Sharing a grounded conductor between two circuits

[jimb]

Hello to all,

I'm looking for help in answering a question that has been posed elsewhere on the 'net. This was mainly asked concerning a single-phase residential install, but it could have applications elsewhere.

Would it be allowable (stictly from a code standpoint) for two circuits to share a common grounded conductor if they were fed from the same leg of the service and if the grounded conductor were sized properly for the maximum load? For example two 15A 120V circuits sharing a #10 copper grounded conductor.

My first thought was no but when I tried to find a specific code reference, I was unable to do so. Since there is no voltage between the two ungrounded conductors in this arrangement, it doesn't satisfy the definition of a multiwire circuit and in my mind the requirements for multiwire circuits couldn't be enforced. Likewise, since the grounded conductor would not be carrying the unbalanced current of two (or more) ungrounded conductors I don't think it could be called a neutral. Everything that I found that I thought might apply to this situation used one or both of those terms. With that in mind, I was unable to find anything that specifically prohibited or regulated this type of installation.

This would all just be a mental exercise, but when thinking about it I began to see some potential problems. For example, IF this is not specifically prohibited and IF it is not required to follow the rules for multiwire circuits (because it doesn't satisfy the definition of a multiwire circuit), then it would be possible for someone to supply split wired receptacles with this arrangement without complying with 210.4(B).

In reality, this doesn't seem like a practical way of running two circuits but I do see where someone could try to apply it. In my mind this should be either prohibited by code or if allowed, it should have to meet the requirements for multiwire circuits.

This may be in the code and I have just missed it. If so, I'll be embarrassed but relieved. Hopefully you guys can straighten me out on this.

Thanks in advance,

Jim

 

[websparky]

Re: Sharing a grounded conductor between two circuits

Hi jimb,

If one were to wire a 15A duplex recepticle with two 15A circuits that were on the same phase, one would be exceeding the ampere rating of the device.
Not to mention #1, 5, 7 and 8 below.

NEC 2002 110.3 Examination, Identification, Installation, and Use of Equipment.
(A) Examination. In judging equipment, considerations such as the following shall be evaluated:
(1)Suitability for installation and use in conformity with the provisions of this Code
FPN:Suitability of equipment use may be identified by a description marked on or provided with a product to identify the suitability of the product for a specific purpose, environment, or application. Suitability of equipment may be evidenced by listing or labeling.
(2)Mechanical strength and durability, including, for parts designed to enclose and protect other equipment, the adequacy of the protection thus provided
(3)Wire-bending and connection space
(4)Electrical insulation
(5)Heating effects under normal conditions of use and also under abnormal conditions likely to arise in service
(6)Arcing effects
(7)Classification by type, size, voltage, current capacity, and specific use
(8)Other factors that contribute to the practical safeguarding of persons using or likely to come in contact with the equipment.

Hope this helps.
Dave

 

[bennie]

Re: Sharing a grounded conductor between two circuits

Two or more circuits, on the same bus, sharing one neutral/ground, is done all time.

Edit: I should write "done every time".

[ June 26, 2003, 06:57 PM: Message edited by: bennie ]

 

[jimb]

Re: Sharing a grounded conductor between two circuits

Dave and Bennie,

Good point and thanks for the replies.

I mentioned the split-wired receptacles and 210.4(B) because that was the first hazard that came to mind. Having said that, would it be acceptable if this installation were used to supply two seperate devices? I'm not trying to be argumentative but rather find a definitive answer about two circuits sharing a grounded conductor.

Thanks again,

Jim

[ June 26, 2003, 07:16 PM: Message edited by: jimb ]

 

[websparky]

Re: Sharing a grounded conductor between two circuits

Hi bennie,

Two or more circuits, on the same bus, sharing one neutral/ground, is done all time.

Hummm........
I don't recall ever using the same neutral wire for more than one "same phase" conductor.

bennie, where is this practice "done all time"?

Thanks,
Dave

 

[bennie]

Re: Sharing a grounded conductor between two circuits

At the service, Dave.

 

[websparky]

Re: Sharing a grounded conductor between two circuits

bennie,

Clever!
The original subject was recepticle circuts.

I thought you were resonding to the subject.

Thanks for the clarification.

Dave

 

[bennie]

Re: Sharing a grounded conductor between two circuits

I was addressing the subject. The neutral conductor is sized according to the maximum possible calculated current.

A conductor with 30 amps ampacity can serve as a neutral for two 15 amp circuits on the same bus.

This would be unconvential, but I don't see anything technical, or code wise, against this.

I have seen this done at the Flir Corp. testing facility. There was a capacitance coupling issue.

[ June 26, 2003, 08:25 PM: Message edited by: bennie ]

 

[websparky]

Re: Sharing a grounded conductor between two circuits

bennie,

Thanks for the explanation.
I was thinking of the typical 15A circuit with 12AWG concuctors used for all wiring in the circuit.
I could see where a larger sized grounded conductor would safely carry the load of both ungrounded conductors.

I wonder if anyone here has used this wiring method? Where would this method be a practical alternitive to the standard wiring practices?

Anyone?

