[RE-wrenches] Conductor Sizing for Supply Side Connection

Sat Mar 9 12:24:57 PST 2013

HI Chris;

Just to be clear, the cables are not going to fail just because they 
aren't code compliant; unless you meant they would fail an inspection.  
I've included a picture of my own service rated at 150 amps.  Using 
table 310.15(B)7 (which I believe is the correct table for sizing the 
service conductors, not 310.B16 or 17), My side is 2/0 aluminum, but the 
utility side is #4 aluminum.  The utility is not held to NEC 
requirements, but a fire isn't starting just because they're grossly 
undersized by NEC standards.  NEC has huge safety factors, which are 
appropriate for wires hidden in walls that are expected to last the life 
of the house.
Finally, to be code compliant with a supply side 60 A connection, I 
believe the service conductors would need to be 3/0 Cu or 250 MCM 
Aluminum.  That would correspond to a 225 A service (x 120%) or 270 
amps, leaving 5 amps to spare.  As Dave mentioned, replacing the service 
conductors would probably not be a reasonable option.  Downsizing the 
main breaker to 175 A, would also work (175 + 60 < 200 x 120%) and since 
some of the loads are being transferred to the 60A breaker, downsizing 
the main should be compliant.

R.Ray Walters
CTO, Solarray, Inc
Nabcep Certified, Licensed Contractor
808 269-7491

On 3/9/2013 8:17 AM, Christopher Warfel wrote:
> 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
>>>
>>>           
>>>

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