[RE-wrenches] Conductor Sizing for Supply Side Connection

Christopher Warfel cwarfel at entech-engineering.com
Sat Mar 9 07:17:14 PST 2013


Had a couple of minutes here, so with the load side connection, the 200 
amp breaker would operate to protect all conductors. With a supply side 
connection, the battery charger could be pulling 60 amps, the main panel 
200, and the service conductors sized for 240 amps would fail. 2/0 in 
free air is rated for 265A T310.15(B)(17), but would you say that 
T310.(B)(16) applies for service conductors and then 2/0 is 175?

I am really starting to hate solar.
On 2/28/2013 10:21 AM, Dave Click wrote:
> 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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-- 

Christopher Warfel, P.E.
ENTECH Engineering, Inc.
Energy Utilization Experts
(401)466-8978

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