Shared Neutrals

[klink]

We have always run three phases in a pipe and shared the neutral, I was told recently from another electrician that this practice is not accepted by the code. I am wandering if this is true and where would I find it in the code book?

 

[roger]

Re: Shared Neutrals

Klink,

In the code book look under article 100 Branch Cicuit Multiwire., then look at 210.4(A), 300.3(B), and 310.15(B)(4)(c).

This multiwire circuit is allowed in cable or raceways.

Roger

 

[david]

Re: Shared Neutrals

Kink;

what three phases are sharing the same neutral?

 

[spsnyder]

Re: Shared Neutrals

My understanding of it is that a neutral can be shared if the phase circuits are directly adjacent to the one another. In other words ckt 3,5,7 can share a neutral, but 3,5,9 cannot. Second example would require (1) neut. for 3,5 and (1) for 9.

 

[roger]

Re: Shared Neutrals

Spsnyder, as long as all three phases are involved the number sequence doesn't matter.

In your example you are correct 3 & 9 are both B phase.

If you were to use 1,15, and 41 you would be fine.

Roger

 

[ryan_618]

Re: Shared Neutrals

I'll go ahead and be the bad guy on this one. Where in the code does it say that the shared nuetral must be for different phases? Can't you share a nuetral for two (a) phases if the nuetral is sized sufficiently?

 

[roger]

Re: Shared Neutrals

Ryan,

Can't you share a nuetral for two (a) phases if the nuetral is sized sufficiently?

yes you can, but that's coming up in the sequel.

Roger

 

[klink]

Re: Shared Neutrals

I would like to thank all of you for your comments. To answer David, I was talking about a,b,c phase sharing a neutral, but if I understand right from the discussion, it is also ok to have two (a) phases on the same neutral if sized sufficiently

 

[ryan_618]

Re: Shared Neutrals

Now, I'm not saying that you should run out and put every similar phase on the same nuetral, I'm just saying that it can be done.

 

[Guest]

Re: Shared Neutrals

Sounds like a job for:
S U P E R N E U T R A L !!!

 

[batch]

Re: Shared Neutrals

How often do you actually put two (a) phase conductors on the same neutral?

My guess is almost never...

 

[hurk27]

Re: Shared Neutrals

You relize that when you share a neutral between two ungrounded circuits of the same phase it can not be called a multiwire circuit anymore as per the NEC artical 100.
and a matter of fact there is no definition of what this circuit would be called?

Branch Circuit, Multiwire. A branch circuit that consists of two or more ungrounded conductors that have a voltage between them , and a grounded conductor that has equal voltage between it and each ungrounded conductor of the circuit and that is connected to the neutral or grounded conductor of the system.

 

[Guest]

Re: Shared Neutrals

Here is a link to an old report on superneutral. Is it still true that a neutral can only be sized to 1.73 times the OCPD? This report is 10-years old. Does it still hold water?

Dispelling the Myth of the SUPER-NEUTRAL.

../Wayne C.

 

[hurk27]

Re: Shared Neutrals

That was the biggest sales hype Bull that I have ever read. The fact is in any true multi wire circuit the neutral only carries the unbalance current the max that will be applied to the neutral will only be the the maximum unbalance current which would be if one leg had no current on it and the other leg had all the current on it and if these circuits are fed by a 20 amp breaker then this would be the limit of the unbalanced current but with harmonics coming into play this would increase the neutral current but how much would depend upon the connected equipment. while the separate neutral for each hot would be the way to go to keep harmonics low, all the other tech-no build up hype. was a bunch of (well you know)
As far as a requirment for a neutral can only be sized to no more than 1.73 times the OCP I have never herd of anything like that and would be news to me too. and does not make any sense.

[ October 27, 2003, 01:56 AM: Message edited by: hurk27 ]

 

[iwire]

Re: Shared Neutrals

You can run as large a conductor as you want.

Could be for voltage drop or harmonics.

Some of the jobs we do call for panel feeders with double neutrals.

So if we use 3/0 for the ungrounded conductors the neutral will be two 3/0s.

I always wondered about this as the Neutral bar itself is not "double"

 

[iwire]

Re: Shared Neutrals

Originally posted by hurk27:
[QB] You realize that when you share a neutral between two ungrounded circuits of the same phase it can not be called a multiwire circuit anymore as per the NEC article 100.
and a matter of fact there is no definition of what this circuit would be called?

It does not fit the definition of Multiwire Branch Circuit but it does fit the definition of Branch Circuit.

I do not think there is any code article to stop you from sharing a neutral between conductors of the same phase.

Say two 12 awg ungrounded conductors sharing a 8 awg grounded conductor.

Unusual? Yes.

Practical? Doubt it.

 

[hurk27]

Re: Shared Neutrals

Bob I agree that it should fit the definition of a branch circuit. but if we look at the way the definition is worded. There is no (s) on the end of "device" as this would mean it be singular, meaning one overcurrent device. But if you go one more after outlet there is the (s) meaning more than one.


Branch Circuit. The circuit conductors between the final overcurrent device protecting the circuit and the outlet(s).

 

[iwire]

Re: Shared Neutrals

Originally posted by hurk27:
Branch Circuit. The circuit conductors between the final overcurrent device protecting the circuit and the outlet(s).

Wayne I am not planing on doing this I am just having some fun

If we take the missing "s" to literally we would not be able to use two fuses to protect a 240 volt single phase load or 3 fuses to protect a three phase load.

One fuse is one overcurrent device.

Bob

 

[steve66]

Re: Shared Neutrals

Quote:

You realize that when you share a neutral between two ungrounded circuits of the same phase it can not be called a multiwire circuit anymore as per the NEC article 100.
and a matter of fact there is no definition of what this circuit would be called?

Only ment to be half serious: If a neutral is shared by two ungrounded circuits of the same phase, isn't it now a feeder? OK, maybe the overcurrent protection doesn't work out the same, but other than that - its a feeder.

Bob: I assume you are aware that square D does make panels with 200% rated neutral buses.

Steve

 

[ryan_618]

Re: Shared Neutrals

Steve: a feeder goes from one OCPD to another OCPD. I don't see how this can be anything but a branch circuit. Not a multiwire branch circuit, just a branch circuit.