Thanks,
Dave

 

[hurk27]

Re: Sharing a grounded conductor between two circuits

I don't see any thing against code. I have seen this done in a differant way when installing a new service in a differant location and running just one neutral and one ground sized for the load, from the old panel to the new but running all the hots back to the new panel. all in conduit. and this was argued not to be code compliant but I forgot the reasone why.

 

[david]

Re: Sharing a grounded conductor between two circuits

Your question is dealing with branch circuits. The code allows multiwire branch circuits. The problem is that the code also defines a multiwire branch circuit. You cannot run a multiwire circuit unless it meets the definition.

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.

210.4 Multiwire Branch Circuits.
(A) General. Branch circuits recognized by this article shall be permitted as multiwire circuits. A multiwire branch circuit shall be permitted to be considered as multiple circuits. All conductors shall originate from the same panelboard.

 

[bennie]

Re: Sharing a grounded conductor between two circuits

There could be 42 circuits on one neutral.

[ June 27, 2003, 03:03 AM: Message edited by: bennie ]

 

[jxofaltrds]

Re: Sharing a grounded conductor between two circuits

Not from the same leg.
1. One would cause the other to trip a breaker.
2. How could you turn off both breakers at one time?
3. No voltage difference.
4. The neutral would be undersized.

Are you asking if you can use the neutral somewhere down line in the circuit? No.

Mike P.

 

[iwire]

Re: Sharing a grounded conductor between two circuits

Originally posted by jxofaltrds:

Not from the same leg.

1. One would cause the other to trip a breaker.

How would this happen?

2. How could you turn off both breakers at one time?

I do not understand this either.

3. No voltage difference.

4. The neutral would be undersized.

That would be true but the opening post asked if you could run one 10 AWG grounded conductor for two 15 Amp circuits on the same phase.

In this case the neutral would not be undersized.

I think Dave brings up a good point as far as doing this with a split outlet, the neutral termination on the device will not be rated 30 amps.

But what if I feed two separate outlets in the same box with two 15 amp circuits on the same phase using two 14 AWG for the "hots" and one 10 AWG for the "neutral"?

Is there a code article that would prohibit this?

Do we need a code article to prevent this?

What would be the electrical hazard in doing this?

[ June 27, 2003, 06:53 AM: Message edited by: iwire ]

 

[iwire]

Re: Sharing a grounded conductor between two circuits

Originally posted by websparky:
I wonder if anyone here has used this wiring method? Where would this method be a practical alternitive to the standard wiring practices?

Anyone?

Dave I ran into this once when I was sent to add an outlet in a commercial garage.

I thought I would tap an existing circuit in a the 3/4" EMT that ran the length of the garage with 1/2" EMT running down to outlets.

When I opened the 4" octagons that were used I found all 12 AWG insulated type TW "hots" and a 8 or 6 AWG BARE pulled straight through the boxes.

The 12 AWG TW neutrals that came in to the box via the 1/2" raceway where soldered to the bare neutral.

I closed the box and ran my own raceway!

The thing that puzzled me most was I thought soldering connections stopped before TW came along?

 

[roger s deas]

Re: Sharing a grounded conductor between two circuits

Common neutrals are specifically allowed for outside lighting, (I can't quote the article right now) and are not specifically prohibited anywhere.

Roger

 

[bennie]

Re: Sharing a grounded conductor between two circuits

Roger: I notice you changed your profile display. I too, like to identify myself, in case I become famous

When you think about short circuit impedance, a large neutral will offer less than many individual smaller ones.

As I wrote previously, the neutral at the service, and on any feeder to a panel, is common to many circuits. The neutral is not sized according to overcurrent devices, but to calculated 120 volt load.

Although somewhat unconvential, this procedure shoud not be forbidden.

 

[jimb]

Re: Sharing a grounded conductor between two circuits

Thank you for the replies,

I think most of us agree that if the grounded conductor is sized properly and the ratings of the devices is not exceeded, this would not create a situation that would damage the wire or devices. Also, there seems to be nothing in the design that would interfere with the performance of the OCPD's for the two (or more) circuits. The only real problem is that it is, as Bennie said, unconventional.

My problem is not with the operation of these circuits as I believe if done "correctly" they could be safe (if not practical). My problem is that I could find no standard of what is "correct". Maybe my imagination is too wild, but to me this qualifies as one of those things that "someone is bound to try".

I didn't mean for this post to be all about receptacles, but they are the easiest example that comes to mind to illustrate some potential problems with this. As Dave pointed out early on, my original example wouldn't be compliant because it would violate the listing of the device. But what if we split the #10 copper grounded conductor at the device, break BOTH sets of tabs, and feed the two halves of the duplex completely seperately? In this case, the loads on the terminals would be limited to 15A which is within the ratings of the receptacle. Although done "correctly" this would create the hazard potential that is addressed in 210.4(B).

Having said all of this, I can visualize some cases where doing this might be advantageous and not necessarily unsafe. Maybe this shouldn't be prohibited, but I'm wondering if it should at least be addressed and regulated by the code.

Jim

 

[jimb]

Re: Sharing a grounded conductor between two circuits

Bennie's comment about short circuit impedence made me think of something else. What about the potential ground fault conditions? Should the size of the EGC be affected by this?

Jim

 

[luke warmwater]

Re: Sharing a grounded conductor between two circuits

Bennie, I thought you were famous