grounding

Thanks for all the helpful information and responses. I am completely clear on that i can run a #6 from my ground rods to my main service. What i am still hung up on is the size of the egc from the main switch to the sub panel. From what i believe i have been told in this forum is that since i have a 3000 amp service I have to run 400 mcm copper in each of my 10 runs of 350 mcm which ties to my ground bar on my main disconnect, which is bonded to my neutral bar, which is fed with 10 runs of 1/0 copper from the transformer (because i have no neutral load, 1/0 is the smallest i have to run). Why do i have to have such a large egc when my neutal (Which i understand is to act for fault clearing) is so small?
 
Crosstown said:
Why do i have to have such a large egc when my neutal (Which i understand is to act for fault clearing) is so small?
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Because the NEC doesn't allow you to run a bunch of smaller EGC's to make a larger one. You have 10 raceways in the feeder which requires a full size EGC (3000 amps from T250.122) in each of the 10 raceways. If you had a single raceway like a wireway for all 10 sets you would only need a single 400 kcmil EGC which would be considerably smaller than the 10-#1/0's in parallel.
 
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infinity said:
Because the NEC doesn't allow you to run a bunch of smaller EGC's to make a larger one. You have 10 raceways in the feeder which requires a full size EGC (3000 amps from T250.122) in each of the 10 raceways. If you had a single raceway like a wireway for all 10 sets you would only need a single 400 kcmil EGC which would be considerably smaller than the 10-#1/0's in parallel.
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does anyone know the "why" behind this rule. Also, im sure 10 1/0's would be greater than a single 400 kcmil, why do i have to run a 400 kcmil in each raceway?
 
Crosstown said:
does anyone know the "why" behind this rule. Also, im sure 10 1/0's would be greater than a single 400 kcmil, why do i have to run a 400 kcmil in each raceway?
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One of life's mysteries I suppose. I don't think that you'll find a logical answer other than the NEC says you need a full size EGC in each parallel raceway.
 
Crosstown said:
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does anyone know the "why" behind this rule. Also, im sure 10 1/0's would be greater than a single 400 kcmil, why do i have to run a 400 kcmil in each raceway?
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There have been multiple proposals to change this, but Code Making Panel 5 has rejected them. It is very difficult to change a long established rule without having some type of testing done to show that reducing the EGC for parallel circuits would be safe.
However, if this was a supply side bonding jumper, and not an EGC, the code would only require a 1/0. The supply side bonding jumper is the fault clearing path. That has been cited as a reason to reduce the size of the EGC, but no successful proposal as of yet.
 
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