Grounding 4000A feeder 1

[cuhead]

I am trying to solve a grounding issue. I have an ATS in down town LA that is burning up and needs to be replaced right now. The existing ATS is internal to the switchgear lineup and the customer would prefer to replace it with an iso-bypass type to mitigate this exposure in the future. The existing ATS is rated at 4000A and the current load was recorded at 2,500A. I would like to install the new switch 25’ away but the feeders will have to be installed through a DWP (Department of Water and Power) transformer vault ventilation system shaft. I believe that shaft would be classified a plenum. I wanted to install cable tray and use DLO in free air but that is out of the question if it is considered a plenum shaft. Due to the time constrictions I will have to install 9 sets of 4” EMT with 3 RHW-2 646kcmil DLO conductors in each (40C with 90C termination crimp on lugs. I need the DLO for flexibility at the termination.). I will not be able to install a grounding conductor (500kcmil for 4000A) inside each raceway and I cannot correctly bond the conduits with ground bushings because the largest conductor that the bushing will accept is a 4/0. If I classify the EMT as a “grounding conductor” (NEC 250.118) does each EMT equal the 500kcmil required for the 4000A OCPD?

 

[dpc]

As far the NEC is concerned using the EMT as the grounding conductor is acceptable. Comparison with required size of a proper grounding conductor is not an issue in the NEC, unless the AHJ feels that the conduit does provide sufficient capacity to allow the overcurrent protection to operate. But I doubt that will come up.

I think it would be a terrible installation and the lack of a decent grounding conductor would only be one of my concerns. Since you only asked about the grounding: The use of conduit as a grounding conductor was shown to be markedly inferior to use of a ground conductor inside the conduit through testing done over 40 years ago. The NEC still allows it out of long-standing tradition and the power of the steel conduit industry.

Let us know what the LA inspectors tell you.

BTW, I'm assuming this is a 480 V service?

 

[rob46]

Please review

http://www.electrician2.com/electa1/ground2.html

 

[dpc]

rob46,

Thanks for that link.

Dave