Neutral sizing

[tadavidson]

We are doing a job which has 14 varible air volume units. each unit is 3ph 480v.
480 is for strip heat in each unit but only uses one leg of each phase (277) to one side of the strip . the other side of the strips are common to a "N" terminal. There is a transformer with contactors for something also needing the neutral. Can 1 neutral be pulled from main ocp and tapped to feed off each unit? This is commonly done with lighting isn't it. I know (i think) since this is not a three wire system technically this is not a neutral only a common.
The OCP is 30a and #10 wire is being pulled for the loads. How would it be sized?

 

[LarryFine]

If you're asking if three single-phase units can share a single (I'm gonna say it anyway) neutral, I can't think of why not. If you mean a single unit with three heaters in it, I'd say of course.

If you're asking if the entire group of 14 units can share a single (okay, I won't say it again) grounded conductor, I'd say no, definitely not. An imbalance could grossly overload any conductor small enough to make it feasible.

The only place this is specifically addressed refers to 'festoon lighting', which makes me envision the bare-bulb lighting we used to see at such places as miniature-golf and used car lots.

This has been discussed before, by the way. Imagine a single neutral running throughout your house, common to every circuit. It would work, but at what cost (not just monitarily.)

The OCP is 30a and #10 wire is being pulled for the loads. How would it be sized?

The closest you'd get to being balanced would be 5, 5, & 4. Therefore, your ampacity wouldf have to be (5 x 30) 150 amps. It could be done, but I wouldn't do it.



P.S. I bet this gets to be another long thread!

 

[websparky]

Hmm...

Be sure the "neutral" you are seeing is in fact a grounded conductor and not the "common" for the 480V resistance heaters. It may be the transformer is a 480V/24V and doesn't require a grounded conductor either.

Just a guess.........

 

[LarryFine]

Hey! What's the big idea being up later than I am?!

 

[websparky]

Hi Larry,

Ah, just one of those late night things!