[RE-wrenches] Point of Connection 705.12D2 Exception

Greg Seelhorst gseelhorst at gmail.com
Mon Dec 31 06:49:12 PST 2012


We recently came across this Square D Side-by-side MMC that uses the same
200A busbar with a 100, 125, 150 and 200A Main Breakers.  They are
interchangeable for swapping the Main Breakers.  I had to go to Schneider to
get this spec sheet which shows the 200A rating for the SC2040M200S series
(is followed by a 100, 125, 150, or 200 after the last 'S').  

Question, using William's calculations shown below, if the 100A Main Breaker
version is used, this would feasibly allow a backfeed 140A of solar, but
wouldn't this be electrically limited by the Main Breaker's capacity of
100A.  Of course, not a likely scenario of wanting to backfeed more than the
main breaker's rating.

Our distributor regularly stocks this because it is so versatile and the
Main Breakers can be ordered separately when the homeowner wants to upgrade
their service and only a breaker swap is required.  

Greg Seelhorst
Designer/Project Manager
NABCEP Certified Solar PV Installer™
  

-----Original Message-----
From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of William
Miller
Sent: Sunday, December 30, 2012 8:28 PM
To: RE-wrenches
Subject: Re: [RE-wrenches] Point of Connection 705.12D2 Exception

Jeremy:

Bingo!  You have discovered the hard truth of grid tie with battery point of
connection compliance (GTWB POC).  Your inverter will through-transfer up to
60 Amps (A), so to use all of that capability, you should feed it with 60A.
However, you are not allowed to feed the inverter with greater than 40A
dedicated over-current protection (OCPD).  The arithmetic is:

Buss rating:  200A
Allowed total buss feed:  200A X 120% = 240A Main breaker: 200A Allowed PV
feed:  240A-200A= 40A*


If you can downgrade the main breaker to 150A you can back feed up to 90A.
The arithmetic then is:

Buss rating:  200A
Allowed total buss feed:  200A X 120% = 240A Main breaker: 150A Allowed PV
feed:  240A-150A= 90A

Downgrading the main breaker is not easy.  Most meter/main/combos (MMCs)
will not allow the downgrade-- there is not a 150A breaker made that is
designed and listed to fit in the 200 A panel.  I have contacted various
manufacturers and requested that 200 (MMCs) be made that will readily accept
a 150A main breaker, but large corporations do not seem to care about the
needs of a small part of the market..

I had found a 200 A MMC that I could downgrade the main breaker to 150 A,
but not easily.  The fit was not perfect -- I had to modify the hold down
bracket, but it passed inspection.  Technically I suppose the MMC was no
longer compliant with the listing once I modified it, but sometimes you
gotta do what you gotta do.

Good Luck.

William Miller

* PS:  I believe the 2011 NEC has an exception that applies, but I do not
believe any jurisdictions are using that code.  I have convinced my local
building officials to pre-adopt certain future code sections.  Maybe you can
make the case to your local inspectors.  Meet with them, explain the
limitations of the XW as far as intertie amperage and maybe they will allow
use of a 60A breaker.

Wm

At 04:27 PM 12/30/2012, you wrote:
>I'm concerned that the inspector will not allow a 60A back fed breaker 
>in the 200A main panel as it exceeds the 120% rule Jeremy Sent via 
>BlackBerry. Sorry for typos and shorthand!
>_______________________________________________

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