Niche underground wiring standards

Jun 27, 2023
6
Kansas City
We purchased a house a year ago with an inground pool. Am slowly upgrading/fixing things as we go, and as a brewer by trade, most of this has been well within my wheel house. However we, and apparently the previous owner, have had issues with the niche light since day 7 or owning the house… yay!
In my troubleshooting I have deduced that the underground wiring is faulty and tripping the gfci and needs to be replaced.
For those that are curious of the steps taken they are as follows:
Replaced gcfi (still trips)
Disconnected light (still trips)
Disconnected all wiring post outlet (doesn’t trip)
Since this has been an intermittent issue that seems to involve moisture, post rain or sprinkler use, I am assuming the sheathing on the old wiring is cracked, and shunting voltage to ground and tripping the gcfi.
So here is my question, what wiring should be used from the niche junction box to the switch? Should it be 3 conductor that is rubber insulated like the light wiring is, or is individual normal conduit wiring sufficient. They had it wired up with individual strand and it’s sheathing visually doesn’t look to great of what I can see above ground.
 
Copper THHN/THWN wires in conduit should be used from the circuit breaker to a light switch to the pool light junction box.
 
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