How to add GFCI to circuit with obsolete breaker box?

[RBM]

The GFCI device senses the current going out on the hot leg and returning on the neutral. If they are not equal, current is leaking to ground, and the device cuts out. Where your landscape lights are concerned, the neutral wire is probably broken, so to make the lights work, he just connected the bare ground as a neutral replacement. That circuit won't work on a GFCI device as there would always be leakage into ground on the bare wire, causing an imbalance. It is also a potentially dangerous situation for anyone that may touch one of these fixtures. If they happen to be touching anything that is better grounded than the ground wire, they could become part of the circuit.

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[Nate Nagel]

no.

the GFCI must be in a box where both hot and neutral are available - it has connections for both. The GFCI compares currents between the two wires, and if they differ by more than 5 mA or so, it'll shut off the power. this is why it also can't be used to replace a switch used in a "switch leg" configuration, because there's no neutral available there.

nate

[Mark]

I simply pulled the old non-gfci circuit that ran to my detached garage out of the panel, then ran a short line to a surface mounted box a few feet from the panel with a gfci outlet. I then pulled the garage line into that so now I have a much needed outlet near my panel and the garage and underground line leading out to it is protected.

[iwdplz]

That wiring makes sense and I tried it just now, but the GFCI trips immediately so the pool light never turns on. I also noticed that the landscape light doesn't turn on anymore - so I jumpered the feed neutral to the pool light neutral like it was before, and now landscape light turns on ok via the other switch. So the pool light and landscape light must be using the same neutral. I am puzzled why the GFCI is tripping and how I should wire it given this new info..

[iwdplz]

Update:

When I add the landscape light to the Load terminal in the GFCI combo device (instead of a separate switch), it works. Now the GFCI works for both the pool light and landscape lights. Except now I don't have independent control over them. That's fine because the landscape lights are next to the pool anyway. So this way I have GFCI protection for ALL of the outdoor lights near the pool.

Why does this GFCI tripping happen with a separate landscape light switch? My guess is that by turning on the landscape switch, it causes the current to drop unevenly in the circuit legs and the GFCI senses a slight mismatch. So by combining them together the current drop is evenly matched.

[iwdplz]

This is the best/correct hookup because now I can use the outlet regardless whether the switch for the lights is turned on or off.

[iwdplz]

I think this is what happened:

"Multiwire circuits have two or more ungrounded conductors sharing a common neutral, as in a 120/240V, single-phase circuit or a 208Y/120V, 3-phase circuit."

"GFCI receptacles can be used on multiwire circuits, but they must be wired such that the neutral on the load side of the GFCI is not shared by two ungrounded conductors. Failure to observe this requirement will result in a differential current any time a load served by an ungrounded conductor not connected to the GFCI is energized, immediately tripping the GFCI."

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