[RE-wrenches] Supply side connection to a dedicated panel

[Philip Boutelle]

Dave,

I think you are reading the output current for 277V. The spec sheet lists AC
max output at 240V as 7700W and 32A. If you take that number at face value,
you can still just use a 40A breaker. If you do the math based on their
listed output (7700W/240V=32.1A *  1.25 = 40.1A), you might need to bump up
to a 45A breaker; probably depends on the comfort level of you as the
installer and the AHJ inspecting.

For your main breaker or fuses, I think you could use 125A no matter how you
interpret the required sizes of the inverter breakers; just don't round up
to the next breaker/fuse size until after you have done your calculations
for all three inverters. 7700W * 3 / 240V * 1.25 = 120.3A, so round up to
125A for the breaker size. Your new total input would be 125A+135A = 260A,
which leaves you at the same place: 225A panel based on a b+w reading of
690.64, or a broader interpretation and therefore a smaller panel?

Around the Bay Area, there are some places that would not let us use the
dedicated panel idea, because it is not spelled out clearly in NEC. Other
AHJs are fine with it, especially with permanent signage installed stating
something about 'for combining inverter output circuits only, do not connect
additional loads', etc. If you are a strict code-stitutionalist and go with
the bigger 225A panel, don't forget that the code reads 'bus bar and
conductors', applying to the conductors in between your combiner panel and
your point of connection.

I've had a few drawn out discussions with John Wiles on this issue, and he
always encourages me to use the bigger panel--we do sometimes, not always.
Someone here.... maybe Bill?.... has mentioned a push to get the inverter
combiner panel exception written into NEC, but no luck yet.

Then there is the issue about if the line side of the ATS is rated for you
to tap/lug on, but I'm glad you didn't ask about that one ;)



Phil Boutelle

Project Engineer
NABCEP Certified Solar PV Installer TM
Real Goods Solar, Inc.

www.RealGoodsSolar.com <http://www.iesolar.com/>

On Mon, Oct 11, 2010 at 9:14 PM, Dave Palumbo
<dave at independentpowerllc.com>wrote:

>  We are installing a 8kW grid tied PV system for a commercial client.
>  There is an auto start generator on the premises, therefore we will be
> utilizing a supply side AC point of connection. Inverter is an SMA SB8000.
> System owner plans on adding to system in the next few years. We are
> designing for a total of 24kW  of PV.
>
>
>
> We will install a 200A fused disconnect off the grid feed (line side of the
> generator transfer switch). My plan is install a dedicated breaker panel for
> the inverter 240VAC output(s). The SB8000 is rated at 29 Amps, so a 40A DP
> breaker will do for the point of connection. This panel will eventually have
> a total of (3) 40A DP breakers each with 8kW of inverter output.
>
>
>
> My question has to do with the interpretation of the 120 percent rule of
> the rating of the busbar in the panel. Do I need to use a 225 Amp rated
> panel with a downsized 150A main breaker? 225A busbar x 1.2 = 270 Amps -150A
> MB = 120A total of inverter output allowed. Or, since this panel will only
> be used for inverter AC point of connection, and not for loads, is it
> allowable to label this panel as a “Dedicated Panel – connect no loads”. If
> that is the case we could use a 200A panel, or even a 150Amp panel. A local
> electrician proposed this “dedicated panel” to avoid the larger load center
> panel. I cannot find any mention of “dedicated panel” in the NEC.
>
>
>
> I think what I am proposing follows 690.64. Any opinions, or advice would
> be appreciated.
>
>
>
> Dave Palumbo
>
> Independent Power LLC
>
> *www.independentpowerllc.com *
>
> NABCEP Certified PV Installer
>
> Vermont Solar Partner
>
> 802.888.7194
>
>
>
>
>
>
>
>
>
>
>
> _______________________________________________
> List sponsored by Home Power magazine
>
> List Address: RE-wrenches at lists.re-wrenches.org
>
> Options & settings:
> http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org
>
> List-Archive:
> http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org
>
> List rules & etiquette:
> www.re-wrenches.org/etiquette.htm
>
> Check out participant bios:
> www.members.re-wrenches.org
>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org/attachments/20101011/bb4ce330/attachment-0004.html>