Just a theroy question! If I have 3 phase 120/208 and on A phase have 5 amps, phase B 12 amps, phase C 10 amps, how do I figure out the balance of the load on the neutral? I have three single pole 15 amp breakers for this or should I have a 3 pole 15?

It's been a few years since power engineering class, and it was never my forte, but if memory serves:

Design should be such that the breakers affect the least amount of equipment necessary. If the loads are all single-phase, it may be best to have each phase with its own breaker, though you will also want a three-phase master breaker on the line as well. If you have any three-phase load involved, you need to have a three-phase breaker for that part of the load. I can't emphasize enough that this may not be the right answer, so don't rely on it for anything important :)

I don't remember enough about load balancing to give you any help on that part.

This white paper (http://www.dataforth.com/catalog/pdf/an110.pdf) offers a good, brief, but comprehensive explanation of the calculations to your question.

The neutral will carry the unbalanced current of Phase 1,2 & 3, your example the current will be 12 amps RMS sinusoidal.


If the loads are 1 phase then 1 pole breakers are allowed, a 3 pole breaker will be required for 3 - phase circuits that are on the same device.

If the load is 3 phase then a 3 pole breaker is required.