Parallel Grounds & GFI

[beanland]

2005 NEC 250.122(F)(2) provided a limited exemption to the "full size EGC in every parallel conduit" rule based on the rationale that where GFI is provided, the EGC is protected by the GFI not the phase OCPD. This makes sense because the argument for EGC in each conduit being "full size" is that if there is a failure in the conduit, the full fault current must be carried, is based on a ground fault current that trips the GFI, not the main phase overcurrent element.

In the 2011 NEC 250.122 that exemption is gone. Which makes me wonder: if I have a 3000A main, which must have GFI, and run eight runs of 500kcmil CU, why do I have to install eight 400kcmil CU EGC if the GFI is set to 400A? If I do not have GFI, then each EGC might, if there is a conductor failure in the conduit, is protected by te 3000A main. But, if I have GFI, doesn't my EGC OCPD (over current protective device) become the GFI rather than the phase overcurrent?

Worse yet, if I run nine sets of 400CU I have to run a 400CU EGC in every conduit! Yeuch!

Where a GFI is provided, isn't it reasonable that the EGC in every conduit need only be what is required based on the GFI setting?

In that case, what is the justification for requiring GFI if everything has to be installed as if the GFI is not present. (Other than to sell wire.)

 

[don_resqcapt19]

The reason the was changed is the fact that it called for a GFP device listed for the purpose of protecting the EGG. There never has been a device so listed.

250.122(F)(2)(3) The ground fault protection is listed for the purpose of protecting the equipment grounding conductor

That is from the 2005 code, previous codes said; "listed for the purpose".

 

[beanland]

Non Existing Device & Service GFI

Non Existing Device & Service GFI

That is the first sane idea I have heard in ages. But, the argument does not hold water. The 2011 NEC 690.11 requires a DC AFCI device be used for PV systems when no such device exists. The lack of such a real-world device did not stop the NEC from making it a code requirement.

If I have a 3000A main with 1200A max GFI per 230.95, since the 1200A GFI is the "Rating or Setting of Automatic overcurrent Device in Circuit Ahead of Equipment" per 2011 NEC 250.122, doesn't that mean that I can size my EGC based on 1200A? If not, why not?

 

[beanland]

Per the AHJ

Per the AHJ

The AHJ has agreed that where GFI is present, the EGC can be sized based on the GFI rather than on the main breaker.

Thus, a 5000A breaker with 1200A GFI allows the ECG to be sized based on 1200A rather than 5000A.

A 5000A breaker with no GFI would require the EGC to be sized based on 5000A.

 

[Strathead]

This makes sense because the argument for EGC in each conduit being "full size" is that if there is a failure in the conduit, the full fault current must be carried,

I have wondered about this rule period. It seems to me to be a waste of money, and I wonder what I am missing. First, if you have a fault in the conduit, then you still have a path back to the point where the other conduits all generate and then back along those grounds to the source, along with metal conduit if available and the myriad of parrallel paths back to the source. In you scenario above, couldn't the same argument be made that a fault on one path of hots could force the entire amperage to be carried on the other conductors, so parrallel should not be allowed at all? I don't have the degree, or the wherewithall to do the math on the return paths, but my Spidey sense tells me that using an EGC based on the ampacity of the wires in an individual conduit for each conduit would easily provide enough current capacity to clear a ground fault.