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,
[This message has been edited by jb (edited 06-26-2003).]
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
Re: Sharing a grounded conductor between two circuits.#85391 06/26/0306:10 PM06/26/0306:10 PM
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.
Re: Sharing a grounded conductor between two circuits.#85392 06/26/0306:15 PM06/26/0306:15 PM
When a grounded conductor is shared as in your example, the load on the conductor could be double of the rating for the conductor. This would cause excessive heat on the grounded conductor and in turn, higher current draw on the ungrounded conductors. I'm still stickin' with "(5)Heating effects under normal conditions of use and also under abnormal conditions likely to arise in service."
Re: Sharing a grounded conductor between two circuits.#85393 06/27/0306:31 AM06/27/0306:31 AM
The only clear reference to this practice that I can think of is 225.7 (B). This pertains to outdoor lighting equipment. Also possibly, 215.4, which pertains to feeders. Although there may be no explicit prohibition of this practice, it certainly runs contrary to conventional method. Also, as you point out, many other questions arise. Where do you draw the line? Could you run a #3 grounded conductor and have five 20 amp circuits tie into it?
[This message has been edited by Redsy (edited 06-27-2003).]
Re: Sharing a grounded conductor between two circuits.#85394 06/27/0310:10 AM06/27/0310:10 AM
I agree. When I was looking for code sections that would validate or prohibit this I ran across 225.7(B) but again, this is one of the sections that refers to a neutral. I may be wrong in my interpretation, but it seems to me that in this case the grounded conductor would not be a neutral because it does not carry the unbalanced current but rather the sum of the currents.
I think everyone would agree that when given the chance it would be much more practical to use a conventional 120/240V multiwire circuit but I can also envision some situations where something like this might happen. Although if done to certain standards this could be a safe installation, I'm concerned by the fact that there are no standards. Maybe I'm still reading too much into some parts of the code and not enough into others but this really has me frustrated