Newbie - Electrician wired two hots of 12/3 to same phase

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Old 09-29-08, 10:33 AM
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Newbie - Electrician wired two hots of 12/3 to same phase

Hi,
I am super new and, in truth, really hope to not have to do this myself. I am seeking to understand what is going on in my home, however, and hope you won't mind the question.

Scenario:
Just built a new house about a year and 4 months ago. In that time the basement has flooded 4 times and the laundry room went out twice at the same time.

I finally got an electrician out that did not wire the house and he found that (and I am just repeating here - please forgive me if I say something incorrectly) that the sump pump and the laundry room are on a 12/3 with a shared neutral (two breakers - one for the laundry room, one for the sump) but that they were both on the same phase. It was his belief that this is what damaged two sump pumps, "burned out" the neutral (the wiring was scorched and he removed it) and damaged the washer.

What I am looking for is:

1. Is the electrician correct in saying that this does not follow code?

2. Does anyone know the actual code reference prohibiting this?

3. Why is this bad? I am an engineer but not electric and I am finding that this is not an all intuitive to me.

Thank you very much,
AJ
 
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Old 09-29-08, 12:40 PM
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The electrician is quite correct. The two legs or "phases" in your panel are 180 degrees out of phase. Thus, if two hots on opposite legs share a neutral, the out-of-phase currents cancel and the neutral current cannot exceed the current on one of the hot wires, and can be zero if the two legs are equally loaded. If the hots are in-phase, the currents add, and the neutral can be badly overloaded, causing it to overheat and possibly to fail at a connection. Once the neutral has failed, loads on the two hots wind up being connected in series to 240 Volts, instead of in parallel to 120V. Google "multiwire branch circuit" and "open neutral" for more information than I can supply in this brief post. See NEC 210.4 and Article 100 for the definition of "Branch Circuit, Multiwire."
 
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Old 09-29-08, 12:56 PM
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Thank you so much! That clarifies everything he was saying to me and more. I will take a look at the code and links as well - I really appreciate it!

Thanks again,
AJ
 
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Old 09-29-08, 01:46 PM
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Originally Posted by AJ0424 View Post
Is the electrician correct in saying that this does not follow code?
The electrician is correct in his determination that the original installation was wrong.

Does anyone know the actual code reference prohibiting this?
There would be several codes that would be in play here.

Why is this bad?
When both legs of a multi-wire circuit are on the same leg, the current on the shared neutral is additive [ I(N) = I(1) + I(2) ]. Therefore, you have the potential of up to 40A on the neutral conductor (20A from each hot leg). The #12 wire is only rated for 20A.

When installed properly on opposite legs, the current on the neutral wire is the absolute value of the difference of the current in the hot wires [ I(N) = |I(1) - I(2)| ]. So instead of a maximum of 40A, the maximum current on the shared neutral wire is 20A (20A from either hot leg) which is within the specs of the #12 wire.
 
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