Super Neutral - ECN Electrical Forums

We were working at a factory installing 120V circuits for receptacles. While we were there we noticed that one of thier ex-plant electricians had done the same in the past only he used (1) #8 Thhn conductor as a neutral for (4) 120V circuits from a 120/240V single phase panel. There were two circuits on each phase. I know that this is not right but I can't think of any specific code section that this violates, other than the JB is overfilled. So, can anyone tell me which section(s) this violates?

Brian

No code has been violated if he has 2ea 20 amp circuits on each leg.

Brian, I agree with Electure, no code has been violated, and it is specifically allowed in 225.7(B)

Roger

Section 225.7(B) Applies to "Lighting Equipment Installed Outdoors". Is there a similar section regarding 120V receptacles?

There was much talk a couple years ago regarding computer and electronic loads. The talk that I heard indicated using a larger neutral. I have not heard much about it lately, but it is possible that is what the ex-electrician had in mind when installing the #8?

I also agree there is no violation and no safety hazard.

Originally Posted by master66

Section 225.7(B) Applies to "Lighting Equipment Installed Outdoors". Is there a similar section regarding 120V receptacles?

The real question is where does the code prohibit what was done?

It does not.

I agree that there might not be anything specifically prohibiting the practice but I would think that if it were allowed that there should be some method of calculating the size of the neutral conductor as well as some rule pertaining to handle ties on breakers. I believe there is a change regarding handle ties in the 2008 code. (210.4(B))
What is the definition of a "Multiwire Branch Circuit"?

Brian

Originally Posted by master66

there should be some method of calculating the size of the neutral conductor Brian

The 12 wire should be adequate for the load

(cheap shot I know)

I think everyone is just site engineering these as they go along but when I've done it we used a #10 with 3 #12 hot.

But it may all be moot anyway since paired MC is being used in most jobs now anyway.

Right from the panel no conduit on the job at all. awful.

CMP 2 does not agree that the NEC would permit the use of this common (super) neutral. text shown in italics below was to be deleted in my proposal

Quote

2-280 Log #2224 NEC-P02 Final Action: Reject
(215.4)
______________________________________________________________
Submitter: Donald A. Ganiere, Ottawa, IL
Recommendation: Revise as follows:
215.4 Feeders with Common Neutral.
(A) Feeders with common Neutral Two or three sets of 3-wire feeders or two
sets of 4-wire or 5-wire feeders shall be permitted to utilize a common neutral.
(B) In Metal Raceway or Enclosure. Where installed in a metal raceway or
other metal enclosure, all conductors of all feeders using a common neutral
shall be enclosed within the same raceway or other enclosure as required in
300.20.
Substantiation: There is no code violation in using a common neutral so the
specific provision to use one is not required.
Panel Meeting Action: Reject
Panel Statement: The panel disagrees with the submitter’s substantiation.
Section 215.4 (A) is the requirement which limits the number of feeders that
may share a common neutral.
Number Eligible to Vote: 12
Ballot Results: Affirmative: 12

Originally Posted by resqcapt19

CMP 2 does not agree that the NEC would permit the use of this common (super) neutral.

I guess if the CMPs handed out red tags they would also have to provide a code section that this installation violates.

215.4(A) limits super neutrals to 3 sets of 3-wire feeders, or 2 sets of 4-wire or 5-wire feeders. 4x 120V circuits would be 2 sets and allowable, would it not?

The size of the neutral would be determined by the maximum current. If this circuit were balanced- EG, 2x one phase and 2x the other, the neutral must be rated for 40A and #8 THHN is acceptable... #10 is limited to 30A by that pesky 240.4(D). If all 4 of those were off 1-phase; well, #1, 215.4(A) would not allow it, and also, it would have to be rated for 80A, and would require #4 THHN.

215.4(A) is for feeders, not branch circuits. I don't think that there is a similar exception for branch circuits.

Bob, does this bring any memories?

All the breakers are 20 Amp, with the exception of the main.

As you posted it:

From the 2002 NEC Handbook

[Linked Image]

and as I modified it:

[Linked Image]

Does anybody have any objection to this?

This subject has come up every couple of years since ECN's inception.

" I don't think that there is a similar exception for branch circuits."
Exception to what? The practice is not prohibited in the first place. Why would it need to be excepted?
I find it to be poor practice, but when I think about it, that's only because someone else trained me that way.

Originally Posted by Gregtaylor

Exception to what? The practice is not prohibited in the first place. Why would it need to be excepted?

I agree 100%, it's not prohibited to start with.

Quote

I find it to be poor practice, but when I think about it, that's only because someone else trained me that way.

I like that open minded point of view.

This subject has come up before and I could not figure out a way that this could save me any money or time, terminations start to get expensive and boxes will need to be bigger.

Originally Posted by electure

Bob, does this bring any memories? :

Definitely, I still post it from time to time.

i believe they call this a multiwire branch ciruit. 210.4 the biggest problem i see with that setup is that it's awfully risky. if that super nuetral ever got cut or unhooked there could end up being 240v on all 4 circuits. a lot of equipment would get fried.

Originally Posted by ALEXT

i believe they call this a multiwire branch ciruit. 210.4 the biggest problem i see with that setup is that it's awfully risky. if that super nuetral ever got cut or unhooked there could end up being 240v on all 4 circuits. a lot of equipment would get fried.

Alex, in reality there could not be 240 volts on any one of the circuits if the neutral were lost, the voltage on each load would be the determined by it's resistance and if they were all equal nothing would happen and the circuits would keep working like nothing was missing.

Below is an illustration of a multiwire circuit with a broke neutral, using the formula I x R = E calculate what the voltage dropped across each load would be, you will see that neither load will see 240 volts.

[Linked Image]

Roger

Roger, that is true if the loads are equal. but chances are with four circuits the loads are not going to be the same. say you have a 1000w appliance on one leg and a 100w on another leg. if the nuetral is lost that 1000w is going have to low a voltage and the 100w is going to have to high a voltage. your right, neither would see 240 volts, but neither would have the correct voltage either.

Originally Posted by ALEXT

your right, neither would see 240 volts, but neither would have the correct voltage either.

In the real world the loads are rarely equal, but they are often close enough that damaging over voltage does not happen.

My own homes service lost the neutral at the utility, I put my recording meter on one leg of the service and had lows of 80 volts and highs of 140 volts, I did not lose any appliances or lamps.

The only way you can actually get a 240 volts on a 120 volt circuit is if the other side of the circuit has a direct short with no resitance.....a very unlikely scenario.

By the way....welcome to the forum.

I'm with iwire on this. Todays price on #8 is $294/M and #12 is $113/M. I'd have to use a deep box ( at least)and the termination or splice would be a pain. Where would I save?