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Old 09-26-2011, 10:04 AM   #1
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Sharing a neutral


What is the effect of hooking up two of the same phases to the same neutral. I know your not Supposed to do it and the code has changed to each circuit needs it's own neutral I'm just wondering what the effect will be?

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Old 09-26-2011, 10:13 AM   #2
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Sharing a neutral


You can overload the neutral without tripping a breaker.
On 2 20 amp circuits, running at full steam, you could have 40 amp on the neutral.
The code has not changed to require each circuit to have it's own neutral.

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Old 09-26-2011, 10:18 AM   #3
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Sharing a neutral


With two hot conductors on opposite legs of the panel the neutral carries the difference of the current. If the conductors are on the same leg the neutral carries the additive current. For example, in a propely wired multiwire branch circuit, leg A has a 15 amp load and B has a 10 amp load. The neutral is carrying 15-10 or 5 amps. Now put both on the same leg and you now have 15+10 for a 25 amp load. This could overload the neutral and cause overheating of the conductor or fires.

However, since many circuits require AFCI protection, the use of multiwire branch circuits will be cut back due to the need for each circuit to have a non-shared neutral.
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Last edited by Jim Port; 09-26-2011 at 01:06 PM. Reason: due to code change
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Old 09-26-2011, 11:19 AM   #4
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Sharing a neutral


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Originally Posted by Jim Port
With two hot conductors on opposite legs of the panel the neutral carries the difference of the current. If the conductors are on the same leg the neutral carries the additive current. For example, in a propely wired multiwire branch circuit, leg A has a 15 amp load and B has a 10 amp load. The neutral is carrying 15-10 or 5 amps. Now put both on the same leg and you now have 15+10 for a 25 amp load. This could overload the neutral and cause overheating of the conductor or fires.

The NEC does not require each circuit to have a dedicated neutral. However, since many circuits require AFCI protection, the use of multiwire branch circuits will be cut back due to the need for each circuit to have a non-shared neutral.
Afci breakers can't share neutrals? Interesting
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Old 09-26-2011, 11:47 AM   #5
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Section 200.4 says 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 feeder conductors unless specifically permitted elsewhere i'n the code. Which means if you use a single pole breaker it needs it own neutral! If you use a two or three pole breaker for your branch circuit it can share a neutral

And I was saying In my question from earlier i'n a three phase panel if you hook 1 and 7 on the same neutral what could happen ?
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Old 09-26-2011, 11:53 AM   #6
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Sharing a neutral


You will overload the neutral, as already stated. 1 and 7 are on the same phase.
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Old 09-26-2011, 12:09 PM   #7
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Sharing a neutral


I know there on the same phase I'm wondering will it burn up a neutral and start a fire or just simply trip the breaker?
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Old 09-26-2011, 12:12 PM   #8
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Sharing a neutral


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I know there on the same phase I'm wondering will it burn up a neutral and start a fire or just simply trip the breaker?
An overloaded neutral will not trip a breaker, since the breaker is protecting the hot. An overloaded neutral is a strong fire risk.
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Old 09-26-2011, 12:12 PM   #9
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I know there on the same phase I'm wondering will it burn up a neutral and start a fire or just simply trip the breaker?
It won't trip the breaker because the line - each one - will have 20 amps (max) but the neutral will have to carry both 20 amp circuits which will be 40 amps....
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Old 09-26-2011, 12:34 PM   #10
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Sharing a neutral


For AFCI and GFCI units the neutral is part of the sensing circuitry. In the case of AFCI and GFCI breakers, the neutral serves only the loads that are served by the hot line(s) going through the same breaker unit. Double pole GFCI breakers are commonly used for 120/240 volt multiwire branch circuits, where both hot lines and the neutral all go into the breaker unit.

The neutral coming from the load side of a GFCI or AFCI unit (including a breaker) may not be tied to a neutral going back to the panel neutral bus some other way (say from a junction box containing more than one branch circuit) and may not serve loads whose hot feed did not come through the same GFCI or AFCI unit. This prevents most AFCI and GFCI units from being in shared neutral situations.
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Last edited by AllanJ; 09-26-2011 at 12:41 PM.
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Old 09-26-2011, 12:46 PM   #11
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Sharing a neutral


Quote:
Originally Posted by Ford214 View Post
Section 200.4 says 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 feeder conductors unless specifically permitted elsewhere i'n the code. Which means if you use a single pole breaker it needs it own neutral! If you use a two or three pole breaker for your branch circuit it can share a neutral

And I was saying In my question from earlier i'n a three phase panel if you hook 1 and 7 on the same neutral what could happen ?
You can use single pole breakers as long as you have a listed handle tie.

As stated above, 1 and 7 are on the same pole, so you can overload the neutral wire and this could lead to a fire.
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Old 09-26-2011, 01:06 PM   #12
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Section 200.4 says 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 feeder conductors unless specifically permitted elsewhere i'n the code. Which means if you use a single pole breaker it needs it own neutral! If you use a two or three pole breaker for your branch circuit it can share a neutral
Thank you for sharing. I just looked at my copy of the 2011 NEC and see the section you posted above. It was not in the 05 or earlier and I have not seen any discussion about this change.

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