Installation of cable tray at an MCC

[lexkyphil]

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

 

[davidbeach]

Why argue with the client? If they have a standard design, they will probably still have that standard design long after they get tired of you trying to change it. They're the ones paying for it aren't they?

 

[waross]

Use gland connectors to enter the MCC. Some folk drop the cable out of the bottom of the (ladder style) tray. Some rol the cable up and over the edge of the tray.

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

 

[gepman]

What you are describing is quite common both in the the oil and gas industry and the mining industry. The one advantage that I have seen is that it makes laying out crowded electrical room conduit easier especially for future installations.

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.

 

[dpc]

Up until the 2008 NEC, the drops from the tray to the MCC had to be in a raceway or continuously protected by "struts, angles, or channels." A bare drop was not allowed - See Article 336.

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.