Cable Tray - Derating Factors - 2008 NEC


I need clarification on a derating issue. When using single conductors, single layer, with spacing in between conductors equal to the conductor diameter, in cable tray you follow Table 310.17 (free air) for the ampacity. Then in Section 392.11(B)(2) for cable range of 1/0 to 500kcmil you immediately have to derate ampacity by .65. (I.E. If I have 1/0 Al, 90 deg, Table 310.17 notes an ampacity of 205A. Section 392.11(B)(2) calls for 65% derating of Table 310.17 values. This equals 205 * .65 = 133.25.

My first two questions are:
1. Why bother using CT if you have such a large derating factor?
2. If derating without a cover is .65 = 133.25, why is that ampacity LESS THAN the equivalent amapacity listed for 1/0 AL, 90 deg, Table 310.16 (conduit) which is 135A? Why would the ampacity of conductor in cable tray be less than in conduit?

Lastly it appears that for adding a solid unventilated cover you have to derate by .60 (Section 392.11(B)(2)). So by adding a solid cover you only derate by 5% over an open tray - that seems low.....but again I don't understand/agree with the 65% derating factor in the first place.

My last questions, based on the above as reference:
3. If I have a vertical cable tray (mounted on the side of a building) what would ampacities be uncovered? Is it 65% derating or something less due to convection?
4. What if i introduce a solid unventilated cover? Is it 60% like above or something else due to convection?
5. What if I introduce a ventilated cover? Code does not address derating of this at all. I contacted a cable tray manufacturer and they said to be safe I should assume 60%.

Any help would be appreciated.

Dennis Alwon

Staff member
Chapel Hill, NC
Electrical Contractor
it sounds like to me you would use 392.11 (B) (3) unless you have a number of other multi conductor cables in the tray also. With no de rating or exception (B) (3) using 310.15 (C)
That was my thought but I assumed the op needed (B)(2). It is curious and I don't know why other than with all the other conductors it will create a lot of heat.


Thanks for the response. I forgot to mention I have a ladder type cable tray. I believe i need to follow 392.11(B)(2) for 3 reasons:
1. Conductors are installed according to 392.10
2. My conductor ranges are 1/0 to 500 kcmil
3. I need to add a cover to the vertical cable tray and this sections references that whereas 392.11(B)(3) is only for uncovered cable tray.

I have a case # opened with NFPA and am awaiting a call back.
When I find out more I will post.

Any other thoughts would be appreciated.