[RE-wrenches] combiners and the 120% rule

Wed Jun 27 08:51:11 PDT 2012

If you install a combiner panel that can supply additional branch circuits,
it becomes part of the building distribution system. Note that Enphase
recommends you install a branch circuit to supply power to the Envoy device
right from the combiner panel. If that is the case, the panel is certainly
part of the building distribution system and is obviously capable of
supplying branch circuits, in which case the 120% rule would apply. I don't
like it and I don't necessarily agree with it, but based on my strict
interpretation of the code I can see why an AHJ would require application
of this section in this case.

Jason Szumlanski
Fafco Solar

On Wed, Jun 27, 2012 at 10:59 AM, Chris Mason
<cometenergysystems at gmail.com>wrote:

> 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****
>>>
>>> ** **
>>>
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>>
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>
>
> --
> Chris Mason
> President, Comet Systems Ltd
> www.cometenergysystems.com
>  Cell: 264.235.5670
> Skype: netconcepts
>
>
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