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Table 250.66

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reboldhack

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Ever wonder why the required Grounding Electrode conductor in Table 250.66 jumps from #2 to #1/0, skipping over #1 wire entirely? Wouldn't you think that maybe 500 kcmil would be OK with a #1 GEC and that the reason for useing #1/0 would be more suited for a 600 kcmil service application?
 
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jerryb

Senior Member
Re: Table 250.66

As a non-technical response, I would think that it has to do with the area (circular mills) of the conductor. The increase in circular mills in the ground wire from #2 to 1/0 is approx. 159%. The increase in circular mills in the service conductor from 4/0 to 400 Kcmil is 189%.

Article 250 says that if a service, feeder or branch conductor is increased (normally for voltage drop) then the ground conductor will be increased proportionally.

The two seem to go together. The whole purpose is to make sure that the ground resistance is low enough to allow a breaker or fuse to open.
 
ryan_618

ryan_618

Senior Member
Location
Salt Lake City, Utah
Re: Table 250.66

Originally posted by jerryb:

Article 250 says that if a service, feeder or branch conductor is increased (normally for voltage drop) then the ground conductor will be increased proportionally.
Click to expand...
This is in 250.122(B) for equipment grounding condcutors.

The two seem to go together. The whole purpose is to make sure that the ground resistance is low enough to allow a breaker or fuse to open.
Click to expand...
I had a hard time with this since it is for GEC's (not used for fault-clearing), but since bonding on the supply side is sized from 250.66 (as per 250.102(C)), this indeed may be the case.
 
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