Ground conductors in parallel, then upsized

[jsbrownpe1]

250.122 (F)(1) and then 250.122(B)?

Take a very long run (over 1000') for a 2000A 480/277V feed from a 4000A main service switchboard to 2000A switchboard. (don't punish the engineer - the owner will not accept medium voltage - just go with me on this)

To make voltage drop work, you move up from (5) 4" C. to (10) 4" C., each with (4) 600KCMIL, (plus ground). An easy 100% increase in the ungrounded conductor size.

Table 250.122 says use 250KCMIL for 2000A device.

Q#1 - Do you need to upsize 100% to 500KCMIL in each of the 10 pipes?

[It does seem extreme, being you already have 250x10=2500KCMIL of copper ground...to have to go up to 500x10=50000KCMIL of copper ground./

Q#2 - Is the only way around this to put ground fault protection on the main switchboard 2000A branch breaker? Using 250-122 (F)(2)

 

[charlie b]

Re: Ground conductors in parallel, then upsized

1. Yes (and yes it is extreme).
2. Yes (given the prohibition against higher voltages).

 

[pierre]

Re: Ground conductors in parallel, then upsized

"It does seem extreme, being you already have 250x10=2500KCMIL of copper ground...to have to go up to 500x10=50000KCMIL of copper ground./"


It is part of ensuring that the "Effective Ground Fault Current Path" is just that, effective.

 

[steve66]

Re: Ground conductors in parallel, then upsized

What type of voltage drop are you shooting for? I don't think I would be trying to get 2% for the full 2000 amps. How often is the load really going to be 2000 amps?

I calculate less than a 3% voltage drop with 6 sets of 600KCM.

Steve

 

[eprice]

Re: Ground conductors in parallel, then upsized

Originally posted by jsbrownpe1:
Q#2 - Is the only way around this to put ground fault protection on the main switchboard 2000A branch breaker? Using 250-122 (F)(2)

Well, in the case you describe, even the ground fault protection doesn't let you get around it. 250-122(F)(2) only works for multi-conductor cable.