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

[Kent Osterberg]

If you back feed at the end of the bus bar where the main is connected 
to the middle of the bus bar, the back fed breaker cannot overload the 
bus bar between the main and the back fed breaker, but it would be 
possible to overload the other side of the bus bar - it's being fed by 
both the main and the back fed breaker. Seems like you shouldn't apply 
the 120% rule to these panels. But realistically, the panel isn't likely 
to be overloaded because most 200-amp panels are fed by services that 
can't really deliver 200 amps, at least not for more than a very brief time.

And what about the SQ D QO or HOM 6-12L100 or 8-16L100 panels that can 
be fed with main lugs that are on opposite ends (left for L1 and right 
for L2)? Seems like you shouldn't apply the 120% rule to these either.

Kent Osterberg
Blue Mountain Solar, Inc.
www.bluemountainsolar.com

On 10/15/2012 9:32 AM, August Goers 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 <mailto:august at luminalt.com>
>
>

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