The outlets in the back of the house are on a 20A GFCI.
(Note to self: are there any other outlets on that same
circuit that I need to chase down?)

Presently using them to light XMAS lights on one of the
citrus trees. IIRC, each string is about 250W. With
~700W on the circuit, turning the breaker ON (i.e., using
it as a switch -- instead of plugging/unplugging the
load) causes it to immediately trip. Repeating the
action in short order appears to get it to latch and
remain latched (ON), indefinitely.

OK, so the surge when all the lamps are cold can increase
the inrush current -- on the short term -- to levels that
probably exceed the 20A limit of the breaker.

"In theory"

OTOH, in years past, I've run the circuit at close to its
capacity (~10 strings) without this problem.

I've changed breakers (swapped with one feeding another
circuit) and the problem persists.

[There are no leakage paths in the wiring OUTSIDE]

This suggests something in the wiring/fixtures. I'll
start isolating outlets, tomorrow (dark here, now).
Perhaps some insect (leaf cutter?) has opted to nest
in one of the receptacles.

Anything else I should explore?


Many many years ago I worked as an electrician in a hospital that had an indoor pool. Every day when the attendant turned on the Metal Halide HID overhead lights over the pool deck, the GFCI circuit breakers would trip. I changed the breakers and tighten the connections, but the symptoms never went away. When the circuit breakers were reset, the lights stayed on until the pool closed when they were then intentionally shut off. The next day would bring the same symptoms. I had surmised that perhaps the high humidity environment was causing condensation on the ballasts, but that was just a best guess. After a while I gave up and just made it my business to turn on the pool lights every day.

John Grabowski
http://www.MrElectrician.TV