NEC 225.7(B) Lighting Equipment Installed Outdoors

[jkim780]

225.7(B) Common Neutral.
The ampacity of the neutral conductor shall not be less than the maximin net computed load current between the newtral and all ungrounded conductors connected to any one phase of the circuit.

What is the intent of this section and what does it mean? Following is my understanding on this section and please correct me if I am wrong.

120/208V, 3phase, 4wire system. If I have (4)-20 amp rated site lighting circuits, I have to have #3 wire to accomodate 80amp of unbalanced current. ( The maximum unbalanced current that can occur is 4x20amps, or 80amps).

Am I correct?

 

[charlie b]

Re: NEC 225.7(B) Lighting Equipment Installed Outdoors

If all four circuits are on the same phase, then yes, you are correct. If the four circuits are on phases A, A, B, and C, then you only need to accommodate a current of 40 amps, and a #8 would answer. The idea is that the lights powered by phases B and C might be turned off, and the lights powered by phase A might all be on. In that case, the neutral will see all 40 amps (i.e., 20 amps from each of the two circuits).