Ground conductor sizes

[lharris104]

need some information regarding sizing a ground conductor.

35kv mv-105 cable in cable tray 4000A with 5 1/c#750 mcm per phase i performed 1 calculation and have a 4/0 ground for each phase.

my Colleague Calculates 400 mcm ground for each phase.

Please advise your opinion and how the calculation is to be applied.

 

[beanland]

250.122

250.122

2011 NEC 250.122 states that for a 4000A, 500kcmil copper would be used.

What NEC section was used to calculate 4/0 AWG and 400kcmil?

If this is not an NEC installation, the size would be based on the damage curve for the conductor, fault current, and OCPD clearing time.

 

[skeshesh]

Assuming you're referring to the "grounding" conductor which it seems like you are, you would size it based on table 250.122 which as you know is based on the rating of the OCPD. For conductors in parallel you would run the size retrieved from that table per 250.122(F).
I'm unsure of what you exactly mean by "calculated" as I've never calculated anything for a grounding conductor unless you count cases of considering the effects of a fault current on the grounding wire as "calculation". If you did perform a calculation to size it please provide your work or a reference to the method used.

 

[Dennis Alwon]

I am not sure if you are talking about the grounded conductor (neutral) or the equipment grounding conductor.

The calculation for the Neutral load is based on 250.61 but can never be smaller than the EGC.