[RE-wrenches] Conductor Sizing for Supply Side Connection

Dave Click daveclick at fsec.ucf.edu
Thu Feb 28 07:21:00 PST 2013 

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*
> Allan at positiveenergysolar.com <mailto: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>
>     [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*
>     Allan at positiveenergysolar.com <mailto: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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