[RE-wrenches] center fed main panel and 120% rule

[Brian Mehalic]

Well the intent is so that no part of the bus bar is exposed to more than
its stated rating.  With the inverter breaker at the bottom and the main at
the top there isn't any spot on the bus that is subject to the sum of the
main and the inverter breaker currents.

With a center fed panel, the section of the bus bar on the opposite side of
the inverter breaker could in fact see the sum of the main and inverter
breaker currents.

I think it is best discussed with the AHJ upfront.

Brian Mehalic
NABCEP Certified Solar PV Installer™ R031508-59
IREC ISPQ Certified Affiliated Instructor/PV US-0132

PV Curriculum Developer and Instructor
Solar Energy International
http://www.solarenergy.org



On Mon, Oct 15, 2012 at 12:32 PM, August Goers <august at luminalt.com> wrote:

> Hi Wrenches,
>
>
>
> Here is yet another 120% rule question as it relates to center fed main
> panels. 2011 NEC 705.12 deals with inverter point of connection and
> 705.12(D)(7) reads:
>
>
>
> "*Inverter Output Connection.* Unless the panelboard is rated not less
> than the sum of the ampere ratings of all overcurrent devices supplying it,
> a connection in a panelboard shall be positioned at the opposite (load) end
> from the input feeder location or main circuit location."
>
>
>
> We are seeing more and more center fed main panels where the main breaker
> is in the middle of the busbars and there are branch circuit breakers
> located both above and below the main breaker. The question is whether it
> is possible to meet the intent of the code if the main breaker is rated the
> same as the panel. Here is a picture to illustrate:
>
>
>
> https://docs.google.com/open?id=0BxRKBvO2EOGudU5KSUJnYnFRYnc
>
>
>
> In this example, is it okay to install our inverter breaker on either the
> bottom or the top? If not, then our only resort is to either upgrade the
> service or downsize the main breaker which isn't always possible to do.
>
>
>
> Thanks for your thoughts in advance.
>
>
>
> Best,
>
>
>
> August
>
>
>
>
>
> August Goers
>
>
>
> Luminalt Energy Corporation
>
> m: 415.559.1525
>
> o: 415.641.4000
>
> august at luminalt.com
>
>
>
>
>
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