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    Photos of the wiring may help us identify anything amiss. FYI the white conductor should be marked with a 'hot' color (black, red, blue, orange, etc) at both ends by means of a marker, tape, etc to indicate that it's not being used as a neutral. – Greg Hill Dec 17 '19 at 0:09
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You're in a QO panel; fortunately has unique busing so you can't misfit a breaker that doesn't belong, you can use either the OEM Square D QO230, or one of the UL-classified competitor models the CHQ230 or ITED230.

Since both wires will be hot, you must use tape or paint to re-mark the white wire a color. Typically this is done by wrapping it with black tape.

Ground first, not last

You should hook up the ground wire first. You said you hooked up the hots, then hooked up ground, then had a problem. You were lucky. You could have been shocked if you tested it without grounds present. The fact that you did grounds last makes it seem like the ground is at fault. It's not. It's simply the "canary in the coal mine" warning you of the problem. Hook it up first, and the problem will be more apparent.

Divide and conquer

Unhook all the wires, except for - anyone, anyone, Bueller? Ground wires. That's right, leave all ground wires attached.

Now, hook up one wire to the disconnect, open the disconnect, and see if the circuit can power up. If so, power down and attach the other wire to the disconnect, and test again. Now close the disconnect and test again. Now attach one onward wire to the water heater. Test again. Etc. etc.

At some point, adding a wire will cause the breaker to trip when turned on. Divide that further if possible. Now you're pretty close to the answer. If you're still stuck, ask.

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Assuming that the water heater is rated for a 30A breaker:

  • You MUST use a double-pole breaker
  • 10/2 is black/white so you use black to one pole of the breaker, white to the other. At the water heater, depending on the water heater you will typically have two identical wires or one black, one red. If one is black and one is red, connect black to black and white to red.
  • A double-breaker should always show ~ 240V between poles. If it doesn't then somehow you have it installed wrong and strange things can happen. So check (a) at the breaker and then (b) at the water heater (i.e., the ends of the black & white wires) for 240V. If you get 0V or 120V then STOP because you have major problems.
  • The ground wire should not be contiguous to either wire. If it is then you have a problem (which would explain the trip). Connect the wires at the water heater but NOT at the breaker and check continuity - you should get a resistance between black & white and no connection at all between ground and black or between ground and white.
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