Cable Tray Fill

[blues]

I have what would appear as a dumb question. In NEC Table 392.22(A) column 1 they list a 6"wide cable tray to have an allowable fill area of 7.0 sq. in. I do not understand how the NEC arrives at that number.

Thanks,
Dan

 

[Julius Right]

In my opinion the maximum permitted filling area is connected with J.Stolpe theory
"Ampacities for Cables in Randomly Filled Trays"/1970
[or P-54-44/WC5 "Ampacities of Cables in Open-top Trays "]/1986.
If we shall divide the "filling area "to" tray width, we'll get 1.165" depth [as per P-54-44/WC5 ch.2.2].
It is true, the above standard is referring to solid-bottom only [and in column 3 of NEC Table 392.22(A) it is about 0.91" depth permitted for solid-bottom]
In order to fit Table 310.15(B)(16) to P-54-44 for solid-bottom tray the depth is limited to 0.91"
and for ventilated tray to 1.165"[so permitting 5% more ampacity than for solid-bottom].