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I have a metal junction box with a main feed. If the ground wires touch the metal box the breaker pops and the ground wires spark. The main feed comes out of a ceiling light fixture box. The ceiling box is two wire BX. I'm using NM to feed the junction box. I grounded the NM to the light fixture mounting stud. Poor ground? Wires touching?

More details...coming out of the junction box I feed a switch for ceiling lights and a feed to a bathroom combo switch. The ohm meter tells me everything is correct at the bathroom combo switch. Everything works perfect. Except when the ground wires touch the metal junction box. Any ideas please?

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    Sounds really bad to me. What's the voltage between the ground wire and the metal box? Can you measure each to some independent ground? – Daniel Griscom Mar 9 '16 at 10:57
  • I'm not sure where to start measuring. Any tips? – Walter Mar 9 '16 at 14:15
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    Myself: I think it's time to call an electrician. – Daniel Griscom Mar 9 '16 at 14:47
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    All fixed. The box was hot. Used my voltage test light to determine. The cause had to be a pinched hot wire inside the metal box. I tightened the BX clamped too much. Inspected all wires and reinstalled. Box is grounded. Thank you very much for your feedback. It helped tremendously. – Walter Mar 13 '16 at 12:37
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Either the junction box has power on it because of some loose hot wire or the ground itself has power on it because of a loose wire somewhere along the circuit.

Most likely though, since the ground doesn't blow the breaker on it's own, the junction box is powered somehow. In one way or another, power is feeding through that box. It could be from the sheathing of the BX or any stripped or loose hot wire touching any of the system. Go back and double check all connections, something isn't right. Be safe.

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    That's what I was thinking. I'm removing the main feed which is romax. Replacing it with BX for better ground. Double check connections. Thanks! – Walter Mar 9 '16 at 14:19
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    I might have read your comment wrong, but do know that 'a better ground' will not fix the problem. The ground that you had been using was perfectly fine because it was tripping the breaker, that's what you want it to do if there is a fault. I don't think the issue is with the ground, it's with one of the hot wires. – TFK Mar 9 '16 at 14:46
  • Thanks for heading me in the right direction. I will check connections. – Walter Mar 9 '16 at 16:49
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    PAll fixed. The box was hot. Used my voltage test light to determine. The cause had to be a pinched hot wire inside the metal box. I tightened the BX clamped too much. Inspected all wires and reinstalled. Box is grounded. Thank you very much for your feedback. It helped tremendously. – Walter Mar 13 '16 at 12:36
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I would look at the problem upside down. Instead of thinking there's something wrong with the ground wires, think there's something wrong with the hot wires. Or a receptacle or something rubbing. I'd start by connecting all the grounds correctly, disconnect everything else in the circuit, and then troubleshoot starting at the panel. Just like wiring it new, except using the existing wire after testing it.

  1. Disconnect all hot and neutral wires inside the service panel (breaker box). Cap off all wires.

  2. Connect all the grounds with great care, so the grounds are solid.

  3. With an ohmmeter, measure each wire's resistance from each other wire - all should be infinity (or well in the megaohms). Obviously if you find a problem, good chance that's it, and fix that before going any further.

  4. Do the same with any switches, receptacles or hardwired devices, realizing that loads will have a low resistance between hot and neutral, but should still be infinity to ground.

  5. Cap off the wires in the first box, and connect hot and neutral in the service panel. Have a helper watch the far end of the wire. Snap the breaker on and listen for any bad sounds (sizzling, helper shouting out, or a trip.) If so, it's that wire (or you failed to cap off the far end).

  6. Turn off the breaker and connect one more thing - receptacle, lamp, next run of wire, whatever and repeat #5. Slow and steady wins the race.

  • All fixed. The box was hot. Used my voltage test light to determine. The cause had to be a pinched hot wire inside the metal box. I tightened the BX clamped too much. Inspected all wires and reinstalled. Box is grounded. Thank you very much for your feedback. It helped tremendously. – Walter Mar 13 '16 at 12:36

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