Three-phase distribution is very common, because it saves money. It takes about 25 percent less copper to transmit the same amount of power…But it’s virtually impossible to perfectly balance a three-phase system and keep it in balance…Fortunately, a three-phase system in the worst case of unbalance is safe as long as the neutral is sized correctly. So let’s say Bozo the Electrician comes in the morning of a load-in where the service in the venue is a single-phase three-wire plus ground system. His dimmer rack is a three-phase four-wire plus ground system, and he doesn’t realize that he’s hooking up a three-wire system to a four-wire system. He connects phase A and B of the dimmer rack to L1 of the system power, phase C of the dimmer rack to L2 of the system power, and the neutral from the dimmer rack to the neutral of the system power. Will he get away with it? Yes…as long as you doubled the neutral feeder cable and you made sure that the dimmer rack complied with NEC Article 520.53 (O)(1), which says, “In portable switchboard equipment designed for use with 3-phase, 4-wire with ground supply, the supply neutral terminal, its associated busbar, or equivalent wiring, or both, shall have an ampacity equal to at least twice the ampacity of the largest ungrounded supply terminal.” You did that because when phase A and B in the dimmer rack are on full and phase C is at zero, then a neutral current would be twice the dimmer phase current.
—Richard Cadena, from Focus on Fundamentals, PLSN, Jan. 2009
