If I'm reading NEC 430 Part III correctly, a 5 HP, 230 V, 3-phase motor with 12.8 A full-load current needs a 16 A circuit (125% of full-load current). So, 12 ga wire with a 20 A breaker should do. The 12 ga THHN wire has an ampacity of 30 A, derated for 7-9 conductors makes it 21 A, so that should be more than adequate, right?
I’m looking to run service for this dust collector. It quotes 12.2 FLA, but looking at a Leeson motors list, I found 12.0, 12.8, and 15 FLA for various 5 HP 3-phase motors.
But my service is 208 V 3-phase. Does that change anything?
Best Answer
When selecting conductors for motors in the US, you do NOT use the motor nameplate FLA, you use values from a table in the NEC, in this case table 430.250. The value in that table for 5HP @208V is 16.7A, so you use 125%, ergo 20.9A. The 12ga 30A rating you quote is for 90C, you can only use that value when de-rating, so assuming you are correct on the de-rate factor (I'm not checking your work), you are still fine.
The only thing you use the nameplate FLA for is in selecting the Overload Protection.