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Hi, I'm Mike G and I'll be assisting you. From your description I suggest renewal of the underground wiring. Not being in conduit I'm not surprised it failed.
Romex isn't rated for other than interior use. Installed in conduit underground is a code violation.
Under a parking lot (vehicle traffic) it should be in GRC at 2 ft deep. If the cable has a paper filler, it's Romex. If the sheath is molded around all conductors, it's UF cable.
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