cable trays

[rbalex]

Bob,
How do you comply with 392.9(A)(2) when the Cableway has a width of less than 6"? I guess we could extrapolate a bit. The volume permitted in square inches is ~90% of the tray width. The Cableway product is 2" wide so we should be able to put 1.8 square inches of cable in the tray. Southwire's 12-2NM is 0.410" wide and 0.179" high. A single cable would have a area of 0.073 square inches. That would permit us to put 24 12-2 cables in the tray. That would be 4 wide and six deep in the Cableway. I would consider that to be "installed without maintaining spacing".

I am not convinced that Article 392 was ever envisioned to be used for applications like this. It wasn't that many code cycles ago that the use of cable tray was limited to industrial applications. Has there ever been any testing with this number of NM cables installed in this manner?


I see that 334.80, specifically says that the ampacity of NM installed in cable trays is per 392.11, so you are correct.

Don,

I wouldn't attempt to comply with 392.9(A)(2) since the Cableway isn't a ladder or ventillated trough cable tray. For the specific Cableway application, I believe I've always directed the discussion to 392.9(C).

Spacing isn't an issue until 392.11. As it applies to NM in Cableway, I believe 392.11(A)(1) is appropriate, but it doesn't mention spacing; 392.11(A)(2) is irrelevant; and 392.11(A)(3), while it mentions spacing, if it were relevant, would permit using the free air rating of conductors in the first place.

 

[rbalex]

BTW the 2? width dimension in the distributer?s ad is wrong; Arlington?s website indicates the Cableway is closer to 3-3/8? wide. I admit that bothers me, but not too much. I can still extrapolate a proper fill.

 

[guschash]

Just looking at this cable tray with all the NM in it, told me it should be derated. This tray was fill to the top, how can it not have to derated. The cables on the bottom have no place for the heat to go. If table 310.15(B)(3)(a) is telling us to adjust if there are more than 3 current-carrying conductors in a raceway or cable. This always has me wonder when I see a 1/2 hole drill through a joist and 5 conductors stuffed through.

gus

 

[rbalex]

Just looking at this cable tray with all the NM in it, told me it should be derated. This tray was fill to the top, how can it not have to derated. The cables on the bottom have no place for the heat to go. If table 310.15(B)(3)(a) is telling us to adjust if there are more than 3 current-carrying conductors in a raceway or cable. This always has me wonder when I see a 1/2 hole drill through a joist and 5 conductors stuffed through.

gus

This is the historic problem with cable trays - the idea that they can be filled to the brim (and often over). Read carefully, the fill restrictions in 392.9 when power conductors are part of the mix is quite severe.

 

[don_resqcapt19]

Don,

I wouldn't attempt to comply with 392.9(A)(2) since the Cableway isn't a ladder or ventillated trough cable tray. For the specific Cableway application, I believe I've always directed the discussion to 392.9(C).

Spacing isn't an issue until 392.11. As it applies to NM in Cableway, I believe 392.11(A)(1) is appropriate, but it doesn't mention spacing; 392.11(A)(2) is irrelevant; and 392.11(A)(3), while it mentions spacing, if it were relevant, would permit using the free air rating of conductors in the first place.

Yes, you did direct us to 392.9(C). I used the incorrect code number but my calculations were based on Column 3 of Table 392.9 which is the correct table.
I can see where the code permits (thanks to your comments) that fill without derating, but I think the code is wrong. There is no way there would not be excessive heating in the Cableway product when it is filled with cables carrying current anywhere near their ampacities.

With the use in NM in commercial occupancies, I can see applications where the cables would be loaded at or near their rated ampacities for hours at a time. Has anyone done any testing to see what the temperature in the Cableway would be under those conditions? I would expect the same to be true with tray cable in industrial occupancies, but many of the loads would be motor loads and the cables will have an ampacity at least 25% greater than the load so that would act to limit the temperature increase.

 

[rbalex]

Bob,
How do you comply with 392.9(A)(2) when the Cableway has a width of less than 6"? I guess we could extrapolate a bit. The volume permitted in square inches is ~90% of the tray width. The Cableway product is 2" wide so we should be able to put 1.8 square inches of cable in the tray. Southwire's 12-2NM is 0.410" wide and 0.179" high. A single cable would have a area of 0.073 square inches. That would permit us to put 24 12-2 cables in the tray. That would be 4 wide and six deep in the Cableway. I would consider that to be "installed without maintaining spacing".
...

Chapter 9, Table 1, Note (9) alters the computed 12-2 cross sectional area to 0.132 sq in. (I knew it was there somewhere). By your calculation with a 2" tray, only 13, 12-2 would be permitted. With the actual 3.8" advertized by Arlington, and using Table 392.9, column 4 as the basis for the reduced tray width, I calculate 5.5-(6.0-3.8) = 3.3 sq in. as the maximum permissible fill. This would still be about 25 cables, but spread 9w x~3 deep. That may not yet be in your comfort zone. But realize we are talking about 90C conductors that are still limited to their 60C ampacities.

With regard to testing, I have no idea. I have some experience with how some cable tray applications were developed, but the problem here is we have THREE (3) CMPs (6, 7 & 8) to coordinate. It's hard enough to coordinate 2.

 

[guschash]

This is turning into every good thread, I'm learning sonething.

thanks,gus

 

[don_resqcapt19]

Chapter 9, Table 1, Note (9) alters the computed 12-2 cross sectional area to 0.132 sq in. (I knew it was there somewhere). By your calculation with a 2" tray, only 13, 12-2 would be permitted. With the actual 3.8" advertized by Arlington, and using Table 392.9, column 4 as the basis for the reduced tray width, I calculate 5.5-(6.0-3.8) = 3.3 sq in. as the maximum permissible fill. This would still be about 25 cables, but spread 9w x~3 deep. That may not yet be in your comfort zone. But realize we are talking about 90C conductors that are still limited to their 60C ampacities.

With regard to testing, I have no idea. I have some experience with how some cable tray applications were developed, but the problem here is we have THREE (3) CMPs (6, 7 & 8) to coordinate. It's hard enough to coordinate 2.

Cable tray is not a raceway and I don't see the notes to Chapter 9, Table 1 applying to this application. Note 9 specifically says that it is for "calculating percentage conduit fill area".
As far as the temperature, if a couple of inches of cables in a hole filled with caulk or insulation can be a safety problem, I still see the potential for issues here, maybe not in residential where the loads are usually no where near the ampacity of the conductors, but that may not be the case for commercial applications.

 

[rbalex]

Cable tray is not a raceway and I don't see the notes to Chapter 9, Table 1 applying to this application. Note 9 specifically says that it is for "calculating percentage conduit fill area".
As far as the temperature, if a couple of inches of cables in a hole filled with caulk or insulation can be a safety problem, I still see the potential for issues here, maybe not in residential where the loads are usually no where near the ampacity of the conductors, but that may not be the case for commercial applications.

I should have quit when I was ahead. You're right; it doesn't apply. But I bet I'd have no problem getting CMP8 to buy into a Proposal to make it apply. (All my best Proposal ideas this cycle have occurred after the deadline ) Filling a bundle with caulk greatly changes the heat transfer - much more than a random lay in a solid bottom tray with an open top would.

The Chapter 9, Table 1, formula for figuring cross sectional areas for eliptical cables has been around since '1978. The only reason I knew it existed was because it was "enforced" on a cable tray job - I just couldn't remember where the citation was.