# 6000 amp service switch



## Buddha In Babylon (Mar 23, 2009)

Do you mean an 800 mcm in place of the 3/0?


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## commuelect (Aug 24, 2009)

yes.


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## Bob Badger (Apr 19, 2009)

> I felt that there should have been a 800 mcm in each conduit based on 250.122 regardless of the ground fault protection in the 6000 amp service switch... am I right on this?


No, the size of the fuse determines the size of the EGC and in this case you would need a 500 copper or 800 AL EGC.



> there was a 6000 amp service switch fused at 4000 amps,,the load side of the switch had 18 sets of 500 mcm's under ground in pvc conduit to a distribution center 60' away


18 sets of 500? AL or CU?

18 sets of copper would be 6800 amps of condutors

18 sets of aluminum would be 5580 amps.

Either way the fact that it is fused at 4000 means larger conductors were run then required that means 250.122(B) kicks in and would require the EGC to be increased in size proportionally to the circuit conductors.



> in each conduit there was a 3/0 wire.


Without a doubt that is a violation.

However there is also 250.122(A) that tells us the EGC is never required to be larger than the circuit conductors.

So based on the little info I have I would say there should be a 500 EGC in each conduit.


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## commuelect (Aug 24, 2009)

Thanks Bob for your response. There were 18 sets of CU, and I was told that everything was oversized for the future, from the service switch to the copper detail in the distribution equipment. It was all rated at 6000 amps and fused at 4000. Good call on the 250.122A. An engineer once told me you could base your EGC on the size of the maximum ground fault setting (1200 amps). I check in the 05 & 08 code book and I don't see it. Does anyone know of any exceptions which might make this legit? Thanks


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## Buddha In Babylon (Mar 23, 2009)

Bob's summation is correct. Perhaps the installer used (table) 250.66 instead of 250.122 and assumed the highest he or she needed to size the "ground" was a 3/0. That's all i can figure.

"An engineer once told me you could base your EGC on the size of the maximum ground fault setting (1200 amps). "

What is this? Mximum ground fault setting? I'm not sure i understand. Is the engineer talking about short circuit calculations here? EGC's have to have a proper withstand rating...something like 1 amp for five seconds per every 42.25 circular mils....But where is that 1200 amp number coming from? I don't understand....
I don't trust everything i hear engineers say...


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## commuelect (Aug 24, 2009)

The 1200amp is comming from 230.95A settings [maximum] I belive what he was saying was you could base your EGC ON that 1200 amp max setting of the ground fault protection ..but I dont get that from what I read in the 05 or 08 code...


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