VAV's on multiwire branch circuits

[justagirl]

Hello,

I have been a lurker for some time now and this forum has been very insightful so just a thank you to everyone who takes their time to help others. Now its my turn to ask a question.

I am working on a project where the designer has single phase VAVs on 3 phase circuits and is providing separate ground and neutral for each VAV and using breakers with handle ties. I am thinking this falls under exception 2 in 210.4(C) but would like some clarification as I have never seen single phase motors on 3 phase circuits.

Thanks in advance!

 

[roger]

If each phase has it's own neutral you do not have a MWBC nor a three phase circuit, you have three individual circuits that just happen to be fed from a three pole breaker.

Welcome to the forums

Roger

 

[dkidd]

OP said handle tie, so it looks like single pole breakers.

 

[roger]

OP said handle tie, so it looks like single pole breakers.

I read right over that so I guess I should have stopped at "you have three individual circuits"

Roger

 

[cpinetree]

It probably is advisable to run a separate neutral to each regardless of handle ties.
Not sure why handle ties would be needed unless for some reason you need to shut off all the VAV's for a particular area when working on them.

If you don't run separate neutrals, be sure to check max neutral load, you may need to up size the neutral, if you choose to run a single neutral.

 

[roger]

It probably is advisable to run a separate neutral to each regardless of handle ties.

Assuming this is a three phase Wye and barring some outrageous additive harmonics, why would individual grounded conductors be an advantage verses using a MWBC?

Roger

 

[cpinetree]

Assuming this is a three phase Wye and barring some outrageous additive harmonics, why would individual grounded conductors be an advantage verses using a MWBC?

Roger

Exactly, we don't know why the engineer spec'd individual grounded conductors, so best practice to just do it. You should have already priced it in your job, why value engineer it now?

 

[roger]

Exactly, we don't know why the engineer spec'd individual grounded conductors, so best practice to just do it. You should have already priced it in your job, why value engineer it now?

The OP is an engineer, not a contractor wanting to VE the project, she's asking if there might be a reason for the designer showing it the way he / she did.

With that said, a MWBC is the most efficient circuit to use and I'm not just talking about initial materials.

Even though the illustration below is single phase it's a pretty good example for visualization, note the total losses in the two scenarios.

Roger

 

[cpinetree]

The OP is an engineer, not a contractor wanting to VE the project, she's asking if there might be a reason for the designer showing it the way he / she did.


Roger

Sorry didn't see any of that in the original post :weeping:

 

[roger]

Sorry didn't see any of that in the original post :weeping:

No need to apologize.

Roger

 

[justagirl]

If each phase has it's own neutral you do not have a MWBC nor a three phase circuit, you have three individual circuits that just happen to be fed from a three pole breaker.

Welcome to the forums

Roger

Thank you. I was mistaken there are no handle ties, it indeed is a 3 pole breaker.