Installation of cable tray at an MCC
Installation of cable tray at an MCC
(OP)
Gentlemen: I am doing work in a NGas compressor station in an unclassified area. My client wants to make all connections to the motor control center using GRS conduit that comes from the yard (C1, Div2, Group D) that rises outside the metal MCC building to pulling ells, transition thru the wall, stubbing to a cable tray, then routing conductors to the top of the MCC. I do not like this approach, since we seem to gain nothing. The runs are just a few feet. The problems I am considering: 1. will have to use TC cable which is larger and will upsize conduits 2. the installation really won't be any neater than well-bent conduit 3. how do we go from the tray to the top of the MCC? (naked wire thru bushings? back to conduit for the drop?) 4. this is just #10 to #1/0 THWN 600v wiring-nothing special 5. Derating of wire will be more complex than using 310.16 and lastly, 6. NEC 392 seems to place a lot of restrictions on minimum sizes, etc. of the conductors.
My thanks in advance for any help or arguments pro or con that you have experience with in this area. Phil from Lexington, KY
My thanks in advance for any help or arguments pro or con that you have experience with in this area. Phil from Lexington, KY





RE: Installation of cable tray at an MCC
RE: Installation of cable tray at an MCC
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Installation of cable tray at an MCC
For question #1 my interpretation of the NEC is that you will need to use TC cable or whatever else meets table 392.3(A) but remember that size 1/0 or larger is acceptable as single conductor in industrial facilities (which yours definitely is).
For question #2 remember future installations where conduit may very likely block future conduit paths.
For question #3 I do not like bare cable hanging and I am certain there probably is some portion of the MCC which gives the maximum unsupported bare cable allowable (if any). I have used both conduit drops from the cable tray and smaller sections of cable tray dropping to the MCC depending on the situation.
For question #4 you can always put the small single conductors in flexible metallic conduit.
For question #5 it seems a lot more complicated but if you basically have the cable tray wide enough and space or single layer the conductors then the ampacity in cable tray should always equal or exceed the ampacity in a conduit.
I always use some very explicit standard details on how to install cable in cable tray in order to meet article 392.
RE: Installation of cable tray at an MCC
The 2008 NEC has a new exception allowing TC-ER cable to be dropped without a raceway for up to 6 feet.
When designing power plants not subject to the NEC, we used to do this all this time, and it is very common outside the US, but in general, the NEC requires some type of raceway, unless you can satisfy the requirements of the exception in Art 336.