I have a mobile home that has an outdoor box which is just used as a main 200 amp breaker and spaces for 6 breakers. One double 20 amp breaker is used for 220v pump circuit which the pump installer used 12-2 romex with ground in PVC conduit. There is just one ground bar where the bare ground wire attached. I have two sheds and wanted to put separate 110v in them. Installing a 20 amp dual breaker(both white and ground to same bar) I ran 12-3 with ground UF cable out between the sheds. I installed an outdoor box with a GFI and 110v (black wire) in the shed for grinder/cordless chargers. All working well. Today I used the red with the ground and neutral to a GFI recpt. in the second shed. I have an octagon box below the first GFI where I did all the connections. I had a float charger in the second shed GFI and as soon as I connected the load red to the red to the second shed with the float charger the black circuit kicked off. Baffled me. Is this a phase issue, ground/neutral or something I'm missing. Thanks and God bless.
Splitting 220v breaker to two separate GFI circuits
gfci
Related Solutions
I'd use a 4x4 box with a raised cover. You can get the cover in a dual GFI configuration. You say a "GFI slave", I assume you just mean another receptacle on the load side. If so then you can get a cover with one GFI opening and one duplex opening:
Just so you know, there is a near zero chance that that old AC cable can safely be used as a grounding conductor. Older AC cable does not have the required bonding strip. See here: How should I connect a grounding wire from a device to this box?
Box:
You can change out the old breaker feeding the pump. This would be the safest way. A surge protector should be located close to your service this would then protect everything not just the pool. I voted up because it is easy to swap an old non GFCI for a GFCI protected breaker and this will make the current pool service much safer.
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Best Answer
You tried to poach the second shed's neutral from the GFCI protection on the first shed, so its no wonder the first GFCI tripped
What happened here isn't as weird-sounding as you are thinking it is. You just need to remember two fundamentals here: first, that electric current flows in loops (it always has to make it back to the utility, generator, battery, or so on) and second, that a GFCI has nothing to do with ground as it simply compares what went through it on the hot side with what went through it on the neutral side and trips if they don't match up.
This all adds up to this being a simple wiring error; namely, that you tried to tap neutral to the second shed from the load side of the GFCI in the first shed. As a result of this, when you tried to use the receptacle in the second shed, you injected the charger's running current, taken from the red wire (which the first shed's GFCI has no clue about) into the load neutral terminal of the first shed's GFCI. Said GFCI sees this inexplicable-to-it flow of current on neutral, goes "WTF?!", and trips, just as it's designed to do, thinking something is wrong, which is true.
Fortunately, the fix here is simple; you just need to move the neutral connection going off to the second shed to the LINE side of the first shed's GFCI. Note that you'll probably want disconnect-rated GFCIs here if you don't have them already (they're denoted by having ON/OFF labeling on their buttons to go with the normal TEST/RESET labeling).