grounding types allowed on 15KV feeder conductors

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Hi y'all,

Where in the 2005 NEC do I find what types of grounding are allowed for feeder conductors? 15KV

I need to site these for spec.

There's talk about grounding the array of feeders to the new substation. We would like to make a case for grounding each feeder's individual conduit. Any thoughts? Also, Is this in the NEC somewhere to back up this best practice? Thanks! :wink:

And are there any other considerations I should take into account to make my system more reliable in the future?

Thanks
 
15kV Grounding

15kV Grounding

NEC does not cover 15kV work well. I suggest NESC as a guide. Also, any adopted State standards for utility work.

Normal utility practice has cable shield/concentric grounded at every splice or termination. However, there are some times that is not advisable and is not done, especially on circuits over 300A. Can be very specialized.
 
The NESC is written completely differently than the NEC, and for many readers is totally inappropriate. For example, relating to grounding, it requires the systems to be "designed to minimize hazards". It relies on the NESC user to design the system and does not provide the information to determine how. It does not say run a 1/0 ground, for example.

You can learn much about how to build grounding systems by reading the NEC, you can't learn nearly as much about the same by reading the NESC.

Can you describe your 15 kv system better?

Jim T[/quote]
 
We're running our new substation's 15kv feeders in non-metallic pipe under concrete.

I believe I found a reference that will back up our desire to run individual grounds per conduit. 352.6 in the NEC 2005.

Do y'all concur?

Thanks!
 
I'm not sure that 352.6 applies, in my book it is refers to listing requirements.

The reference that comes to mind is 250.122(F). Applying it is a bit of a stretch because you would not use 250.122 to size ground wires in your 15 kv system. One place to start looking is 250.X

What type of system do you have, grounded or ungrounded? What type of cable are you running? Shielded cable needs a separate neutral/ground, but URD cable for instance would not. Are you combining 3 phases+neutral in one conduit?

I don't believe it is necessary to run a neutral and a ground in multigrounded systems, although a single point grounded system requires two separate conductors.

Jim T
 
What type of system do you have, grounded or ungrounded? Grounded

What type of cable are you running? Shielded cable needs a separate neutral/ground, but URD cable for instance would not. Medium Voltage, Shielded (not URD)

Are you combining 3 phases+neutral in one conduit? No Neutral

Thanks!
 
Based on that, you certainly would want to run a neutral/ground in each conduit. In a multiple grounded system there is little or no difference between neutral and ground.

Are all of your downstream transformer connections delta on the high side? They will need to be if you haven't planned a current carrying neutral.

If it serves only to ground, it would need to be sized to provide safe step and touch potentials at equipment, etc. and provide adequate fault clearing capability. Unfortunately, there is no table to look up the appropriate wire size.

Jim T
 
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