Cable trays and grouping questions

[dm9289]

I have been reading and re-reading 392 and still have some questions maybe someone can clarify.

We recently had a fire by what I believe to be a classic case of age and tray overloading. 6 -1000kcm cables buried by many conductors of various sizes tray cover very tough to get on. Many reasons this would fail in 392


New build condition calculation (trying to avoid past problem) 6-750kcm conductors 480v delta 2thhn-750kcm per phase

6x area in sq in of 750kcm THHN(new cable)
6x1.0496=6.29
Table 392.22B1 says 6 in tray can hold 6.5 sq in so 6in tray would be fine???? Do you agree??


750kcm cable diameter x # of cables is wider than 6 in tray 6x1.156=6.936 would this violate 392.20 D???


Is trade size 6in tray wider than 6in???
Is tray height assumed to be 3inches for all calculations involving tray??

 

[starbolin]

Can't answer your question unless you specify the cable tray type.

392.20(D) applies to conductors less than 4/0 and more than 1/0. I think you were thinking 392.22(B)(2) which your fill would violate should you be using Ventilated Channel type cable trays.

 

[dm9289]

Yes I was using vented or ladder tray

 

[dm9289]

My question was related to 392-22-B-1-b for the calculation. My first post was off

 

[Smart $]

Note 392 Tables specify inside width. Marketing descriptions typically use outside width. So if you are going by 6in tray in the Tables, you are likely referring to 8in or wider tray by marketing descriptions.

Also note you will have to take how you want to determine ampacity into consideration. If you want to use the higher ampacity allowance permitted by spacing conductors in uncovered tray, under 392.80(A)(2)(c), you will need an inside width not less than double the sum of cable diameters.