Stupid Question About Sharing Neutrals

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Bob Kraemer

Bob Kraemer

Senior Member
Location
Ohio
I never thought about this till today, or maybe I have & forgot.:confused:
Lets say you have a single or 3 phase service in a commericial application.
You have numerous conduit runs & MWBC's
CKT #1 is for EMRG lighting & #3 is for fixtures, they share a neutral in the same conduit. These ckts are phase A&B
Now in a different conduit you have CKT #1 for EMRG lighting & #7 is for outlets they share a neutral in the same but different conduit from the first example. These ckts are phase A&B
Are you allowed to do this? Sorry but I went brain dead today, fighting a sinus infection or summer cold :mad:
I think I can but not sure.
Sorry for the dumb question.
 
Please clarify one aspect of your question. Are you asking about having an emergency circuit and a non-emergency circuit sharing a neutral? If so, that answer is no.

But if both circuits are emergency (or both are not emergency), then you can do both of the things you suggest. However, an upcoming change in the 2008 NEC will essentially prohibit your second suggestion. It will say that, for a MWBC, you have to be able to turn off all ungrounded conductors with one throw. That is, you must use a multi-pole breaker or use handle ties. If the two circuits are not side-by-side, then you won’t be able to install a handle tie that turns off both circuits, without also turning off the unrelated circuits in between them.
 
charlie b said:
Please clarify one aspect of your question. Are you asking about having an emergency circuit and a non-emergency circuit sharing a neutral? If so, that answer is no.

But if both circuits are emergency (or both are not emergency), then you can do both of the things you suggest.
Click to expand...
Would that be the case for battery powered (self contained) emer. lights?

EDIT:
Like this light:

daymaker.gif
 
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charlie b said:
Please clarify one aspect of your question. Are you asking about having an emergency circuit and a non-emergency circuit sharing a neutral? If so, that answer is no.


Sorry I am absolutely brain dead tonight. I am sharing a neutral in a conduit for an EMRG ckt & a non EMRG ckt. This is a no no? under 2005 code? if so I just made a big mistake. No gen set, just utility power. Breaker lock for EMRG ckt.
Click to expand...
 
Bob Kraemer said:
I am sharing a neutral in a conduit for an EMRG ckt & a non EMRG ckt. This is a no no? under 2005 code? if so I just made a big mistake. No gen set, just utility power. Breaker lock for EMRG ckt.
Click to expand...

Do both circuits originate from the SAME panel?
 
Bob Kraemer said:
Yes they do.
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How is it that they are " EMRG ckt & a non EMRG ckt"?

An emer. ckt will originate from an emer. panel...

700.9(B)
(B) Wiring.
Wiring of two or more emergency circuits supplied from the same source shall be permitted in the same raceway, cable, box, or cabinet. Wiring from an emergency source or emergency source distribution overcurrent protection to emergency loads shall be kept entirely independent of all other wiring and equipment, unless otherwise permitted in (1) through (4):

........
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You do not have an "emergency source" ...you have a panel that powers an emergency light w/battery back-up...along with other loads in the occupancy.
 
celtic said:
How is it that they are " EMRG ckt & a non EMRG ckt"?

An emer. ckt will originate from an emer. panel...


You do not have an "emergency source" ...you have a panel that powers an emergency light w/battery back-up...along with other loads in the occupancy.
Click to expand...
Ooops on my part, no EMRG panel, All exit & emrg lights are battery back up.
So back to my question, am I allowed to share neutrals in this case?
Sorry folks, I am sick & should be in bed, but being self employed, I am up trying to solve my question/problems.
 
Bob Kraemer said:
So back to my question, am I allowed to share neutrals in this case?
Click to expand...

This circuit is just another circuit - matters not that the battery back-up emer. light is on that circuit.

...MWBC away.
 
Bob Kraemer said:
Now in a different conduit you have CKT #1 for EMRG lighting & #7 is for outlets they share a neutral in the same but different conduit from the first example. These ckts are phase A&B
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Problem! Ckts 1 and 7 are both A phase (as are 13, 19, etc.), so your neutral is in danger of being overloaded.

Instead, feed 1 an 3 on one neutral, and 1 and 5 on the other.

The other guys have already addressed mixing emergency and non-emergency circuits on a shared neutral.

Those issues aside, your question boils down to two things:

1. Can a single circuit supply wires in more than one conduit? Yes.

2. Can one circuit be split and each part share a different neutral? Yes.
 
LarryFine said:
Problem! Ckts 1 and 7 are both A phase (as are 13, 19, etc.), so your neutral is in danger of being overloaded.
Click to expand...

What if it's a single phase panel?
 
celtic said:
What if it's a single phase panel?
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Then I would say that 1, 5, 9, etc. are the same phase. Okay, I was accurate only on the 3-phase example.

However, the real question is whether one hot from each of two separate MWBC's can share a breaker.

The answer is yes. In fact, I did exactly that today.

I pigtailed two wires onto a breaker, each one heading out of the panel in a different conduit, each part of a MWBC.
 
If both neutrals come off the neutral bus I don't see why not... Or better put, I can't think of any code reason not to. If it were splices from a single (say #12) conductor - then no...
 
e57 said:
If it were splices from a single (say #12) conductor - then no...
Click to expand...
Right, because that portion of the neutral would be carrying two hots' worth of current from the same phase.
 
LarryFine said:
However, the real question is whether one hot from each of two separate MWBC's can share a breaker. The answer is yes. In fact, I did exactly that today.
Click to expand...
I agree that it is OK today. But you won't be able to tomorrow (or whenever thereafter that the 2008 NEC is adopted in your area). ;)
 
celtic said:
This circuit is just another circuit - matters not that the battery back-up emer. light is on that circuit.
Click to expand...

I agree. I think a lot of people get confused when trying to apply 700.9(B) in situations where battery backup emergency lights are used. 700.9(B) only applies to the wiring between the emergency source (the battery pack in this case) and the load. It doesn't apply to the wiring between the utility and the battery pack. I assume that's why you said what you said, but many try to apply 700.9(B) to the whole circuit.
 
Why do you have a separate circuit for emergency lights? As I remember, unless you have a generator, the emergency lights MUST be on the same circuit as the lights in the area that is served.

This is so that if there is a fault in the local lighting branch circuit, there will be emergency lighting for evac purposes.

A separate circuit for the emergency lights defeats that purpose.

(where is my code book...it was in there the last time I looked...?)
 
Is 700.12(F) what you're looking for?: "The branch circuit feeding the unit equipment shall be the same branch circuit as that serving the normal lighting in the area and connected ahead of any local switches."
 
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