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11-17-2008, 11:37 AM   #1
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## multiwire branch circuit question

I'm installing an automatic transfer switch for a standby generator. I'm not an electrician, just a DIYer, but I have completely required a bathroom and such. As part of this transfer switch project, I've moved selected circuits from the main panel to the transfer switch panel (these are the circuits that will be powered by the generator). A few of these circuits appear to be "multiwire branch circuits", that is, there are black and red hot wires with a shared white (neutral) wire. I have 2 questions:

1) Every authoritative thing I've read says that the potential between the circuits that share a neutral must be 240v (that is, they are connected to different phases). This is the way they were wired in the main panel, and this is the way I've done it in the transfer switch panel. However, an electrician (who I actually had considerable faith in), insisted that it must be wired so that the potential across the circuits is 0v. I'm wondering if there is any possible scenario where this would be the correct answer.

2) In one case, one half of the multiwire branch circuits stays in the main panel, while the other goes in the transfer switch panel. Is this acceptable? If so, does it matter whether the neutral is connected to the neutral bar in the main panel or in the transfer switch panel?

Thanks.

11-17-2008, 11:42 AM   #2
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Each leg of the MWBC should be on opposite phases to avoid overloading the neutral.

 11-17-2008, 11:49 AM #3 Member   Join Date: Feb 2008 Location: Atlanta, Ga/Hamilton, Al Posts: 2,487 Rewards Points: 2,350 You are correct that the potential across the MWBC must be 240 V. If it is 0 V, then the neutral will carry the combined current in each leg, a recipe for danger. As to the second part, whether the neutral can be in one panel and one hot be in another, the answer is no. If you have to power one half of a MWBC from the transfer switch, then you must power both halves from the transfer switch.

11-17-2008, 11:56 AM   #4
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Quote:
 Originally Posted by Jeremy Hillary Boob, PhD I'm installing an automatic transfer switch for a standby generator. I'm not an electrician, just a DIYer, but I have completely required a bathroom and such. As part of this transfer switch project, I've moved selected circuits from the main panel to the transfer switch panel (these are the circuits that will be powered by the generator). A few of these circuits appear to be "multiwire branch circuits", that is, there are black and red hot wires with a shared white (neutral) wire. I have 2 questions: 1) Every authoritative thing I've read says that the potential between the circuits that share a neutral must be 240v (that is, they are connected to different phases). This is the way they were wired in the main panel, and this is the way I've done it in the transfer switch panel. However, an electrician (who I actually had considerable faith in), insisted that it must be wired so that the potential across the circuits is 0v. I'm wondering if there is any possible scenario where this would be the correct answer. 2) In one case, one half of the multiwire branch circuits stays in the main panel, while the other goes in the transfer switch panel. Is this acceptable? If so, does it matter whether the neutral is connected to the neutral bar in the main panel or in the transfer switch panel? Thanks.
If they are on the same leg of the panel, and you put a meter across the 2 hot leads, it will not read anything, as it has no path. It is like touching the same hot wire with a meter - no reading. THIS IS WRONG for a MWBC. I just recently wired my fist MWBC's but I am positive that they must be on seperate legs of the panel and the meter needs to read 240 if you test on both hot wires.

2008 code will require double pole breakers to ensure the power is from seperate legs of the panel.

Jamie

 11-17-2008, 02:04 PM #5 Member   Join Date: Jul 2008 Location: Iowa Posts: 1,543 Rewards Points: 1,000 240 across both hots, 120 between each hot and neutral.
 11-17-2008, 03:00 PM #6 Member   Join Date: Jul 2008 Location: Canada (s/w ON.) Posts: 2,294 Rewards Points: 1,000 Is the generator one with a 240 V/2 pole center tapped neutral output? If so, there is no problem with MWBC wiring. But, if the generator is 120V/1 pole, one of the poles of the MWBC should be dropped to avoid neutral over-loading!
11-18-2008, 01:25 PM   #7
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Quote:
 Originally Posted by InPhase277 You are correct that the potential across the MWBC must be 240 V. If it is 0 V, then the neutral will carry the combined current in each leg, a recipe for danger. As to the second part, whether the neutral can be in one panel and one hot be in another, the answer is no. If you have to power one half of a MWBC from the transfer switch, then you must power both halves from the transfer switch.
Thanks for all of the responses.

I'm not clear as to why both halves of the MWBC must be powered from the transfer switch. Can you explain the issue here? Thanks.

11-18-2008, 02:24 PM   #8
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Quote:
 Originally Posted by Jeremy Hillary Boob, PhD Thanks for all of the responses. I'm not clear as to why both halves of the MWBC must be powered from the transfer switch. Can you explain the issue here? Thanks.
They must have power from different legs of the panel (or generator) otherwise the shared neutral has no protection from overcurrent. You can MELT your neutral and start a fire, if you don't have the power fed from seperate phases.

If you have half fed from the generator and half from the panel and service is on from the power co, then your feeding that neutral with power from the generator and your feeding the neutral with power from the power co.

If you hook up both legs of a MWBC to the generator, then you must make sure that the generator is putting put power on 2 different phases. 120 each and 240 between hots.
Jamie

Last edited by jamiedolan; 11-18-2008 at 03:05 PM. Reason: clarification

 11-18-2008, 02:40 PM #9 Member     Join Date: Mar 2005 Location: Welland, Ontario Posts: 13,275 Rewards Points: 13,346 Blog Entries: 11 Power comes back on and you have power from two different sources feeding through the neutral. They are not in phase and who knows what would happen.
11-18-2008, 03:16 PM   #10
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Quote:
 Originally Posted by joed Power comes back on and you have power from two different sources feeding through the neutral. They are not in phase and who knows what would happen.
Yes, this makes sense. Thanks.

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