[RE-wrenches] Conductor Sizing for Supply Side Connection

Thu Feb 28 08:04:51 PST 2013

Great point Dave.  It crossed my mind that it was technically possible, but
figured since I would be removing loads off the main panel it was highly
unlikely.  Thanks for the code reference.  I'll go load side, and be
prepared for some discussion.

Thanks,

Garrison 

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Dave Click
Sent: Thursday, February 28, 2013 9:21 AM
To: re-wrenches at lists.re-wrenches.org
Subject: Re: [RE-wrenches] Conductor Sizing for Supply Side Connection

As a note, supply side connections have an extra complication with battery
backup systems. With a regular GT inverter you're just pushing current into
that interconnection point and you can work out easily that you won't have
any overcurrent issues unless you made a big mistake and your inverter
output exceeds the rating of your service conductors. However, with a
battery system you're also potentially pulling current from that point.
Before the battery inverter is installed your main breaker protects your
service conductors, but if the inverter is pulling in 60A and the main
breaker is also operating near its capacity, you could have >240A running
over 200A service conductors with no breakers tripping. You would overwhelm
the capacity of the service [230.23(A)] and no breakers would trip. You
could fix this by replacing your service conductors back to the transformer
(I'm just saying that it's an option) or downsizing the main breaker, and at
that point you're probably better off just making it a load side connection.
I'd go load side and argue with the AHJ to use the 2011 update mentioned
earlier.

Dave

On 2013/2/28 9:55, Garrison Riegel wrote:

Allen,

The benefit I see would be to allow for a code compliant load side
connection on a 200A panel with a 200A MB, where the AHJ is on the 2008 NEC
or older and will not listen to your good logic.  I don't think this would
always be the best option, but if backup loads and inverter output were less
than 32A then a 40A OCPD in the main panel should be fine?  Since it sounds
like the 60A breaker in the main panel is not a safety issue, but a design
consideration, I suppose I would just prefer flexibility when possible.

That said, this AHJ is on the 2008, and the loads will be less than 30A, but
based on this conversation I plan to go with a 60A and try to convince the
AHJ that it will be code compliant in their future!

Thanks,

Garrison

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Allan
Sindelar
Sent: Thursday, February 28, 2013 8:14 AM
To: RE-wrenches
Subject: Re: [RE-wrenches] Conductor Sizing for Supply Side Connection

Garrison,
I don't know why it's not listed as 60A max, but my own internal logic would
ask why it should be. The only benefit I could see for using smaller than a
60A breaker would be to allow use of #8 conductors (allowed with a 40A or
50A breaker) instead of the #6 necessary with a 60A breaker. And of course,
you could use a 40A breaker with #6 conductors, so theoretically it would be
fine. I just fail to see any benefit to doing so.
Allan

Allan Sindelar
 <mailto:Allan at positiveenergysolar.com> Allan at positiveenergysolar.com
NABCEP Certified Photovoltaic Installer
NABCEP Certified Technical Sales Professional
New Mexico EE98J Journeyman Electrician
Founder and Chief Technology Officer
Positive Energy, Inc.
3209 Richards Lane (note new address)
Santa Fe, New Mexico 87507
505 424-1112
www.positiveenergysolar.com <http://www.positiveenergysolar.com/>  

On 2/28/2013 6:27 AM, Garrison Riegel wrote:

Thanks Allen for clarifying why a 60A is required.  I was wondering.  The
spec does list a surge current of 9000W so I thought that may be the
rationale, but even that would only require a 50A, and since this surge
occurs during 'stand-alone mode' it didn't seem to apply to the OCPD at the
main panel.  Your explanation makes more sense, but I wonder why then they
don't list the AC input breaker size as 60A max.  If you have few backup
loads, and are not on the 2011 NEC, a 40A could theoretically be fine?

Thanks all for your thoughts, much appreciated.

Garrison

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Allan
Sindelar
Sent: Wednesday, February 27, 2013 6:08 PM
To: RE-wrenches
Subject: Re: [RE-wrenches] Conductor Sizing for Supply Side Connection

August,
The 60A breaker is intended to allow grid power to pass through to the loads
in excess of the inverter's stand-alone output. The point of 705.12 (moved
in the 2011 NEC from 690.64 (B)(2)) is to differentiate between load
pass-through current and sell current. The amount of current fed into the
grid is (4500/230 =) 19.56A, while the amount that can be taken from the
grid and passed through to the load is much greater. If you were limited to
a 40A breaker in order to maintain 120% of a 200A main bus, you'd be prone
to nuisance trips under large cumulative loads.

Allan

Allan Sindelar
 <mailto:Allan at positiveenergysolar.com> Allan at positiveenergysolar.com
NABCEP Certified Photovoltaic Installer
NABCEP Certified Technical Sales Professional
New Mexico EE98J Journeyman Electrician
Founder and Chief Technology Officer
Positive Energy, Inc.
3209 Richards Lane (note new address)
Santa Fe, New Mexico 87507
505 424-1112
www.positiveenergysolar.com <http://www.positiveenergysolar.com/>  

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