Grounding in conduit

[nsbelectrical]

Say you have 4000amp feeder breaker feeding 4 conduit sets of three 500mcm cables at 34.5KV on a 800 ft run (lets assume pvc buried 6 ft in the ground). So, we have 4 conduit runs with three 500mcm cables per conduit.

My question is what size grounding cable does the NEC require to run in each conduit?

Be Safe.

 

[alehman]

Sorry, let me clarify. By "common practice" I mean that I have personally seen it and believe it has been accepted by inspecting authorities. I did not mean to imply that I believe it complies with the present NEC.

 

[nsbelectrical]

Gianoli,
I assure you it is challenging but very possible.

I have a new question:

We have a resistance grounded neutral that limits ground fault to 1200 amps. So Im thinking we should use that for sizing the ground cable rather than the 4000 amps of the breaker rating? The grounding grid of the substation is sized for 1200 amps rather than 4000. Are their provisions in the code for this? Considering "common practice" what would you guys do?

Thanks.

Be Safe.

 

[Gianoli]

“We have a resistance grounded neutral that limits ground fault to 1200 amps. So Im thinking we should use that for sizing the ground cable rather than the 4000 amps of the breaker rating? The grounding grid of the substation is sized for 1200 amps rather than 4000. Are their provisions in the code for this?”

There are no provisions (that I am aware of anyway) for this type of logic in the Canadian Electrical Code. The size of the bonding conductor is determined by the rating or setting of the overcurrent device ahead of the equipment without consideration for the type of system grounding that may be used.


"Considering "common practice" what would you guys do?"


If this system really has to be installed as you have described I would, at a minimum, specify that one #3 AWG copper bonding conductor be installed in each conduit that contains ungrounded circuit conductors as per my previous post.

 

[alehman]

It depends on the code in your area. The 2008 NEC does not recognize ground fault trip settings, so 500 kCMIL would be required. As I said, earlier versions of the NEC would recognize your ground fault relay setting. You should consult your local authority having jurisdiction as to the adopted code version and local amendments that would change your design.

 

[nsbelectrical]

Thanks for the info.

Be Safe.