[RE-wrenches] combiners and the 120% rule

[Chris Mason]

I don't think this is expressed in the code, but in my opinion, the 120%
applies to the building distribution equipment, not to parts of the solar
system. In the case where a panel is being used to combine multiple
inverter outputs, the panel is part of the solar system only. The 120% rule
was an accommodation to allow solar to feed a building distribution panel
and is not applicable to solar system components. It would be good if the
code could indicate this more clearly.

On Wed, Jun 27, 2012 at 8:29 AM, Jason Szumlanski <jason at fafcosolar.com>wrote:

> Kirk,
>
> That's basically what I said. Unfortunately, your opinion holds no weight
> with my local AHJ's. I've argued the point till blue in the face. Although,
> I have never had the instance where all available slots were filled in the
> combiner panel - I might be able to argue that case successfully.
>
> Jason Szumlanski
> Fafco Solar
>
>
>
>
> On Tue, Jun 26, 2012 at 4:53 PM, Kirk Herander <kirk at vtsolar.com> wrote:
>
>> Jason,****
>>
>> ** **
>>
>> In your email below you state:****
>>
>> ** **
>>
>> “You DO need to observe the 120% rule for the combining subpanel,
>> regardless of whether there are loads present, at least in jurisdictions
>> where I have worked. I've heard that some inspectors will allow you to
>> ignore it if it is labeled as a PV combiner with "add no loads" notation.
>> ”****
>>
>> ** **
>>
>> NEC 705.12 (D) states that the distribution equipment (in this case the
>> combiner panel, fed by multiple inverters and a utility source) must be
>> “capable of supplying multiple branch circuits or feeders or both” for
>> (D)(1) through (7) to apply. If you fully populated a combiner panel with
>> inverter breakers, leaving no slots for load breakers, it is not capable of
>> supplying branch circuits or feeders, and IMO the 120% rule does not apply
>> to the combiner buss or the conductors back to its point of utility
>> interconnect. I have argued this point as well as label combiners “load
>> circuits prohibited” (with or without available slots) and received AHJ
>> approval.****
>>
>> ** **
>>
>> You could also just lock shut a combiner that had spare slots as a
>> deterrent to adding load breakers.****
>>
>> ** **
>>
>> ** **
>>
>> Kirk Herander****
>>
>> VT Solar, LLC****
>>
>> dba Vermont Solar Engineering****
>>
>> NABCEPTM Certified installer Charter Member****
>>
>> NYSERDA-eligible Installer****
>>
>> VT RE Incentive Program Partner****
>>
>> 802.863.1202****
>>
>> ** **
>>
>> *From:* re-wrenches-bounces at lists.re-wrenches.org [mailto:
>> re-wrenches-bounces at lists.re-wrenches.org] *On Behalf Of *Jason
>> Szumlanski
>> *Sent:* Tuesday, June 26, 2012 8:28 AM
>> *To:* RE-wrenches
>> *Subject:* Re: [RE-wrenches] Enphase grid tie question****
>>
>> ** **
>>
>> I'll email you off-list a 1-line diagram from a system with 164
>> microinverters broken down into 8 strings in a 208V system. This particular
>> system used two subpanels to accumulate PV, but that was only because we
>> had to backfeed two existing subpanels due to the size of existing 480/208V
>> transformers. You will have to look at the utility service and all existing
>> equipment.****
>>
>> ** **
>>
>> Regarding the breakers in the subpanel, you will only need a maximum of a
>> 20A breaker for each string. The max inverters per string is 25 and the
>> calculation for OCPD is:****
>>
>> ** **
>>
>> 215W / 208V x 25 inverters / 1.732 x 1.25 = 18.65A****
>>
>> ** **
>>
>> “You DO need to observe the 120% rule for the combining subpanel,
>> regardless of whether there are loads present, at least in jurisdictions
>> where I have worked. I've heard that some inspectors will allow you to
>> ignore it if it is labeled as a PV combiner with "add no loads" notation.
>> ”****
>>
>> ** **
>>
>> Use a MLO panel with a fusible disconnect between the subpanel and the
>> interconnection point. If you use a 225A panel, you can feed it with 270A.
>> With eight 20A backfed PV circuits, you would need to protect the line side
>> of the panel with a 100A fusible disconnect. That probably isn't going to
>> work. You may be best off from a cost perspective using two 225A subpanels
>> and two 60A fusible disconnects. Anything larger than a 60A 3P disconnect
>> and the price skyrockets. It all depends on your circuit calculations and
>> the existing equipment. Of course, you would need two spaces for your
>> interconnection point.****
>>
>> ** **
>>
>> Jason Szumlanski****
>>
>> Fafco Solar****
>>
>> ** **
>>
>> _______________________________________________
>> List sponsored by Home Power magazine
>>
>> List Address: RE-wrenches at lists.re-wrenches.org
>>
>> Options & settings:
>> http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org
>>
>> List-Archive:
>> http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org
>>
>> List rules & etiquette:
>> www.re-wrenches.org/etiquette.htm
>>
>> Check out participant bios:
>> www.members.re-wrenches.org
>>
>>
>>
>
> _______________________________________________
> List sponsored by Home Power magazine
>
> List Address: RE-wrenches at lists.re-wrenches.org
>
> Options & settings:
> http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org
>
> List-Archive:
> http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org
>
> List rules & etiquette:
> www.re-wrenches.org/etiquette.htm
>
> Check out participant bios:
> www.members.re-wrenches.org
>
>
>


-- 
Chris Mason
President, Comet Systems Ltd
www.cometenergysystems.com
Cell: 264.235.5670
Skype: netconcepts
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org/attachments/20120627/b9ff891d/attachment-0003.html>