On 12/03/2015 10:45 AM, Don Y wrote:
....

We know it isn't -- because I moved the extension cord to a non-GFCI
circuit and the circuit had no problem holding the load.

But have you done the simple expedient of swapping out extension cords yet?

....

I'll try plugging the extension cord into a kitchen outlet (unloaded
circuit)
as that would test a different GFCI with the existing extension cord
BUT DIFFERENT WIRING (as we've already tried a different GFCI with the
existing extension cord and THE SAME WIRING).

I repeat--

But have you done the simple expedient of swapping out extension cords yet?

[This is easy to do whereas tying the existing wiring to a nonGFCI
breaker is a significant effort]

Just swap a standard breaker into the slot in the box.

--