Why not share neutrals.......... I don't get the 2-wire mentality either, but don't knock it.
As for the school of it, somewhere in the infamous "Ugly's" the is some refferance to the "Dangers of the multi-wire circuit". It must be those persons who don't twist wires before applying the wire-nut.
![[Linked Image]](https://www.electrical-contractor.net/ubb/smile.gif)
Which is the only real danger in 3 and 4-wire circuits.
Back to the "Super-size neutral", I worked for a guy way back that used them a lot... #8 for 4 20A single phase, and #6 for 6 20A 3 phase. Use a split bolt for tap at the junctions. He only did this on circuits that were 120+' long for lighting in warehouses and shops, where the loads would be stable and even, which I thought was not cool. So one day, after me questioning it for the umpteeth time he loaded up the 6 circuits about 15A each, amped out the neutral pretty low, at under 10A. Said the only reason to make it larger than the hots was in case it got fed by only one phase for any reason. It's not a pratcice that I favor, but it does work and seems as safe as anything else to me. The only reason I don't favor it is that if the loads were not well balanced, say one or more banks of lighting were off, it would raise the amperage on the neutral. Seeing that these were usually off a contactor, it wouldn't happen unless the contactor or breakers were failing, or, some electrician didn't know the circuit was like that, and modified it for some reason..... The reason I don't do it, who knows who'll be there long after I'm gone.
Anyway I found some commentary on it an it appears that these circuits need to have the neutral sized at least 140%.
Multiwire branch circuits may be derived from a 120/240-volt, single-phase; a 208Y/120-volt and 480Y/277-volt, 3-phase, 4-wire; or a 240/120-volt, 3-phase, 4-wire delta system. Section 210.11(B) requires multiwire branch circuits to be properly balanced. If two ungrounded conductors and a common neutral are used as a multiwire branch circuit supplied from a 208Y/120-volt, 3-phase, 4-wire system, the neutral carries the same current as the phase conductor with the highest current and, therefore, should be the same size. The neutral for a 2-phase, 3-wire or a 2-phase, 5-wire circuit must be sized to carry 140 percent of the ampere rating of the circuit as required by 220.22. See the commentary following 210.4(A), FPN, for further information on 3-phase, 4-wire system neutral conductors.
If loads are connected line to line (i.e., utilization equipment connected between 2 or 3 phases), 2-pole or 3-pole circuit breakers are required to disconnect all ungrounded conductors simultaneously. In testing 240-volt equipment, it is quite possible not to realize that the circuit is still energized with 120 volts if one pole of the overcurrent device is open. See 210.10 and 240.20(B) for further information on circuit breaker overcurrent protection of ungrounded conductors. Other precautions concerning device removal on multiwire branch circuits are found in the commentary following 300.13(B).
[This message has been edited by e57 (edited 03-12-2005).]
