ECN Forum Forums Code Related Discussion NEC & other Code issues Neutrals tied together

Can anyone site a code violation for splicing two neutrals together in a junction box, each from a differant branch circuit. Is there a safety issue if this is done?

I think the safety issue is if you have circuit "A" and "B" sharing a neutral, if you disconnect power to "A", and a load is still present on "B", you could get current to ground if you provide a path for it on the joined neutral.

Except on multiwire branch circuits, where all hots are d/c'd by one breaker, I'd stick to the "one hot - one breaker" principle.

And you might weant to check with your AHJ - a lot of municipalities require separate neutrals for each branch circuit.

The wording in 310.4 seems to imply that only conductors size 1/0 and larger may be paralleled, although it doesn't actually say so directly.

Considering that 310.4, "Exception No. 2" specifically deals with applications where conducters smaller than 1/0 are allowed to be paralleled, I think it's safe to say that 310.4 applies exclusively to 1/0 and larger unless otherwise noted.

ElectricIan
Why do you want to splice the neutrals from independent branch circuits to begin with?


-John

As has been stated 310.4 along with the conductors being smaller than allowed for paralleling, they would also have to meet all the requirements of 310.4


Parallel conductors have to have the same characteristics as each other so each gets an equal share of the load.

If you have tied the neutrals from two or more branch circuits together you have the potential for the neutrals to have to carry two or more times the current rating of one branch circuit.

Example.

You have tied together the neutrals of two 14/2 NM, 15 amp branch circuits on the same phase together.

One of these cables takes a direct route to the panel, the other cable goes all over the house before getting back to the panel.

Now load both of the hots of these circuits to 15 amps and the two paralleled neutrals have to deal with 30 amps.

Because of the different lengths the current will not split equally and you may have 20 amps on the short 14 AWG and only 10 amps on the longer 14 AWG.

There is also 300.3(B) Conductors of the Same Circuit.

Bob

I don't want to do splice the neutrals together. I agree with all that this is in general a bad thing to do. I just counldn't find it in the code book and am just asking if anyone has come across a ruling against it specifically stated in the code book.

Thanks to all for the feedback on this question, especially Bob for such an excellent and thorough explanation along with the code reference.