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

[Bill Brooks]

August,

 

I'm not really worried about residential center fed buswork since the 120%
exception only applied residential prior to the 2008 NEC and the location of
the OCPD was not specified.

 

The location of the OCPD was specified in the 2008 NEC to address commercial
panelboards that are run at full load routinely.

 

An AHJ could raise a problem in which case you would have to do a supply
side connection or replace the panelboard. Otherwise, don't ask, don't tell
works for me since it is consistent with 20+ years of code interpretation.

 

Bill.

 

 

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of August Goers
Sent: Monday, October 15, 2012 12:38 PM
To: RE-wrenches
Subject: Re: [RE-wrenches] center fed main panel and 120% rule

 

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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