My question has to do with ampacity of feeder conductors in parallel.

Parallel feeds must follow 310.10(H), last time I checked.

310.10(H) states that it's only allowed if one follows 310.10(H)(2) through 310.10(H)(6)

310.10(H)(4) requires an ampacity adjustment per 310.15(B)(3)(a)

ASSUME NEUTRAL IS A CONDUCTOR

310.15(B)(3)(a) states that for four (4) conductors, the ampacity is reduced to 80%

I have someone trying to install an 800-amp parallel feed (800-amp CB) using 2 sets of 500MCM.

To me, 500MCM is good for 380-amps (assume 75-degree C rated terminations on equipment)

380+380=760

760 x 80% = 608 amps

Next size larger breaker permitted per 240.4(B) = 700-amps

SOOOOOOOOOOO basically,

If the load calculation is greater than 608-amps, they will need larger wire.

If the load is less than 608-amps, they can stay with the 500MCM provided that they use a 700-amp breaker.

Would you agree? Am I right? I think I'm right.

NEC 310.15.5. Neutrals don't count in 3 phase application. Next size up is allowed up to and including 800A. Paralled 500Cu for 800A is OK as long as long as the load calculation is under it's rating. Also the 90 degree table can be used in some situations for ambient temperature derates