Metal Wireways vs Cable Trays 1

[OhioAviator]

We are in the engineering phase of a new construction project where we have a motor control center (mcc) inside a building controlling motors outside the building on various platforms. The plan is to extend conduits up from the top of the mcc to either a metal wireway (enclosed trough) or a cable tray that will run horizontally to an exterior wall. From there the motor conductors will transition to individual conduits and go out to the various motors. We are aware that if we use open cable tray then the motor conductors from the mcc all the way to the motors must be tray cable or similar conductors labelled for cable tray use.

Question: Can we use a non-ventilated solid bottom cable tray with attachable covers, essentially making it a wiring trough, and then use single-conductor cable instead of using three-conductor tray cable?

Thanks!

 

[l3city]

Hi "OhioAviator"!!

NEC 392.3 Tells you the permitted uses of cable tray.
392.6(D) Permitted covers
392.8(E)Cable installation, single conductors (I guess this is not going to apply to your set-up, since you are using a "solid bottom")
392.10 Quantity of Single-Conductor Cables permitted in Cable tray
This section doesn't prohibit (at least I don't see it now)that having a solid bootom cable tray and covering it will be a problem (please, any one correct me) Regards, hope this helped!

P.D. NEC 392.9(C) Talks about solid bottom cable tray, but for multicondutor cables

Maybe I forgot to ask this question (I shall done it at the start of this) Why you need to cover this single conductor cables? Usually they are listed to be outdoors, weatherproof, insulated, listed for cable tray use, etc, etc. Please, check out NEC 392.3.

On the other hand, I think that at the end, building-up your set-up as you want it (covering a solid bottom cable tray will be almost the same as a "wireway"), and NEC 376,378 might apply instead of NEC 392.

 

[dpc]

Basically - no you can't do that. Tray requires "TC" cable. You could run wireway, but then you'd be subject to all of the restrictions imposed on wireway in the NEC.

 

[OhioAviator]

Thanks, I3city for your reply. Much appreciated.

dpc... Thank you also for your reply.
You said that I could run wireway but "...then you'd be subject to all of the restrictions imposed on wireway in the NEC." I think you are correct, but when I compare the two sections of the NEC dealing with metal wireways and cable trays (Articles 376 & 392, respectively) it appears that there's much less restrictions on conductor types when using wireways vs cable trays. It appears that as long as I don't violate the 20% total cross-sectional area rule (NEC 376.22), I can pretty much use whatever listed wire I want, including single conductor THHN. Is my interpretation correct?

The whole reason for this exercise is to try and make it easier for the electrical contractor to pull the motor circuits through the buried PVC conduits. It's been my experience that a group of single-conductor THHN's are much easier to pull through PVC conduit than are several tray cables.

If someone has a better approach to this problem I'd sure appreciate hearing your ideas.

Thanks for all the help!

 

[dpc]

You can use single conductor wiring in the wireway, but you must derate the ampacity if you have more than 30 current-carrying conductors and must limit fill to 20%.

But I don't think you can put in a covered solid-bottom tray and call it a wireway. Wireways are generally UL-listed I believe.

FWIW, THHN conductor is very easy to damage when pulled through conduit in singles, at least in my experience.

 

[OhioAviator]

Well, after much consternation and study, I guess we're gonna' abandon the metal wireway approach and just use cable tray together with tray cable. We'll just oversize the conduits to help make the tray cables easier to pull through RGS and PVC conduits.

Thanks for the help!

 

[dpc]

Just a word of caution about oversizing - you need to consider the jamming ratio as well as the fill when you have certain cable combination. Sometimes bigger can be worse than smaller.

Okonite has some good information on jam ratios and cable pulling in their Conductor Installation manual, which I'm sure you can get for free from one of their reps.

Good luck.

 

[OhioAviator]

Thanks dcp, You dredged up a painful memory of just that exact circumstance I got into about 15 years ago. And it was the jam ratio that made our job difficult.

Thank you very much for mentioning that 'cuz I completely forgot about that little pitfall. Much appreciated and I gave you a star for reminding me of that.

Again, thanks!