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I just rewired my garage using what was there and have non-functional brand new GFCI outlets. They both come out of one box and feed separate circuits in my garage (one to a wall and other to my openers). They worked before supplying one circuit to my openers. Is having 2 GFCIs in parallel causing this? There is also a mysterious ground coming into the box feeding them that I tied together with the incoming ground.

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    I am not sure you are using the word "circuit" properly. A circuit is everything connected to a breaker i.e. that loses power when the breaker is cut. If a circuit splits and goes off two directions, I call those branches or forks (not to be confused with branch circuit). "Parallel" is a term that is equivalent to "bad". If you know the tricks, you only need 1 GFCI per circuit . We need more info to understand what you are doing. – Harper Jun 10 '18 at 23:27
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"mysterious grounding" may be the problem. A GFCI is very sensitive to grounding problems - by design. A few questions:

1 - Do the GFCIs trip and not reset? Or do they NOT trip but simply "don't work"? There is a big difference. If they trip and won't reset then there is a real grounding problem. If they don't trip then they may be bad, though unusual to have two brand new bad outlets.

2 - I'm a little confused about the circuits. "feed separate circuits" implies 2. "They worked before supplying one circuit" implies 1. Do you have 2 separate circuits - i.e., two separate circuit breakers in the panel? Or do you have 1 circuit that originally supplied the openers and now also supplies an outlet? If you only have one circuit then you should only have one GFCI installed.

In addition to any other "mysterious ground", it is possible you have a problem with the openers - i.e., a fault that is minor enough to not trip the regular circuit breaker but is enough to trip the GFCI.

I don't know whether an opener circuit by itself (i.e., no outlets on the same circuit) needs GFCI protection according to code or not. Harper or ThreePhaseEel or one of the other code experts can answer that.

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    GFCIs are not even aware that ground exists. However the presence of ground makes it easier for faulty devices with current leakage to leak it somewhere it'll complete a circuit, which means fault current is more likely to flow, which means it's more likely to trip a GFCI. If no device is defective, grounded or ungrounded makes no difference. – Harper Jun 10 '18 at 23:24
  • True. But "mysterious grounding" could be something gong to ground that shouldn't as opposed to just the ground itself being "bad". – manassehkatz Jun 10 '18 at 23:30

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