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Old 09-10-2011, 08:05 PM   #16
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Two Circuits Sharing a Neutral


So what's the answer to the original question? Can two same-phase circuits share a neutral?

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Old 09-10-2011, 08:23 PM   #17
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Two Circuits Sharing a Neutral


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Originally Posted by uconduit View Post
So what's the answer to the original question? Can two same-phase circuits share a neutral?
If they do you run the risk of overloading the neutral. In this case the neutral current is additive, not the difference like on a correctly wired multiwire branch circuit.
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Answers based on the National Electrical Code. Local amendments may apply. Check with your local building officials.
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Old 09-10-2011, 08:38 PM   #18
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Two Circuits Sharing a Neutral


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Originally Posted by uconduit
So what's the answer to the original question? Can two same-phase circuits share a neutral?
Highly not recommended
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Old 09-10-2011, 08:52 PM   #19
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Two Circuits Sharing a Neutral


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Originally Posted by uconduit View Post
So what's the answer to the original question? Can two same-phase circuits share a neutral?
2011 NEC addresses this specifically.

200.4 Neutral Conductors. Neutral conductors shall not
be used for more than one branch circuit, for more than one
multiwire branch circuit, or for more than one set of ungrounded
ungrounded feeder conductors unless specifically permitted
elsewhere in this Code.

225.7(B) is one that allows this.

225.7 Lighting Equipment Installed Outdoors.
(A) General. For the supply of lighting equipment installed
outdoors, the branch circuits shall comply with Article
210 and 225.7(B) through (D).

(B) Common Neutral. The ampacity of the neutral conductor
shall not be less than the maximum net calculated load
current between the neutral conductor and all ungrounded
conductors connected to any one phase of the circuit.
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Old 09-10-2011, 09:38 PM   #20
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Two Circuits Sharing a Neutral


I'm the original poster. And I realized the answer was right in front of my face.

At the suggestion of rjniles I tested the voltage across the breakers. From 12 to 16 (the circuits sharing a neutral) I got what I think is 208 (it's one of those old needle multimeters, not a digital). From 14 to 16 I got 0. From 12 to 14 I got 208. From 10 to 12 I got 0. I moved up the line and over to the odd side and wound up with this pattern:

A A
A A
B B
B B
A A
A A
B B
B B

And then I realized it's right there on the door of the panel: (Forgive me if the attachment doesn't work right, it's a schematic that i didn't really understand until now)
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Two Circuits Sharing a Neutral-panellabel.jpg  
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Old 09-11-2011, 09:03 AM   #21
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Two Circuits Sharing a Neutral


If you measured 208 volts your building may have 3 phase where your panel received just two of the phases let's say A and B.

For the purpose of multiwire branch circuits, the A and B* phases of a 120/208 volt (wye; symmetric neutral) 3 phase system can share the neutral the same way as the A and B legs of a 120/240 volt single phase system, no special treatment needed.

* Nitpicking: Or the A and C phases, or the B and C phases, or all 3 phases.
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Stop wasting time re-adjusting the pattern. Have several lawn sprinklers, one for each pattern.

Last edited by AllanJ; 09-11-2011 at 09:09 AM.
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Old 09-11-2011, 06:01 PM   #22
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Two Circuits Sharing a Neutral


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I recently bought a home in Brooklyn, NY, and in the process of changing out a receptacle, I noticed that I had two 20 amp circuits sharing the same neutral.

It's my understanding that this is fine as long as the 2 circuits are on different phases. What I don't know is whether my circuits are on different phases. They are circuits #12 and #16, meaning that they are in the same column in the panel, with one breaker (#14) in between.

If this means they are on the same phase, am I OK leaving it as is? The circuits get a very light load. If I switched the 20 amp breakers to 15 amp, would that help minimize any risk of overloading the neutral?

Thanks in advance.
I am not exactly sure what code allows,
As it tends to vary from place to place.
Technically if the two circuits are different phases it will work.
provided the current capacity is correct for the cable used.

You can check to see if the breakers are on different phases
by carefully opening up the panel, and looking at the set up
on the power input side of the breakers.
But please be real carefull.
Do not stick your fingers anywhere near the wires !
Eyes only !
If you are not 100% confident, DONT DO IT !

I do not like the idea of sharing neutrals,
It can lead to problems.
And I suspect most electricians would feel similair.
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Old 09-12-2011, 09:40 PM   #23
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Two Circuits Sharing a Neutral


I have always done it right, because that's how I was taught. I just never really questioned it before.

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