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

[Dave Click]

Garrison,

If you're doing a small commercial grid-tie system, your inverter GEC 
requirements are determined not by the service size but by your 
inverter. To illustrate, if you were installing a single Enphase, 
running an unspliced #3/0 copper wire from your pipe up to your inverter 
would be overkill, right? 690.47(B) is for systems with DC requirements 
only (e.g. small inverter-free stand-alone systems).

If you're on the 2011 NEC, the simplest way to do this is to run a 
combination EGC/GEC from the inverter GEC terminal *unspliced* through 
your AC conduit to the ground bus of your interconnection panelboard, 
sizing it to meet DC GEC and AC EGC requirements. If you're on the 2008 
and your inspector won't allow you to use the 2011 method, you'd run and 
size your AC EGC as you normally would. For your GEC you would size that 
based off the larger of 250.66 or 250.166, and note that 250.166(B) 
doesn't apply since you have a pipe electrode (250.166(C) overrides 
166(B)). So you can get away with a #6 copper GEC unspliced from your 
inverter to your pipe. Run that GEC in PVC conduit if you can, but if 
the site requires you to use metal, you'll need to bond both ends.

To answer your questions:
1. Per 2008, you'll size the GEC per 250.66 and 250.166, and the .166 
requirement will likely win out. Per 2011, you'll size your combined 
EGC/GEC as no smaller than 250.122 or 250.166.

2 & 3. "Largest conductor" applies to the largest conductor in the PV 
system (likely your homerun DC).

Dave

On 2012/7/20 15:06, Garrison Riegel wrote:
> 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 Installer™
>
>
>
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