×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

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

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

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

(OP)
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...

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

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

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

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.

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

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

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources