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Joined: Mar 2003
Posts: 849
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392.8 (d) conductors in cable tay Paralleled Shall be Grouped.
392.11 (B)(2) single conductors 1/0 to 500 kcmil grouped shall be derated at 65% per tables 310.17 -310.18 .
tray no cover free air copper 500 kcmil=(700amp at 90%c) (for derateing only)x 65% = 455 amps per group.

Unless installed with free air space per 392.11 (b)(4)then table 310.20 (500 kcmil=580amps at 90c )

Wanna check my work guys ???
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Joined: Mar 2003
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??????????
Nobody does this type of work???

Joined: Mar 2005
Posts: 1,213
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Just use a bigger tray laugh Grouping doesn't mean they have to be bundled, you just need to run the whole group together in a single tray (or parallel trays are OK too, I believe). If you can leave a cable diameter between the 500kcmil cables, you don't need to derate at all. I've often seen them tie-wrap cables to the trays to maintain spacing. Looks cheap with all the tie-wraps (IMHO), but very clean, and is very effective at maintaining spacing.

I really don't see why NEC would specifically disapply 310.15(B)(2) though, it doesn't make sense, and 65% is a HARSH derating. Ask your AHJ, they may be lenient about minimum cable space to avoid derating- I know I sure am on cable spacing in venetilated trays.

Alas, I won't have to worry about this anymore- as of 2008, 708.10(C) specifically prohibits it for my sites, even when the EMI environment will allow open tray. All RMC from now on, I suppose. Maybe the occassionally SCH80 duct bank...

Last edited by SteveFehr; 03/28/08 01:35 PM.

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