Shared Neutrals

rjken1969

Member
Location
Princeton, NJ
What section in the code will give me direction on shared neutrals? I'm doing some work in an older building and they have as many as 11 20A/1P breakers on a single neutral conductor, on a 3PH panel. They are trying to aviod rewiring the entire building, but some of this work clearly needs to be redone.

Is there a section I can reference when stating my case.
 

roger

Moderator
Staff member
Location
Fl
Occupation
Electrician
For code cycles before 2011, article section 225.7 was the closest you will find, it specifically allowed it for outside lighting but, with this being the case and nothing else where prohibitting it it seemed it could be used for other applications as well.

Roger
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Electrical Contractor
I stumbled upon 215.4, I think I found what I'm looking for.
So is this a feeder or a branch circuit. Certainly you can feed a panel with one neutral however there are limits to branch circuits.

For the 2011 look at 200.4 but these installs were done before then as Roger was mentioning.
 

Smart $

Esteemed Member
Location
Ohio
For code cycles before 2011, article section 225.7 was the closest you will find, it specifically allowed it for outside lighting but, with this being the case and nothing else where prohibitting it it seemed it could be used for other applications as well.

Roger
I agree. Nothing specifically prohibited a common neutral for MWBC's prior to 2011 NEC.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
What section in the code will give me direction on shared neutrals? I'm doing some work in an older building and they have as many as 11 20A/1P breakers on a single neutral conductor, on a 3PH panel.
Can you explain how this didn't burn up over time? Was the single neutral oversized?
 

kwired

Electron manager
Location
NE Nebraska
Can you explain how this didn't burn up over time? Was the single neutral oversized?
Depending on the loads:

Loads were lucky enough to be reasonably balanced and no significant overheating occurred?

Circuits were lightly loaded, or maybe little demand most of the time.

If there would have been 12 circuits instead of 11 and all were identical load, the neutral current should be zero. Assuming they are all in demand simultaneously - which could possibly happen if this is lighting circuits.
 

GoldDigger

Moderator
Staff member
Depending on the loads:

Loads were lucky enough to be reasonably balanced and no significant overheating occurred?

Circuits were lightly loaded, or maybe little demand most of the time.

If there would have been 12 circuits instead of 11 and all were identical load, the neutral current should be zero. Assuming they are all in demand simultaneously - which could possibly happen if this is lighting circuits.
Until somebody turns on exactly the right (wrong) combination of 6 of the 12 lighting circuits.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
I guess that would depend on what type of lighting loads. Do you think that if they all were fluorescent lighting loads the neutral would have burnt up a long time due to triplen harmonics?
 

kwired

Electron manager
Location
NE Nebraska
Until somebody turns on exactly the right (wrong) combination of 6 of the 12 lighting circuits.
Maybe they were always all on at same time.

I guess that would depend on what type of lighting loads. Do you think that if they all were fluorescent lighting loads the neutral would have burnt up a long time due to triplen harmonics?
Would that be a problem with magnetic ballasts? Remember those?;) I can see that with electronic ballasts. We were not told how old this installation is or what the loads actually are.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Maybe they were always all on at same time.



Would that be a problem with magnetic ballasts? Remember those?;) I can see that with electronic ballasts. We were not told how old this installation is or what the loads actually are.
I know, the OP was lacking in information that's why I asked him to provide some detail. :)
 

rjken1969

Member
Location
Princeton, NJ
I have no idea what type of circuits these are, it's a rat's nest of an installation. It's probably the worst I've seen. Forty years and tens of contractors, most of which were not licensed.
 

kwired

Electron manager
Location
NE Nebraska
I have no idea what type of circuits these are, it's a rat's nest of an installation. It's probably the worst I've seen. Forty years and tens of contractors, most of which were not licensed.
You can't tell if they are branch circuits or feeders?

If they are feeders they will be supplying other overcurrent devices such as a panelboard or taps to fused disconnects or something similar that ends up being overcurrent device(s).

If they are branch circuits they will be supplying outlets - lighting, receptacles, other utilization equipment.
 
Top