[RE-wrenches] Point of Connection 705.12D2 Exception

Mon Dec 31 07:39:02 PST 2012

Greg:

Two part answer...

1) Yes, downsizing the main breaker is one scenario permitted by NEC to increase the current of the sources backfiring the panel.  If the bus bars in the panel are rated for 200 amps then you are allowed 120% or 240 Amps feeding from all sources.  Decreasing the main to 100 Amps (the grid source) would allow you to have 140 Amps of "other sources" and not overload the bus bar in the panel.

2) As you correctly noted the amount of current from the other sources would then exceed the rating of the main breaker.  While the bus bars would be ok you would still have a problem because you need to design all the breakers in the "upstream" current path to support the 140 Amp load.  If you were to order the panel with a 125A main this would be the best configuration.  240 Amps from all sources - 125 A main breaker = 115 Amps available for the other sources and, since 115 Amps from the other sources would not overload the 125 Amp main, you would probably be ok.   The other approach would me to install a secondary service (often incorrectly referred to as a line side tap) and interconnect on the utility side of the main.  This is what I have at my house with a 100 Amp fused disconnect feeding the Sunny Island backup/Gird tied PV system).

Another thing to consider and that is the size of the utility transformer feeding the service. While not a Code issue it could be an issue for the utility.   In residential services it is relatively common to see a transformer that is smaller (sometimes much smaller) than what would be needed to support the full load of a 200 Amp main.  At my house I requested a larger transformer and they installed a 25 kVA instead of the standard 20 kVA.  In the 115 Amp scenario a 20 kVA transformer would be marginal at peak back feed (115A * 240 = 27.6 kVA / 1.25  = 22.08 kVA continuous).
Lastly, the inspector >>might<< want you to show a calculation to prove the 125Amp main is large enough to support all the loads onsite.  Pointing out the limitations of the transformer feeding the service >> might<< be helpful in overcoming this objection (assuming the transformer is big enough to support your onsite generation).

Best Regards,

John Berdner
General Manager, North America

3347 Gateway Boulevard, Fremont CA 94538 USA  (*Please note of our new address.)
T: 510.498.3201 ext 747
M: 530.277.4894

-----Original Message-----
From: re-wrenches-bounces at lists.re-wrenches.org [mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Greg Seelhorst
Sent: Monday, December 31, 2012 6:49 AM
To: 'RE-wrenches'
Subject: Re: [RE-wrenches] Point of Connection 705.12D2 Exception

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 InstallerT

-----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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