Most of the time this ends up being water in a box somewhere.
I start by splitting the circuit in half. Pick a box in the middle of
the run, remove the device and terminate the wire there with wire
nuts, being sure the conductors are not touching the box.
Let it run that way for a while and see if you still trip.
Then work in the direction you have identified as the problem.
When you are putting boxes back together, it helps to point all wire
nuts up and keep them toward the top of the box.
I did exactly that with several boxes. Only a couple had wire nuts pointing
down, so I fixed them. Also used silicone sealant to seal around the gray
conduit where it entered the top of any boxes.
Good idea about how to find the culprit -- start at the halfway point. Then
depending on the result, go to halfway along one of the two legs, and keep doing
that until you find it. In computer jargon it's called the binary search
method, and is one of the fastest ways to find something. There is a game that
works the same way -- have someone think of a number between 1 and 100, then
tell you if your guess is low or high. Using the binary technique you can
always guess the number in about 7 tries.
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