Multiwire branch circuit definition & implementation

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  #1  
Old 01-25-08, 09:16 AM
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Multiwire branch circuit definition & implementation

I am running a MWBC to an outbuilding. I want to qualify for Exception to 250.32, which allows NOT having a grounding electrode for the out-building provided it is a single branch circuit feeder (including MWBC). The supply is a 240v 3-conductor 10 awg, if it matters. There will be a cut-off switch at the outbuilding.

As for the actual circuitry within the outbuilding, I am wondering what the pros/cons are for running several 2-wire conductors from the cut-off switch (some on each hot wire --- i.e. some run from black feed, some from red feed) versus running 3-wire conductors to everything in the outbuilding.

It seems easier to me to run 2-wire from the switch as outlined above and divide the load that way. But, does that still constitute a MWBC from the perspective of the250.32 exception? (There is no fusing or circuit breakers in the outbuilding.) Is this acceptable? desirable? frowned upon?

If it is preferred to run 3-wire to everything, what is the usual procedure? I am thinking I would snake one or two 3-wire runs to every device and connect every other one to red or black... is that how it is done?

Thanks,
Dave
 
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Old 01-25-08, 09:33 AM
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I sure hope you are running four conductors to the building, with one being a ground and the other three being insulated.

The code requirement is for one circuit to the outbuilding. Once you reach the outbuilding you can do whatever you want with the one circuit (as long as you do it properly).

One you reach the outbuilding you can separate the multi-wire circuit into two runs. One run connects to the red, white and ground wires, the other connects to the black, white and ground wires.

The advantages of this method are as follows:

You can place a single GFCI on each run and have it protect the entire run.

You can connect the neutral wire directly to the device without having to pigtail when the neutral also leaves the box.

You can use 12-2 cable which reduces box fill.

Multi-wire circuits carry some risk. Wire something wrong or have a problem and you could get 240 volts across a 120 volt device, probably destroying it. Once you separate into two runs you reduce the risks.



The disadvantages of this method are as follows:

You cannot make changes in the future without rewiring if you want a particular device to be on the other half of the circuit.




Continuing the multi-wire run has it's advantages, as follows:

You have flexibility at each location and can change which half of the circuit is used without running new wire.

But it also has disadvantages:

You must pigtail the neutral wire to each device when the neutral continues on.

Each receptacle must itself be a GFCI receptacle, and all connections must be on the LINE terminals.

Box fill will be greater because of the extra conductor.

Multi-wire circuits carry some risk. Wire something wrong or have a problem and you could get 240 volts across a 120 volt device, probably destroying it.



If it were me, I would separate into two separate runs at the entrance to the building.
 
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Old 01-25-08, 11:24 AM
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Yes, it is 3-wire w/ ground. Racraft, thanks for your thorough answer--very helpful!

Can I use the disconnect switch box as a junction box or is there some restriction (beyond box size) as to how many splices can be made within it? I might want to split off two physical runs from one hot, let's say, one run from red and two runs from the black hot wire--just for convenience.
 
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Old 01-25-08, 11:30 AM
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If your box is large enough you can put all this in one box.
 
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Old 02-06-08, 11:09 AM
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Failed!

Sigh. OK, just had my inspection and the inspector said that I had to have a main lug panel w/ separate breakers for the two runs/circuits in the shed.

As you all have advised, it seems the inspector is incorrect. I politely referred to the multiwire branch circuit exception in NEC but didn't go so far as to get out my copy and show her--maybe I should have? But I feel like I'm dealing with the IRS so...

Anyway what do you all recommend? Is there an appeals process generally? (I live in Seattle.) Is it worth going thru or I am better off simply biting the bullet and replace the cutoff switch with the subpanel that the switch had replaced. Hmmmmm seems like I am going in circles... wonder why!?

Thanks for your advice on how to proceed.
 
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Old 02-06-08, 11:30 AM
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It has been a long time since I looked at the Seattle Electrical code but as I recall there were several additions and deletions from the NEC.

You must remember that the national codes have no power of law. That power comes about ONLY when the local governmental agency enacts the code into law within their jurisdictional boundary.

I live in Bothell and Bothell uses the Washington state electrical code which has some deviations from the national code. The interesting thing about Bothell is that it is a city that spans the line between King and Snohomish counties. I live in Snohomish county yet often I am held to regulation that is enforced in King county because the city council has made the King county regulations law in ALL of Bothell.

To answer your question of appeal...yes, you may appeal to the chief electrical inspector and if you don't his (or her) ruling then you may appeal to the Director (I can't remember if it is still the Building department or if the title has been changed) all the way to the Mayor.

But I will advise you that your chance of the original inspector being overruled is slim. It will be far easier to just install the panel. I think that you can purchase a panel for less than twenty bucks and the breakers are about four bucks a piece.
 
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Old 02-06-08, 01:02 PM
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According to the NEC, a multi-wire branch circuit can be considered both as two circuits and as one circuit, depending on the situation. Although I think it should be considered one circuit here, and most inspectors would agree, your inspector has the (almost) final say. I would try to speak with your inspector again and emphasize that the NEC allows you to treat a MWBC as one circuit, and thus would not require a subpanel. If that fails, ask your inspector what the appeals process is.

Here are the most compelling codes you can cite (from the 2005 NEC):

225.30 Number of Supplies
Where more than one building or other structure is on the same property and under single management, each additional building or other structure that is served by a branch circuit or feeder on the load side of the service disconnecting means shall be supplied by only one feeder or branch circuit unless permitted in 225.30(A) through (E). For the purpose of this section, a multiwire branch circuit shall be considered a single circuit.

225.33(B) Single-Pole Units Two or three single-pole switches or breakers capable of individual operation shall be permitted on multiwire circuits, one pole for each ungrounded conductor, as one multipole disconnect, provided they are equipped with handle ties or a master handle to disconnect all ungrounded conductors with no more than six operations of the hand.

250.32(A) Exception: A grounding electrode shall not be required where only a single branch circuit supplies the building or structure and the branch circuit includes an equipment grounding conductor for grounding the conductive non–current-carrying parts of equipment. For the purpose of this section, a multiwire branch circuit shall be considered as a single branch circuit.

(The handbook provides further explanation at this point, including showing a diagram of a multiwire circuit to an outbuilding, without a subpanel. Although the handbook explanations do not have the force of code, they should be a significant factor in determining the interpretation. Note that this exception is new to the 2005 code, so if your area is on the 2002 or earlier, the inspector may not be aware of it.)
 
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Old 02-06-08, 01:25 PM
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My $.02 worth: as someone who has done extensive work in my own residences, IMO it's well worth the few extra bucks to stay in the good graces of the building department.
 
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