[RE-wrenches] combiners and the 120% rule

[Jason Szumlanski]

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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