sharing a neutral

[erfitz1208]

i am a 1st yr appr. workin w a j-man. we are pulling wire for 277v commercial lighting, my question is1) what is the rule about sharing the neutral. ive been told u can share a neutral for 2 or 3 circuits only if there different phases. ex a,b; a,b,c; a,c; . is this true? i understand balancing phase loads..(2) my other question is wouldnt this create a potential danger w neutral current flow on a circuit that has a cb in off position?

 

[augie47]

if I understand your question, you are correct (a), you can share a neutral as long as the ungrounded conductors are of different phases.
If you share on identical phases the neutral ends up with the sum of the phases.
as far as (b), yes, there can be a back-feed on the neutral and that is why the '08 code requires simutaneous disconnection of all circuits on a MWBC.

 

[infinity]

If one of the MWBC's were off there would be no dangerous current flow on that conductor. The current would flow, as normally, through the energized circuit(s) and the neutral. Is this what you're asking?

 

[erfitz1208]

thanks

thanks

thanks Gus. that answers my questions.