Sharing neutrals - Electrician Talk - Professional Electrical Contractors Forum

rossski · 06-26-2009, 07:34 PM

Hi guys I'm a third year apprentice. I was looking at a drawing that showed two circuits supplied by two circuit breakers from the one chassis and shared the neutrals before reaching the neutral bar for the chassis. If you turn off one breaker the neutral could still be live, I can't find a rule saying this is not ok. Is there a rule about this?
Thanks to anyone who can help in advance.

raider1 · 06-26-2009, 07:49 PM

What you describe is a multiwire branch circuit. In the NEC this is permissible, but due to the issues that you have raised the latest edition of the NEC requires that all the ungrounded conductors of the multiwire branch circuit be equipped with a means to simultaneously disconnect all the ungrounded conductors at the same time.

Here is what the NEC has to say in regards to multiwire branch circuits:

Quote:

210.4 Multiwire Branch Circuits.
(A) General. Branch circuits recognized by this article shall be permitted as multiwire circuits. A multiwire circuit shall be permitted to be considered as multiple circuits. All conductors of a multiwire branch circuit shall originate from the same panelboard or similar distribution equipment.
FPN: A 3-phase, 4-wire, wye-connected power system used to supply power to nonlinear loads may necessitate that the power system design allow for the possibility of high harmonic currents on the neutral conductor.
(B) Disconnecting Means. Each multiwire branch circuit shall be provided with a means that will simultaneously disconnect all ungrounded conductors at the point where the branch circuit originates.
(C) Line-to-Neutral Loads. Multiwire branch circuits shall supply only line-to-neutral loads.
Exception No. 1: A multiwire branch circuit that supplies only one utilization equipment.
Exception No. 2: Where all ungrounded conductors of the multiwire branch circuit are opened simultaneously by the branch-circuit overcurrent device.
FPN: See 300.13(B) for continuity of grounded conductor on multiwire circuits.
(D) Grouping. The ungrounded and grounded conductors of each multiwire branch circuit shall be grouped by wire ties or similar means in at least one location within the panelboard or other point of origination.
Exception: The requirement for grouping shall not apply if the circuit enters from a cable or raceway unique to the circuit that makes the grouping obvious.

Chris

rossski · 06-26-2009, 07:57 PM

Thanks Chris.

"NEC requires that all the ungrounded conductors of the multiwire branch circuit be equipped with a means to simultaneously disconnect all the ungrounded conductors at the same time".
Does this mean that the main switch on the chassis can be the isloator for these circuits or do the circuits themselves have to be mechanically or electrically interolcked?

raider1 · 06-26-2009, 08:00 PM

Quote:

Originally Posted by rossski

Thanks Chris.

"NEC requires that all the ungrounded conductors of the multiwire branch circuit be equipped with a means to simultaneously disconnect all the ungrounded conductors at the same time".
Does this mean that the main switch on the chassis can be the isloator for these circuits or do the circuits themselves have to be mechanically or electrically interolcked?

Yes, the breaker can be the disconnecting means by either being a multipole breaker or being equipped with an identifed handle tie.

Chris

rossski · 06-26-2009, 08:03 PM

Thanks again.

InPhase277 · 06-26-2009, 08:12 PM

Quote:

Originally Posted by rossski

Thanks again.

But hold up. This fellow is in Australia, so the NEC doesn't apply to him. Rossski, you need to ask someone about this at school or maybe your boss, because there are few, if any, people here that have knowledge of the Australian electrical regs.

rossski · 06-26-2009, 08:46 PM

I know these are different regulations / standards. I've asked a few people about this scenario and get different answers for it. I also haven't found an answer for it in the Australian standards so here I am.

InPhase277 · 06-26-2009, 08:53 PM

Quote:

Originally Posted by rossski

I know these are different regulations / standards. I've asked a few people about this scenario and get different answers for it. I also haven't found an answer for it in the Australian standards so here I am.

Well, the Australian system is similar to the British system, at least until they changed to that fruity European crap, so maybe a British sparky can answer. Post your question over at http://www.electriciansforums.co.uk

It is a forum very similar to this one, except with some pretty sharp limey electricians

Docara · 06-28-2009, 05:23 PM

hi Rosski

If your standards are the same (or close) to ours (British) then its an absolute no no to 'borrow' neutrals. The primary reasons is akin to what you stated especially as the neutral is now considered a 'live' conductor. I do seem to remember another reason that you must run L and N together to ensure stray emf is cancelled out within a structured wiring system. A requirement I could never work out as lighting circuits (to switches) never have a neutral. I presume it becomes more apparent over long cable runs.

Also CPS's (Circuit Protective Conductors) also must be included in the same cable route.

CPC is an Earth - the correct term in our wiring standards (but we still use earth when talkimng to each other!!!)

Please note that within an installation it is illegal to join Neutral and Earth together

Regards
Docara

Dennis Alwon · 06-28-2009, 05:45 PM

I believe this forum is for you guys down under. Check it out.

InPhase277 · 06-28-2009, 06:29 PM

Quote:

Originally Posted by Docara

A requirement I could never work out as lighting circuits (to switches) never have a neutral. I presume it becomes more apparent over long cable runs.

In a switch loop, the currents are flowing in opposite directions, canceling each other. Just as if a neutral was present.