Cable Trays, Conductors, and Plenums - NEC 2011 Change

[Erikkinn]

NEC 2011 added 300.22 (C) (2) (b) on 'solid side and bottom metal cable tray systems' within other spaces used for environmental air (plenums). As I read this, it appears to now allow the installation of single conductors such as THHN/THWN-2 within a solid enclosed cable tray system (w/ solid top cover) within such 'other type' plenum space, i.e. similar to the allowance for using metal wireways with covers, as it's completely enclosing the wiring method above the ceiling.

I am looking at this installation method for a commercial project, not industrial, above lay in ceiling, large feeder with multiple parallel sets. With existing utilities and a long feeder run, I cannot utilize conduits effectively within the building above the ceiling, and I have to use existing conduits provided in another project by others to get out of main gear (underground) and into the building. Thus my interest in this code change. Comments appreciated, thanks.

 

[pete m.]

If you are looking at this for a "commercial" project I do not believe single conductors such as THHN/THWN will be permitted. See 392.10(A). Only the wiring methods in table 392.10(A) would be permitted in a "commercial" occupancy.

Pete

p.s. Welcome to the forum.

 

[Erikkinn]

One item is that the NEC 2011 Handbook on page 483 indicates that not all specific cable types that may be installed in commercial and industrial cable tray systems are identified within that table (392.10(A)). HB quote appears to indicate that the "Other factory assembled..." line item is meant to state that other cable types that are approved for installing within a cable tray system would fall under that heading. Thus, my thought on ability to use THHN/THWN-2.

Then back in NEC 2011 Handbook on page 316, its summary, it indicates by a separate sentence "Solid bottom cable trays with solid metal covers and solid sides can support any system allowed in 392.10 because they would completely enclose the wiring method.", which this sentence being separate and distinct from the previous Handbook sentence that references wiring methods having to be specific for air handling spaces, thus the reason for the (b) article item.

Thus, I'm seeing two things in the handbook that appear to support this. My main thought being, if this were not the case, one could just delete item 300.22 (C) (2) (b) and just leave statement (a) only.

Thanks for comments, I've read this forum a long time, finally decided to throw my hat in the ring as well! And this topic had me wanting to get input from others.