So I have a Grounding Question - with Motors/cable tray cable

[gilly]

In 2008 the code changed a little on how to size the equipment grounding conductor to motors, I am refering to article 250.122(2). According to this article if you are using an instant trip breaker on a motor then you need to size the EGC as a dual element time delay fuse (@175% of amps). As long as I am correct here this is what I am up against.
We are doing an Industrial job with Alumn. ladder Cable tray as the main HR and Rigid conduit Drops to the motors. We are using ERTC multi conductor cable, to feed each motor. All motors are feed from 1 MCC building. So my problem is on my larger motors say a 250hp 3phase 480v, 1000' from the mcc room. We are pulling parallel 350's to the motor (this is a Spec job, engineered by another firm), in the cable we are pulling the EGC is a #3, pretty standard thru different manfactures. So if I take a 250hp @ 302amps and apply the 175% that gives me 528.5amps. Then look at Table 250.122 go up to the next standard Size - 600amp - this says I need a #1 for my EGC.
My question would be - what is everyone else doing when it comes to cable tray jobs and using tray cable to there motor loads? Am I missing something?
It seams that I am running into this on most motors 100hp and up. 100hp 3phase 480v - 124amps phase conductors #2/0, 124ampsx1.75=217amps go to table 250.122 look acrossed from the 300amp and you need a #4.. Standard ground wire in the #2/0 Tray cable is a #6..

Can someone please shed some light on this for me.

Also I am using southwire to give you the standard tray cable sizes

 

[Dennis Alwon]

First off How can you use a #3 egc and parallel it if art. 310.4 demands 1/0 minimum for parallel runs? Also with 1000' I assume you are dealing with VD and have upsized the cables- Did you consider that the egc must also be upsized- 250.122(B)

392.8 (D) is for cable tray in parallel

 

[Dennis Alwon]

I see that the EGC is not included in 310.4

 

[Dennis Alwon]

Someone must make tray cable with larger egc.

 

[don_resqcapt19]

You buy 4 conductor cable and use the 4th full size conductor as the equipment grounding conductor, or you order custom cable with the EGC sized as required by the code for your application. This is an issue anytime you use a cables in parallel. The standard EGC in cable assemblies is sized based on the ampacity of the conductors in the cable and table 250.122. Any application that requires an EGC that is larger than that, will require special attention.