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#94234 - 07/19/05 06:24 PM 250.66  
Dnkldorf  Offline
Member
Joined: Dec 2004
Posts: 1,064
nowhere usa
Just need a refresher here.

3 phase, parallel 500mcm service.
Total of 6 ungrounded conductors.

Is a 3/0 Gec from building steel to service equipment, the correct size?

2X500mcm=1000.

Next question, Is the best way to install a GEC to a 480x208/120Y sds, is to run it with the feeders to the transformer, or just run the feeders alone and run a GEC to the buiding steel.If it is run with the feeders, is has to be sized for the secondary, correct?


Dnk....


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#94235 - 07/20/05 05:12 PM Re: 250.66  
Roger  Offline
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Joined: May 2002
Posts: 1,716
N.C.
Dnk, in your first question (I am assuming all are copper for this conversation) 2/0 is good.

The second question, you would need to run a GEC from the XO tap to the building steel, the feeders would still need a properly sized EGC (250.122 or one recognized in 250.118) installed with them, and this would isolate from the neutral in the equipment served.

2002 NEC referenced

Edited to add a word.

Roger



[This message has been edited by Roger (edited 07-20-2005).]


#94236 - 07/20/05 08:05 PM Re: 250.66  
Dnkldorf  Offline
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Joined: Dec 2004
Posts: 1,064
nowhere usa
Roger, in the above post I made, this is the situation:

800a MDP, 480/277V, parallel 500mcm with parallel 350mcm nuetral. No bond to building steel in MDP, or bond anywhere for that matter. Under 250.66, don't you take the largest ungrounded conductor in each parallel set and add the circular mills together, and using the chart in 250.66, that comes out to a 3/0? Where did I go wrong?

Next question, If I bond this service to the steel, and water pipes, sprinkers ect, and this GEC terminates in the MDP, where I bond it to the nuetral, I can't use this bus to bond the nuetral of a 480X208/120Y? The Egc would be the Emt and Flex, but I can't run a GEC from the bus of the MDP to the transformer, I have to run its own seperate GEC to building steel?

Make sense I hope.

Thanks.

Dnk.......


#94237 - 07/20/05 08:42 PM Re: 250.66  
Roger  Offline
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Joined: May 2002
Posts: 1,716
N.C.
Dnk, I think I'm on the same page as you now.

Concerning the first question again, look at the line in the first column of 250.66 Over 600 through 1100 this would align with the 2/0 copper GEC in the third column.

In the second question, if you connect the GEC to the Neutral/Ground bar in the MDP (NOTE; the neutral bar must be bonded to the MDP enclosure in this scenario, hence where I said Neutral/Ground bar) you will not bond any conductor to the XO tap in the transformer, the Neutral will be the only conductor(s) landed on that terminal.

The EGC however will be bonded to the transformer enclosure.

Roger

[This message has been edited by Roger (edited 07-21-2005).]


#94238 - 07/21/05 07:41 AM Re: 250.66  
electure  Offline


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Joined: Dec 2000
Posts: 4,260
Fullerton, CA USA
Are 2 - 500 kcmil conductors sufficient for your 800A MDP?



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