ECN Forum Forums Code Related Discussion NEC & other Code issues 1200A Panel EG

John: Since the raceway and the EGC will be bonded (directly or indirectly) at each end, you basically have the pipe and the EGC in parallel with each other, creater a lower impedance EGC.

For example: You could install 3" IMC with 350's and a 1/0 for 540 feet and still get a 4800 amp fault.

If you had 3" PVC with 350's and a 3/0 EGC and were trying to get a 4800 amp fault, you could only go 131 feet.

These numbers are assuming a fault of 4 times the rating of the OCPD to initiate the OCPD under ground fault.

I think that is why there is a note to table 250.122 that tells you that following the table values doesn't allways achieve compliance with 250.4(A)(5).

A also don't think this type of allowance is really stretching the code, since 250.4 is main requirement of article 250, and 250.4 is in fact a performance based requirement, not a prescriptive one. That tells me that the AHJ should be on his/her "A" game when sizing EGC's.

[This message has been edited by Ryan_J (edited 07-06-2004).]

Ryan,

I am in agreement on the general point of calculating the fault current to see if it is sufficient, and presumably calculating the energy delivered to the conductors in a fault situation to make sure that the OCPD protected the conductors. However I believe that the point of this exercise is to size the _parallel_ EGCs so that they correctly function even when _not_ effectively in parallel.

In the example situation that started this thread, 4 parallel 1/0 conductors were used as the EGC for a 1200A panel. Table 250.122 calls for a single 3/0 conductor. The total cross section of the parallel 1/0 conductors is in excess of 2.5x the cross section of the 3/0 conductor; so _taken as a set_ the 4 1/0 conductors are clearly a better EGC than the single 3/0 conductor. Clearly then, the issue is 'What happens when these conductors are _not_ effectively in parallel?'

The situation that I was suggesting requires separate calculation is as follows:

Equipment ground bonded at _both_ ends of the metallic raceway.

Phase to ground fault _inside_ of the raceway.

Now you cannot consider the two conductors to be in parallel, but would instead have to consider the following two paths in parallel: the 'short' path through the EGC to the supply end of the raceway, and the 'long' path through the EGC to the load end of the raceway, and then back through the raceway to the supply end.

-Jon

Ryan_J, I believe the total lenght of this circuit is roughly 300 FT. The EC lead man is saying that 1/0 is fine and they could have gone with a smaller EGC. I think he's using 250-66, but that art. is for sizing the grounding electrode conductor, is there any way someone could use this table to size the equipment grounding conductor. Beside they are two different conductors. Does anyone believe this may be the case. I believe 3/0 is what is needed according to 250-122, can anyone disagree with that. Or how should I go about letting them know how they should size an equipment grounding conductor?