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oc250r 04-22-2008 08:40 PM

Two Circuits-Shared common
Hello, I have a question regarding the possibility of common wire overload. I have had a large garage for several years that has metal conduit throughout. Two 20 amp -120 volt circuit breakers lead to one double metal box that included one circuit for one outlet, and the other circuit powers a lighting circuit. My concern is that only three wires came into the box from the main panel- One Red, One Black, and One common (the box is grounded). The two circuits had always shared the common wire without problem. I have recently removed the outlet and expanded that circuit with another lighting circuit of approximately 1300 watts assuming that the “hot” wire and separate circuit would easily meet that demand. Now I am questioning if the shared common between the two circuits could become a problem if both circuits are nearing capacity.

joed 04-22-2008 09:01 PM

The two hot must be on different legs of the incoming service. This way the neutral only carries the differnce in current not the sum. It's called a multiwire branch circuit.

Termite 04-22-2008 09:14 PM

What you have is called a multiwire branch circuit...Actually you have two of them. As you stated, they're separate circuits that share a neutral.

The shared neutral is fine as long as the two circuit breakers are not on the same phase. There's an A and a B phase in your house. Two breakers pulling power from the same phase essentially double the juice going through the neutral, which ain't good. With the breakers on separate phases, the neutral doesn't know that it is even being shared, and things work fine.

In most panels, breakers that are on top of one another are on adjacent phases. The only way to be sure is to look at the buss bar under the breakers and see which way the buss bars run. You'll have two...An A and a B.

Simple enough, avoid having the two phase wires (black and red) landing on breakers that are on the same phase. If they're both on A phase or both on B phase, your neutral is waaaay overloaded. If not, you're ok.

You're really heads-up for recognizing a potential problem.

Termite 04-22-2008 09:15 PM

JoeD snuck in and beat me to it while I was typing! :thumbsup:

oc250r 04-23-2008 06:06 PM

Thank You for your help,

The breakers are one on top of the other. I pulled them and they are on different bars :thumbup:

jrclen 04-23-2008 06:29 PM

On a multi wire circuit, properly connected to each polarity of your service, the most current the common or neutral wire will carry is the ampacity of one of the circuit breakers. If the other breaker is carrying current, the load on the neutral will actually decrease. For instance, if both circuit breakers are conducting 20 amps of current, the neutral will be conducting 0 amps. If one circuit is carrying 20 amps and the other is carrying 15 amps, the neutral will carry 5 amps. The neutral only carries the unbalanced load between the 2 circuits. It will never be more than the rating of one of the circuit breakers. But can be less.

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