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

[Garrison Riegel]

Hi Dave (and all)

Thank you for your response.

I appreciate your perspective and hope the AHJ will accept the logic that
the "largest conductor supplied by the system" is limited to the PV system.
Would you say the "system" stops at the first inverter output circuit OCPD?
In this design we have #10 PV output circuit conductors to each inverter,
and then #8 inverter output conductors to a 40A breaker in a dedicated
subpanel.  This panel has a 250A MB and 250kcmil cables to the MDP.

So...is the #8 the largest conductor in this system, or would it be the
250kcmil since it is a dedicated inverter panel?

Also, you note that 250.166(B) doesn't apply.  It is my understanding that a
water main is not considered a "pipe electrode" as in 250.52(A)(5), so the
#6 maximum GEC size unfortunately does not apply here, and we're back to the
"largest conductor supplied by the system."  Have you been able to argue
that the water pipe is in fact a pipe?  It seems logical, but sometimes
logic just doesn't seem to matter.

To note, the City of Chicago is back in the stone age (1999 NEC), and
requires the GEC to be run independently and in metal conduit.  That said
they have been known to allow certain exceptions if it meets the latest NEC
cycle (and they are in a good mood).

Thank you again for your time and knowledge.  It is much appreciated.

Garrison

Garrison Riegel
Project Manager 

Solar Service Inc
[p] 847-677-0950
[f] 847-647-9360
www.solarserviceinc.com

NABCEP Certified Solar PV and Thermal InstallerT


-----Original Message-----
From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Dave Click
Sent: Friday, July 20, 2012 3:23 PM
To: RE-wrenches
Subject: Re: [RE-wrenches] DC GEC Sizing Issue - Water Main GE

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...
>