Help with electrical problem

[Marshal Mo Hare, SASS #45984]

45 years ago I did some GFCI to Underground Feeder wiring, two circuits, both for swimming pools.  One of the circuits was long and after the pool and pump were removed from the circuit there were only two lights, one 150W and one 75W, the GFCI was in the panel. Sometimes it would trip. I did the math and the circuit was long enough that the leakage from the circuit was almost at the limit of the GFCI. Wires use current especially if they are underground.