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?