multiconductor cable in cable tray

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ThomasZimmerman

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I have a question with regard to the 2005 edition of NFPA70.

Specifically 392.11- Ampacity of Cables, Rated 2000 Volts or Less, in Cable Trays.

Application: Type MC, multiconductor cable with not more than three current carrying conductors, installed in accordance with 392.9, in open ladder type tray.

Both 330.80(A) and 310.15(B)(2)(a) exception 2 refer to 392.11.

392.11(A)(1) applies the derating factors of 310.15(B)(2)(a) only to cables with more than three current carrying conductors.

392.11(A)(3) limits the ampacity to comply with 310.15(C) when spacing is maintained.

Question: Will derating factors apply to Type MC, multiconductor cable with not more than three current carrying conductors without maintained space in lengths exceeding 24 in.?

The wording, as I read it, does not require derating, though I suspect this is not the intent.

Your help will be much appreciated.
 
Re: multiconductor cable in cable tray

Is this a no-brainer? Am I missing something?

I refuse to believe the NEC will allow an unlimited no. of cables(up to fill percentage of course)without spacing, possibly in multiple layers, without derating.
 
Re: multiconductor cable in cable tray

I'm looking at the 2002 NEC handbook, and they show an illustration with multiconductor cables in a cable tray (not more than 3 conductors per cable). Their illustration shows a single layer with one cable diameter between each cable. I would think that would allow the heat to dissipate so no derating would be required.

Are you sure you can put these cables in multiple layers? (It looks like control and signal conductors are treated different than power and lighting conductors).


Steve
 
Re: multiconductor cable in cable tray

I have seen the illustration in the handbook as well.
Yes, I agree that space would allow for heat dissipation but I can't seem to find the requirement in the text.
This question applies to power circuits in this instance.
 
Re: multiconductor cable in cable tray

Originally posted by ThomasZimmerman:
...I refuse to believe the NEC will allow an unlimited no. of cables(up to fill percentage of course)without spacing, possibly in multiple layers, without derating.
Click to expand...
It won't, but it will allow an unlimited no. of 3/c or less cables(up to fill percentage of course)without spacing, possibly in multiple layers, without derating.

The circular references are cumbersome, but in an open tray environment, with the 3/c per cable restriction, there is sufficient heat dissipation available as long as the maximum cable tray fill restrictions are also maintained. Remember, most smaller conductors already have ampacity restrictions below their actual capability and ambient temperature correction is still a requirement.
 
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