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What is the depth of a 6" cable tray to be used in a cable tray fill calculation? 5" ?

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bdn2004

Electrical
Jan 27, 2007
797
2011 NEC Section 392.22(A)(2) says that 6" is to be used in the calculation for any tray with a usuable depth of more than 6"...but what about a standard 6" cable tray? Most standard cable tray has a 6" side rail, and the rungs take up inside space so should you always use 5"?

This makes a difference in the allowable fill calculations...
 
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Well the conduit tables use actual areas, not nominal areas. It may be safe to use actual dimensions for cable tray. If you use nominal dimensions for tray you may want to consult the AHJ.

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
You are right, waross. NEC sais: "cable tray having a USABLE INSIDE depth of 150 mm [6 in] ".If the depth is more than 6" NEC recommends the filling percent will be calculated as only 6" in it is the depth.
I don't know how a modern program could calculate the ampacity. In power station we used only solid bottom open top tray and the ampacity is calculated as per the Stolpe/ICEA Cable Tray Model [1970] and adopted in ICEA P-54-440 standard "Ampacities of cables in Open-Top Cable Trays."
I my opinion it will be very conservative to follow Stolpe for ventilated cable tray.
Sandia National Laboratories publicized a very interesting article [July 2000]:
"Ampacity Derating and Cable Functionality for Raceway Fire Barriers"
However, in my opinion, NEC is simpler to follow and is not so conservative.
By-the-way, NEC 2014 keep art.392.22(A).2 unchanged.
 
Solid bottom trays will limit air flow and reduce the ampacity of power cables that are spaced to facilitate air cooling and the use of free air ratings. Once the tray is filled enough to prevent free air circulation you should consider the derating tables for multiple conductors in a raceway. The cables themselves prevent free air circulation and the tray bottom becomes relatively unimportant for ampacity ratings.

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
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