I saw this on a lighting circuit. A 2 pole breaker, in a 3 phase panel, feeding 2 120v circuits sharing one neutral. Knew the neutral wouldn't be 0 and it wasn't, it was less than either hot though. Anything wrong with this practice?

In this case the neutral sees the geometric sum of the current in both phases. If only one phase of a 3 phase system were loaded the neutral would see the full phase current.

A simple rule anytime you run 1Ø off from 3Ø WYE system is that you have to have netural conductor the same size as phase conductor.

I don't excatally recall how I did that forumla it was a while seice I done that but corret me if I am wrong.,

( L1-L2 ) / N * 30°

I am not sure if that work right due the phase shift affect on it.

A true single phase MWBC will cancel netural current due 180° shift as long L1 - L2 load is the same the netrual current will read zero { not excactally but it will show near 0 on it }

Merci,Marc

Pas de problme,il marche n'est-ce pas?"(No problem, it works doesn't it?)

Think of it this way: assume you have a balanced 3 phase wye load and the currents are 10 amps at 0 degrees, 120 degrees, and 240 degrees. Add the 10 amps at 0 and 120 degrees and you get 10 amps at 60 degrees, which is 180 degrees from the 240 degree load and that adds to 0 amps in the neutral. So the highest load on the neutral can not be higher than any of the phases. Nothing wrong with a shared neutral circuit.

A neutral like this will have less current than if it was a single phase off the 3-phase panel, but typically more than if it was in a residential 120/240. It's normal and OK, and will never exceed 100% ampacity. Well, not in normal circuimstances, at least...

One abnormal circumstance is low pf, leading on one phase and lagging on the other.

I must say, that's a new one! I was more referring to 3-phase 6-step power supplies, but I can your PF problem being especially bad on a circuit with two 208V legs. There's an easy solution, though: swap the legs if that occurs, and the currents cancel, problem solved!

One abnormal circumstance is low pf, leading on one phase and lagging on the other.

I must say, that's a new one! I was more referring to 3-phase 6-step power supplies, but I can your PF problem being especially bad on a circuit with two 208V legs. There's an easy solution, though: swap the legs if that occurs, and the currents cancel, problem solved!

But if the pf is very low, you are only reducing the problem, not eliminating it. And this one can be bad in those extreme cases with 180 degree legs at 120/240V. Fortunately, such cases are unheard of in residential situations.

#182492 - 11/28/0809:30 PMRe: MWBC with 2 out of 3 phases
[Re: pdh]

In Europe larger companies are having all sorts of trouble with large numbers of switching mode power supplies on 3 phase supplies which can raise the neutral current above that of the largest phase current.