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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