[RE-wrenches] sizing a sub-panel used to combine multiple inverter outputs

Wed Mar 30 09:59:19 PDT 2011

This is diagram is based on a 3-phase 208 V service, as clarified in the
text accompanying the diagrams. There is a mistake of sorts, however. The
authors¹ original manuscript called out a ³generic² 7500W inverter connected
to 50A, 2-P breaker. We then added more detail, calling out a Fronius 7500W
IG Plus inverter. However, according to the most recent product data from
the manufacturer, the max AC OCPD rating for this inverter is 45A, not 50A.
The example still works w/ 45A breakers: 400A + 90A = 490A, which is larger
than the 480A allowed under the 120% rule. Apply the subpanel & that problem
is eliminated.

RE: the question about which breaker to count. Code experts like Bill Brooks
& John Wiles have clarified that when a dedicated inverter aggregation panel
is used, the breaker on the output of the panel can be considered ³the
output of the utility interactive inverter(s).² The intent of the language
in 690.64(B)(2) in NEC 2008 about series connected panels is not to make you
count all of the breakers in a dedicated inverter aggregation panel. Rather
it is to clarify that it is unnecessary use the progressively larger
upstream breakers ratings, which can be ³fed² by the inverters, in your
point of connection calculations.

RE: the original question. I think the most correct answer is ³It depends.²
It depends on your jurisdiction and your PE. A literal interpretation of the
Code would appear to have you upsize the inverter aggregation panel and the
feeder conductor. But there is certainly precedent for AHJs not requiring
this. If you propose labeling the inverter aggregation panel ³PV Only ‹ Do
Not Add Loads² and mark the feeder conductor with warnings ³PV Only ‹ Do Not
Tap² wherever it is exposed, you may find that the AHJ will not require you
to upsize these. 

Hope that¹s cleared than mud,

David Brearley, Senior Technical Editor
SolarPro magazine 
NABCEP Certified PV Installer 
david.brearley at solarprofessional.com
Direct: 541.261.6545

On 3/30/11 11:12 AM, "Kent Osterberg" <kent at coveoregon.com> wrote:

> Jason,
> 
> Now you are opening up a can of worms.
> 
> It's bad enough that 705.12(D) doesn't say anything about a panel that can't
> (or shouldn't, or won't) have anything connected but interactive inverters.
> But when that is the case, a bus rating of 100% of the source circuits should
> apply. For PV systems a 125% factor will already apply to each inverter
> circuit.
> 
> While it seems logical that the conductors are an extension of the bus bar and
> should be treated the same way, 705.12(D) isn't about the conductors - it's
> about the bus bars. The conductors are protected by the breakers -- 80-amp
> breakers and wire with 80-amp ampacity.
> 
> Getting into the example further, I see flaws in it. If the inverters are
> 7500-watt 240-volt, the output current would be 7500/240 = 31.25 amps and
> 40-amp breakers would be adequate. Then there would have been no issues, even
> going directly into the main panel. If the inverters are 7500-watt 208-volt,
> the output current is 7500/208 = 36 amps and the 50-amp breakers make sense.
> That means the bus bars and feeder conductor have a continuous current of 72
> amps.  That means neither the 80-amp breaker nor 80-amp wire is sufficient
> because 72 x 1.25 = 90 amps. Now the example doesn't resolve the limitation of
> backfeeding at the main. Opps!
> 
> Kent Osterberg
> Blue Mountain Solar
> 
> 
> 
> 
> 
> 
> Jason Szumlanski wrote:
>>     
>>  
>> 
>> Another key is to remember that this discussion also applies to the conductor
>> between the main panel and subpanel. In a large PV system, this could result
>> in a pretty large wire between the two panels, and a significant cost that is
>> often overlooked. In some cases it makes sense to locate the subpanel close
>> to the main panel and run multiple sets of smaller wires from the inverters
>> to the subpanel.
>>  
>>  
>>  
>> And because the calculation is based on the first OC protection connected to
>> the inverters, adding a main breaker (theoretically 80A in this example) in
>> the subpanel doesn¹t change things. Even though this wire would be
>> theoretically protected by an 80A breaker at each end, you can¹t size the
>> wire for 160A / 1.2 = 133.3A. You have to size for 180A/1.2 = 150A. (not that
>> it makes much of a difference in this example, but it still must be
>> considered)
>>  
>>  
>>  
>> At least that¹s how I understand itŠ
>>  
>>  
>>  
>>  
>> 
>> Jason Szumlanski
>>  
>> Fafco Solar 
>>  
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>> 
>> From: re-wrenches-bounces at lists.re-wrenches.org
>> [mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Mark Frye
>>  Sent: Wednesday, March 30, 2011 1:33 AM
>>  To: 'RE-wrenches'
>>  Subject: Re: [RE-wrenches] sizing a sub-panel used to
>> combinemultipleinverter outputs
>>  
>>  
>>  
>>  
>>  
>> Opps!
>>  
>>  
>>  
>> My bad, I was thinking of a single phase system, not the three phase system
>> shown in the article.
>>  
>>  
>>  
>> For the three phase system Kent is correct in counting 180A of supply per
>> bar.
>>  
>>  
>>  
>> Mark Frye 
>>  Berkeley Solar Electric Systems
>>  303 Redbud Way 
>>  Nevada City,  CA 95959
>>  (530) 401-8024 
>>  www.berkeleysolar.com <http://www.berkeleysolar.com/>
>>  
>>  
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>>  
>> 
>> From: re-wrenches-bounces at lists.re-wrenches.org
>> [mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Mark Frye
>>  Sent: Tuesday, March 29, 2011 10:17 PM
>>  To: 'RE-wrenches'
>>  Subject: Re: [RE-wrenches] sizing a sub-panel used to combine
>> multipleinverter outputs
>>  
>> I think Kent and I agree. For the case where the subpanel is not dedicated a
>> PV sub-panel he is calculating for 2 - 50A breakers and I calculated for 3 -
>> 50A breakers.
>>  
>> 
>>  Mark Frye 
>>  Berkeley Solar Electric Systems
>>  303 Redbud Way 
>>  Nevada City,  CA 95959
>>  (530) 401-8024 
>>  www.berkeleysolar.com <http://www.berkeleysolar.com/>
>>  
>>  
>> 
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>> From: re-wrenches-bounces at lists.re-wrenches.org
>> [mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Kent
>> Osterberg
>>  Sent: Tuesday, March 29, 2011 9:26 PM
>>  To: RE-wrenches
>>  Subject: Re: [RE-wrenches] sizing a sub-panel used to combine multiple
>> inverter outputs
>>  
>> Per 705.12(D) the sub-panel could be any distribution equipment on the
>> premises. So the question becomes: is the sub-panel capable of supplying
>> branch circuits or feeder loads? If yes, then the sum of the breakers
>> (potentially) feeding the bus is 180 amps so a 150-amp rating is required and
>> the inverters would have to feed the opposite end of the bus bars. If no, the
>> code is not clear on the requirement, but obviously the 80-amp breaker in the
>> main panel limits the maximum current flowing through the sub-panel.
>>  
>> Kent Osterberg
>> Blue Mountain Solar
>>  
>>  
>>  
>>  
>> 
>> 
>> 
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