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

[August Goers]

Kent, Brian,



It still seems like a grey area to me. I think that from a conservative
point of view that we cannot apply the 120% rule to a center fed panel. In
my experience, AHJs around the Bay Area know a little about the 120% rule
but haven't thought about it to this level of detail. In other words,
they're probably fine with it in part perhaps due to ignorance. If we point
out the issue to them then we might very well have a problem.



For the vast majority of our projects we can find a solution but for a
small portion of projects we cannot downsize the main breaker and we're
really stuck. In the example photo that I sent the only option might be
switching out the main service panel.



Best,



August



*From:* re-wrenches-bounces at lists.re-wrenches.org [mailto:
re-wrenches-bounces at lists.re-wrenches.org] *On Behalf Of *Kent Osterberg
*Sent:* Monday, October 15, 2012 12:07 PM
*To:* RE-wrenches
*Subject:* Re: [RE-wrenches] center fed main panel and 120% rule



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
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