[RE-wrenches] DC GEC Sizing Issue - Water Main GE

[Garrison Riegel]

Wrenches,

 

I have GEC sizing issue and would greatly appreciate any advice you can
share.

 

We have a small commercial grid-tie system installing on a facility with a
4000A, 240V, 3Ph service that uses the water main as the Ground Electrode.
My engineer is referencing NEC Table 250.66 (and the 4000A service entrance
cables) to size the GEC from the inverters to the GE, and therefore is
requiring a 3/0 (maximum size required by this table).

 

Since this is actually a DC GEC, I think it should be sized according to NEC
690.47(B) which references 250.166.  Since the GE is a water main it seems
that 250.166(B) would apply, requiring the GEC "shall not be smaller than
the largest conductor supplied by the system."  If taken literally, the
largest conductor supplied by the system could be the 4000A service entrance
cables, which would require a GEC even larger than the seemingly excessive
3/0.  For obvious reasons I hesitate to mention this to my engineer, but I
in the end I want to do what is best.

 

My questions:

 

1.    Is my engineer correct, and we should size the GEC according to
250.66? 

 

2.    If #1 is no, and 250.166(B) does apply, is "the largest conductor
supplied by this system" the 4000A service entrance cables, and the GEC size
should match these?

 

3.	Or, since 250.166 is written for DC systems, is this "largest
conductor" the DC source circuit conductors (#10 in this case), and
therefore the GEC can be a #8 (smallest size allowed)?

 

Thank you in advance,

 

Garrison 

 

Garrison Riegel

Project Manager 

 

Solar Service Inc

[p] 847-677-0950

www.solarserviceinc.com <http://www.solarserviceinc.com/> 

 

NABCEP Certified Solar PV and Thermal InstallerT

 

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