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My pool system has a 3-wire connection to the main breaker box: 2 Hot and Ground. The wires go from the main panel to a 2 pole GFCI (pig tail not connected), from there to the timer and the pump. In order to connect a 120 V unit, I ran a neutral from the main breaker box through the same conduit into the GFCI sub-panel and connected it to the neutral bar (I made sure it's not bonded to ground). I connected the pig-tail to the neutral bar and ran a wire from there to a regular 120 V outlet (with one hot lead and ground).

The outlet measures 120 V and the pump system runs fine. Pressing the test button trips the 2-pole GFCI. Everything looks fine. However - as soon as I plug something into the outlet, the GFCI trips.

I could wire a GFCI outlet and take the hot before the 2-pole GFCI. But it is important to understand that the 120V outlet powers a pump and if it fails without the pool equipment shutting down, the pool pump will flood the equipment. So there's a relation that requires a fault in the 120 V circuit to cut the power to the pump as well. That's why I wired it this way in the first place.

Here's my question: Is it normal behavior for a double pole GFCI to trip in this configuration? How else can I get the desired behavior so that a fault on the 120V circuit will shut down the pool equipment?

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The GFCI breaker is working as designed. As far as it can see, with neutral run directly to the neutral bar, there is a massive fault that might be trying to kill a person, so it trips.

If you want to feed 120V loads from that breaker you must connect its pigtail to the neutral bar and connect the load neutral wire to the load neutral terminal on the breaker. That way the breaker will be able to see all power coming and going and only trip on actual faults.

  • Yikes. I wasn't even aware of this neutral connector. So the power now flows from the neutral bar through the pigtail to the neutral connector on the two-pole GFCI. I actually went out in the dark to rewire and it works fine. Thank you. – Michaela.Merz Jul 19 '20 at 3:13
  • @Michaela.Merz -- this'll take more work to make fully safe. Can you post photos of the inside of the GFCI's box please? – ThreePhaseEel Jul 19 '20 at 3:21
  • @ThreePhaseEel Sure can. When the sun is back up. What in particular are you interested in? – Michaela.Merz Jul 19 '20 at 3:33
  • @Michaela.Merz -- clear photos of where the wiring enters/exits the box are the most important things we're looking for – ThreePhaseEel Jul 19 '20 at 4:13
  • @ThreePhaseEel Hmm .. ok - it's just a waterproof plastic 1/2" conduit that is screwed into the metal housing and it carries the incoming and outgoing wires.4 wires in, 4 wires out. The box is grounded. Ah yes .. it has a gasket and a lock nut from the inside. – Michaela.Merz Jul 19 '20 at 5:49

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