On Wed, 02 Dec 2015 00:25:12 -0700, Don Y
wrote:

On 12/1/2015 7:57 PM, wrote:
On Tue, 01 Dec 2015 19:19:01 -0700, Don Y
wrote:

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?

Since you are swapping stuff around, try it on a non-GFI circuit. That
will tell you if it is a ground fault.

Moving the strins of lights to another circuit would take the
existing wiring *and* GFCI out of the calculus.

OTOH, I could see if there's a breaker nearby that I can
"borrow" onto which to move the existing wiring. I.e.,
I've tried changing JUST the GFCI breaker with another
GFCI breaker (keeping everything else constant); this would
allow me to try nonGFCI vs. GFCI (again, keeping everything
else constant).

Trick will be to see how far I can reach in the electric panel
to pick up a different breaker.

Usually this tracks back to water/bugs in a box.

Boxes are recessed into block/cement walls. Faces sealed with
foam gaskets. Outdoor "in use" covers keep out direct rain/water.

Bugs, OTOH, can always crawl in through the hole in the cover intended
for the cords to exit.

But, bug would lead to suspected GFCI issue. And, would persist
(not "clear" itself after the first breaker trip/reset)

I have one nightmare GFI circuit here that is longer than the design
spec for GFIs but it works when everything is dry. When it fails, I
end up splitting the circuit up to isolate the failure. Bear in mind,
it can be a ground fault on the neutral.

Again, see above. IMO, it has something to do with the initial
transient. I need to find a load that is more PURELY resistive
to see what it's like when the load is X from the moment the
breaker is FIRST closed.

On outside boxes, make sure all of the wirenuts are pointed up, near
the top of the box and that they are not too close to the box. It
mitigates the water they all collect eventually.

Make up a "soft start box" getr an electric heater. take a short
extension cord (or make one uo) and cut the live wire, inserting a
second plug in series with the existing one. Plug the heater into the
second plug, the lights into the first. This will reduce the current
on startup. Wire a switch across the second plug so you can short the
heater out of the circuit after the lights come on. This will tell you
for sure if it is a cold surge problem. If you get a 750/1500 heater
you can select how hard or soft the start is.