Septic system:
Gravity flow into 2-500 conc. tanks, then
into 4x6' conc. culvert w/ 4/10hp sump pump & check valve that
sends water uphill via 1.25-1.5" PVC 100 yards, 8-10' head to
a 300 gal. fiberglass tank that gravity distributes effluent to subterranean septic lines

When the house was built in 1980, the electrician brought power to the pump from the HVAC disconnect fuse box via
underground
10/2 "Romex", conduit only at above ground locations,
55' to a GFI J-box into which the pump was plugged
(covered by a weather cover and a plastic hood from rain and soil poisoned against ants shorting it out)

Problem:
The less than one year old GFI 15amp outlet flipped with the beginning of a gully washer and continued to flip when ever
the pump (but not a simple light) was plugged in during the following days of rain.
The normal temporary solution has been to plug the pump into a 100' 12/2 cord, then into the 15amp GFI weatherproof
house outlet on the covered porch.
This worked.

Assumption was that flooding from the rain entered the culvert and the pump was overloaded, requiring a drier, perhaps
larger amp outlet.
The 15amp outlet was replaced with another GFI, but 20amp.

This solution did not last long, as the next rain flipped the outlet again under pump loads but not light loads
Again, the (inferior) extension cord solution worked.

It was assumed that the 22 year old underground power line had insulation issues that allowed its bare wire contact with
the soil, causing a GFI reading and flipping the outlet (breaker) in very wet, soil penetrating weather - perhaps a
landscape "steel edge" had driven down enough to slice the insulation.
The 10/2 underground cable was replaced with a 12/2 cable.
The outlet again performed under light loads of a light, but flipped under the pump loads.
The pump continued to work with the extension cord to house.

The lid was removed from the pump culvert and a water hose with power spray was used to clean off all exposed surfaces
of tank, pump, etc. and under the pump area. Repeatedly, the pump emptied the clean water.
Thereafter, the pump successfully operated on the new 20amp GFI outlet supplied by the new underground 12/2 cable UNTIL
another lightning storm moved in and "flipped" it. Since then, I have used the extension cord to covered porch GFI
(dry) outlet.

Other evidence I should supply is that a description of the power box/HVAC disconnect fuse box that is mounted on the
redwood siding wall that originally supplies the underground wire to the GFI yard outlet into which the septic pump
plugs.
It is metal and has the original plastic conduit protecting the underground "septic/GFI" wire into the ground a few
inches. However, a few years ago, we added a whirlpool tub GFI outlet, so the electrician used metal conduit that runs
near the ground, along this redwood wall and connects into this metal disconnect fuse box, in order to supply power
through the exterior bath wall. All wiring in the metal disconnect box grounds to the box. Although I have a generous
38" roof overhang/soffit at least 8'0" above grade in the location of the disconnect box, it does get wet .. so does its
metal conduit .... could this "wet connection" to grade be affecting a new 20a. GFI outlet under heavy pump loads, but
not under tiny light bulb loads?

A description of the outlet box is that is metal, 1980 on a plastic conduit stuck into the ground to protect the
underground wire.

A description of the pump is 4/10hp, new as of Jan '01.

Local electricians do not have an answer, so
Help!