MWBC lighting circuits at 277V

[Isaiah]

I just want to be sure on this - NEC 210.4 allows me to use a multi wire branch circuit with a common neutral for all three phases provided a 3 pole branch CB or handle tie breaks all 3 phases simultaneously. In this case it’s for street lighting pole mounted LEDS at 277V.


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[electrofelon]

All good!

 

[infinity]

In this case it’s for street lighting pole mounted LEDS at 277V.

For street lighting you can also use a properly sized single common neutral for all of the circuits.

 

[don_resqcapt19]

For street lighting you can also use a properly sized single common neutral for all of the circuits.

If you are talking about the rule that was in 225.7(B), that rule in not in the 2023.
Also, the term "common neutral" is not defined and a lot of people have no idea what it means. I tried to get a definition in the code as it did appear in Article 225 and 215 until the 2023 code.
My definition was "a neutral conductor used with two or more phase conductors where the phase conductors do not have potential between them" or something like that.

 

[Isaiah]

If you are talking about the rule that was in 225.7(B), that rule in not in the 2023.
Also, the term "common neutral" is not defined and a lot of people have no idea what it means. I tried to get a definition in the code as it did appear in Article 225 and 215 until the 2023 code.
My definition was "a neutral conductor used with two or more phase conductors where the phase conductors do not have potential between them" or something like that.

Would this now be calculated as a three phase circuit since there is a shared neutral involved?


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[Dsg319]

Would this now be calculated as a three phase circuit since there is a shared neutral involved?


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I would say yes for VD

 

[don_resqcapt19]

Would this now be calculated as a three phase circuit since there is a shared neutral involved?


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The common neutral is not the shared neutral used with a multiwire branch circuit. The code has never given any guidance on the sizing of a "common neutral". The common assumption is that the common neutral is sized by the sum of the OCPDs that protect the conductors of the same phase.
You could have a circuit with three 12s each for A, B and C and an a neutral sized at 6AWG, based on the code requirement that the neutral be sized for the maximum unbalance current.

 

[infinity]

The 2020 and earlier cycles had a decent "definition" right in Article 225 with the method required to calculate it. Just take the phase with the most current and that is the minimum size.

225.7(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.

 

[Isaiah]

The common neutral is not the shared neutral used with a multiwire branch circuit. The code has never given any guidance on the sizing of a "common neutral". The common assumption is that the common neutral is sized by the sum of the OCPDs that protect the conductors of the same phase.
You could have a circuit with three 12s each for A, B and C and an a neutral sized at 6AWG, based on the code requirement that the neutral be sized for the maximum unbalance current.

But why would the circuit be unbalanced if each circuit is carrying the same number of lamps with the same current?


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[electrofelon]

For street lighting you can also use a properly sized single common neutral for all of the circuits.

You can do that for branch circuits? For some reason I thought you can only do that with feeders.

 

[infinity]

You can do that for branch circuits? For some reason I thought you can only do that with feeders.

Outdoor lighting permits a common neutral. At least it did in the 2020.

 

[don_resqcapt19]

But why would the circuit be unbalanced if each circuit is carrying the same number of lamps with the same current?


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It might not be, but code requires the neutral to be sized for worst case...that would be with no load on two of the 3 phases, so the neutral would carry the same load as the remaining phase.

 

[pipe_bender]

Outdoor lighting permits a common neutral. At least it did in the 2020.

why was that removed from the code?

 

[infinity]

why was that removed from the code?

Maybe Don will know if it was completely removed or somehow incorporated into a different code section. I read somewhere that it was only removed from Article 225 because it was already contained in Article 210 so it is still permissible.

 

[don_resqcapt19]

Maybe Don will know if it was completely removed or somehow incorporated into a different code section. I read somewhere that it was only removed from Article 225 because it was already contained in Article 210 so it is still permissible.

The panel statement said "This section is redundant and unnecessary because these requirements already exist in Articles 210 and 220." However there is no permission in 210 for the use of a common neutral. Without a specific permission the use of a common neutral is prohibited by 200.4. The only specific provision remaining in the code is 215.4.

In the previous cycle a PI was submitted to delete 225.7(B) and was rejected with a panel statement that said: "The requirements in 225.7(B) provide direction where the permissive requirements of 215.4 are applied. The suggested revisions in 225.7(C) do not add clarity."