Table 250.122 2008 NEC

[Mike01]

:-? Table 250.122
1600A OCPD
4 sets of 3-750kcmil al conductors+grd.
Or
4 sets of 3-600kcmil cu conductors+grd.

250.122(f) Conductors in parallel:
?Each parallel equipment grounding conductor shall be sized on the basis on the ampere rating of the overcurent device protecting the circuit conductors in the raceway or cable in accordance with table 250.122?

So this indicates that for a 1600A OCPD a #350kcmil al or 4/0cu conductor is required for the equipment ground conductor. The statement above indicates that each conduit must contain the conductor, so each set of 750?s al would have a 350grd, and each set or 600kcmil cu conductors would contain a 4/0 conductor. I wanted to make sure I was reading this correctly. This is the way I interpet it, however someone in our office indicated that for paralleled conductors you would take the 350,000 and divide it by the number of sets (Ex.4) 350,000/4 = 87,500 per set of conductors and that would work out to a 1/0 in each set inleau of a 350kcmil in each set I believe he is incorrect. Looking foreward to you opinions.

 

[don_resqcapt19]

Each cable or raceway used for a parallel circuit must have a full sized EGC based on the rating of the OCPD an Table 250.122.

 

[Mike01]

250.66

250.66

Then 250.66

So then I would assume it works the same with 250.66

If I have 2-sets of 350kcmil cu. The ground would be one 4/0 per set

And for 2-sets of 350kcmil al the ground would be one 3/0 per set

For paralleled conductors you would add together the cmills and use that total in the table correct? Thanks again..

 

[charlie b]

This is the way I interpet it, however someone in our office indicated that for paralleled conductors you would take the 350,000 and divide it by the number of sets . . . .

They are wrong. You (and Don) are right. Reference 250.122(F)(1) (2005 version).

 

[charlie b]

So then I would assume it works the same with 250.66

I think not, though I have had this discussion in my office, with no clear consensus. My thoughts are that if you are talking about the Grounding Electrode Conductor, and sizing it per 250.66, then you only need to have one of them, and it can go into any one of the parallel conduits. I would be interested in hearing other opinions on this point.

 

[don_resqcapt19]

I agree with Charlie, in part, that the rules for GECs are not the same...the GEC if run in parallel is sized based on the phase conductors in that raceway or cable and not the total of all of the parallel sets. It is my opinion that you have a 310.4 violation if you only put a GEC in one of the parallel conduits.

 

[Mike01]

??

??

Wait so CharlieB you are saying not the Grounded but the groudning electrode when sized using 250.66 (new service or seperatley derived systems) you believe you only need one conductor in one conduit for paralleled sets? so like stated above 1600A cu, would be 4-600's, the ground conductor per 250.66 would require a 4/0 conductor but what CharlieB indicated is that one (1) 4/0 installed one conductor of the paralled set as opposed to one in each set? intresting I think i will do a little more research on this. thanks.