Cable Tray in Open Top Utilidor

Ty.Jackson68

Member
Location
Texas
Occupation
Engineer
Looking for guidance. Have an instance where we need to run multiple cable trays in a utilidor.
The utilidor will have an open grating cover - freely allowing heat to dissipate (nominally 1/2" wide x 1" openings, continuous along the length of the utilidor).
The utilidor is pre-cast concrete on three sides (base, and 2 walls).

There would 3 cable trays mounted off one wall in a stacked configuration. 12" spacing between cable tray siderails.

Would this arrangement be considered impinging on "free air"?

Obviously, MC cable stacking in the cable tray would also have to be considered. The question is - does the placement of the tray in the open top utilidor add any considerations beyond normal calculations for cable spacing (random vs. spaced) and the ambient outdoor conditions. In this case - the ability for a "breeze" to be present is diminished in comparison to cable trays mounted in traditional fashion to structure above ground.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
I don't know of any allowance for cable tray to be treated as free air. However Annex B is allowed to be used under engineering supervision. Not sure if that is helpful to you or not
 

Ty.Jackson68

Member
Location
Texas
Occupation
Engineer
392.80 allows for cables in cable tray to be considered in "free air" and not subject to derating ampacity if spaced correctly.
If not spaced - then you are in a "random" fill. Code requires derating in that condition.
And code requires that if you place a cable tray cover (solid) then you have to derate the cables to 95%. (in my case above - there is no cable tray cover).

Question is - with an open top to the utilidor - is any additional derating required (or prudent) over and above the normal deratings that may be required due to cable spacing within the cable tray, cable tray covers, ambient temperatures, etc. - simply because the cable tray is not "out in the breeze" of a conventional exterior overhead cable tray installation.
 
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