Need Quick Answer Cable Tray Fill???

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sonny1957

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I have an interesting question...Am working on a 2MVA 3 PH 480VAC XFMR secondary...the branch circuit is comprised of 15 - 1000MCM cables, 5 per Phase...layed out in open ventillated cabletray...the field people want to use 18" tray and get around the single layer with spacing requirement by bundling the 15 conductors into 5 bundles of 3 conductors each. 392.10 (A) says conductors can be bound but shall comprise each circuit group...am not sure what is meant by each circuit group???? At any rate need quick answer if possible..is this legal??

Thanks

Sonny Felts
 
A, B, and C phase would be bound together to form a group. According to my calculations the 18-inch wide tray does not comply.
 
Sonny,
If you use the triangular configuration in uncovered tray, and you have a spacing between bundles of 2.15 times a single cable OD, then you can use 5 sets of 600kcmil for this application, but this still wouldn't fit in an 18" tray.
Don
 
Sonny-

Yes it is legal but I think you may have the cart before the horse. When you talk about these different methods of utilizing your cable tray, you have a serious impact on the ampacity rating on your cable. There are a few other factors to consider in addition to the method you use.

From my experience, cable "bundling" is the most effiecient method to use. If you look at how it's laid out, each bundle is spaced 2 on bottom, 1 on top leaving a space for 2.15 outside diameter in between. In my applications, we also include a fully rated neutral. So if I took the 2 cables off the top, I could fit them essentially in between them. The difference is in the cable ampacity rating that you get. I will have to make some assumptions here that may be different for your situation.

Ambient temp = 40C
Transformer connections and/or MCC rated at least 75C
All the cables in the vented or ladder cable tray (uncovered) are for the transformer output and the same length.

392.11 lists 3 methods for cabling and how to determine the ampacity for each:

1. Single layer of cables, no space in between.
1000 MCM found in Table 310.17 (Free Air rating) = 935 A
935 A x .88 (temp derate) = 822.8 A per cable
822.8 A x 75% = 617.1 A per cable
617.1 A x 5 = 3085.5 A total.

2. Single layer of cables maintaining 1 cable space between.
1000 MCM found in Table 310.17 (Free Air rating) = 935 A
935 A x .88 (temp derate) = 822.8 A per cable
(Good news: no further derating required. Bad news: Your cable tray is 2X the size!)
822.8 A x 5 = 4114 A (I think this would be over-kill for your situation. And given the price of copper today. . .:confused: )

3. Bundling one cable of each phase together.
1000 MCM found in Table 310.20 (3 insulated cond'rs. on messenger) = 748 A
748 x 5 = 3740 A
This method best utilizes the space on the cable tray AND has the best ampacity. So you might be able to drop some of those cable and save $$$$

One last thing: If you do use that last method, you can calculate the minimum size of the cable tray = [Outside Diam. of conductor] x 2.15 x 2 x [number of runs]. AND THEN, bounce that off the Table 392.10(A). Some of the other in this forum might have a more effiecient and technically correct method of calculating the size but this will ensure that you have enough spacing in between the cables. I'd rather have 2 extra inchs that being 1 inch short.

Let me know what you decide.

Edit to note that two replies were sent since I started--I take too long!
 
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If the termination temperatures are 75 degrees, how can you use Table 310.17 and 310.18? Section 110.14(C) requires the use of Table 310.16.
 
RB1-

Good point. I would check to see what the transformer terminations are rated for before going down that path. Most likely they DO have a rating but I only gave my recommendation based on the information presented by the OP.

Good point non-the-less. Sonny, any additional info you can provide on the terminations?
 
RB1,
I never looked at that part of 110.14 before. It looks like there would never be a use for Table 310.17. I have always assumed that we could use the 75 degree column of 310.17 for this type of work.
Don
 
Cable Tray Reply...

Cable Tray Reply...

I really appreciate the help on this cable tray issue....the cable I am using is Okonite 112-24-2651 with an outside Dia. of 1.44"...our problem is the this is a fast track project and we are having trouble getting cable tray quick enough...24" tray is available but from calculations the same problem exists pertaining the cable spacing...with triangular bundles it still would require the tray to be at least 27" wide...unfortunately we are stuck with this cable because it has already been purchased..still looking at options though..

Sonny Felts
 
Cable Tray Reply...

Cable Tray Reply...

Transformer lugs....this is interesting...The lugs are not big enough to handle 5 1000 MCM cables each...the transformer is coming from a rebuild shop and they are supplying a termination panel and fabricating some Cu buss bars on stand off insulators to connect the feeders to...The transformer lugs are 75Deg. C but the Cu buss would not have a rating...am curious if the 75Deg. C rating would be required since we are terminating to the buss bars????


Sonny Felts
 
Sonny,
How long is the run? Have you looked at bus duct in place of the cable tray? If the run is short matching the length of the conductors becomes more critical.
Don
 
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