You must install an underwater luminaire so the top of the lens is at least 18 in. below the normal water level of the pool, unless it's listed and identified for use at lesser depths [680.23(A)(5)]. Transformers used to supply underwater luminaires must be listed as a swimming pool and spa transformer — and be of the isolating-winding type with a grounded metal barrier between the primary and secondary windings [680.23(A)(2)]. Branch circuits that supply underwater luminaires greater than 15V must be GFCI protected [680.23(A)(3)].
In 680.2, the NEC defines three general categories of underwater luminaires. Specifics, such as the wiring methods allowed to the forming shell and bonding requirements, are included for each type as follows:
•Wet-niche [680.23(B)].
•Dry-niche [680.23(C)].
•No-niche [680.23(D)].