Sharing a nuetral for 2 phases of 3 phase panel

[NorthSparky]

I'm looking for where in the NEC specifically it is permitted for 2 phases of a 3 phase panel to share a neutral, and where it says those multiwire branch circuit conductors have to be from separate phases.

I understand they have to be on a 2 pole breaker, or have the handles tied together, and the ungrounded conductors need to be grouped with their associated grounded conductor.

The question has come up because the phase differences in split phase and 3 phase panels. It makes sense on split phase, where they are 180 degrees out of phase that the sum of the neutral currents would always be less than the highest current on one of the conductors, but for a 3 phase panel the phases would only be 120 degrees out of phase. I've seen other posts where they run through the math to show even 120 degrees out of phase they wont exceed the highest current of the most heavily loaded conductor, but just wanted to read it in the NEC for myself that this was permitted. (i.e. to show to the boss)

Thank you for your time.

 

[jumper]

310.14(B)(5)(b) In a 3-wire circuit consisting of two phase conductors
and the neutral conductor of a 4-wire, 3-phase, wye connected
system, a common conductor carries approximately
the same current as the line-to-neutral load currents
of the other conductors and shall be counted when applying
the provisions of 310.15(B)(3)(a).

 

[NorthSparky]

Awesome

Awesome

Thank you very much for your time and prompt response!

So…You can share a neutral between 2 phases of a 3 phase panel BUT! You have to count the neutral as a conductor for the purposes of article 310.15(B)(3)(a), which says if you have more than 3 conductors in a raceway you must de-rate the ampacity of the conductors.

The table in that section says to de-rate by factor of .8 for 4-6 conductors in a raceway. That puts us at 16 amp capacity for 5 12 AWG conductors, which means a 20 amp breaker is just fine per 240.4(B) which allows you to use next breaker size up for loads <800 amps.

 

[jap]

Thank you very much for your time and prompt response!

So…You can share a neutral between 2 phases of a 3 phase panel BUT! You have to count the neutral as a conductor for the purposes of article 310.15(B)(3)(a), which says if you have more than 3 conductors in a raceway you must de-rate the ampacity of the conductors.

The table in that section says to de-rate by factor of .8 for 4-6 conductors in a raceway. That puts us at 16 amp capacity for 5 12 AWG conductors, which means a 20 amp breaker is just fine per 240.4(B) which allows you to use next breaker size up for loads <800 amps.

:?.


JAP>

 

[kwired]

Thank you very much for your time and prompt response!

So…You can share a neutral between 2 phases of a 3 phase panel BUT! You have to count the neutral as a conductor for the purposes of article 310.15(B)(3)(a), which says if you have more than 3 conductors in a raceway you must de-rate the ampacity of the conductors.

The table in that section says to de-rate by factor of .8 for 4-6 conductors in a raceway. That puts us at 16 amp capacity for 5 12 AWG conductors, which means a 20 amp breaker is just fine per 240.4(B) which allows you to use next breaker size up for loads <800 amps.

If you have more then 3 "current carrying conductors" in the raceway you must apply ampacity adjustments to those conductors.

What Jumper posted is where it is telling you how to determine when a neutral conductor is considered "current carrying" for this sort of application.

 

[Smart $]

...
The table in that section says to de-rate by factor of .8 for 4-6 conductors in a raceway. That puts us at 16 amp capacity for 5 12 AWG conductors, which means a 20 amp breaker is just fine per 240.4(B) which allows you to use next breaker size up for loads <800 amps.

Typical wiring in raceway is THHN/THWN-2, which is 90°C-rated. That means you can derate from the value in the 90°C column: 30A × 80% = 24A.

 

[NorthSparky]

Typical wiring in raceway is THHN/THWN-2, which is 90°C-rated. That means you can derate from the value in the 90°C column: 30A × 80% = 24A.

Thanks Smart Money. As far as breaker sizing, am I correct in thinking the terminations as well as the wire have to be rated for 90°C to use that column? Since the terminations on the loads (a fan and some lighting fixtures) are not marked i'm assuming they are 60° C.

 

[JFletcher]

"where it says those multiwire branch circuit conductors have to be from separate phases."


What tells you that they have to be from different phases is that you cannot parallel small conductors, 310.4. Even if you managed two #14 hots from different 15A breakers on the same leg and a #10 neutral to get your MWBC, you'd be violating handle tie rules, plus probably a few other NEC rules.

also, the definition of a MWBC precludes same-phase installs.

 

[winnie]

See Article 100 'Definitions' specifically 'Branch Circuit, Multiwire'.

However to add confusion look for 'Common Neutral' circuits in article 225, or 'Feeders with Common Neutral' in article 215. We used to have long arguments about using 'common neutral' setups for ordinary branch circuits, since such were not explicitly allowed nor prohibited. I believe such were explicitly prohibited in 2014 but can't dig that up now.

-Jon

 

[Smart $]

Thanks Smart Money. As far as breaker sizing, am I correct in thinking the terminations as well as the wire have to be rated for 90°C to use that column? Since the terminations on the loads (a fan and some lighting fixtures) are not marked i'm assuming they are 60° C.

It's a multi-part determination. You can use the 90°C value for adjustment and correction, but the lowest termination temperature limit puts a cap on circuit ampacity. If that fan or lighting fixture is not labeled/marked, the 60°C value caps the circuit ampacity at 20A.