[RE-wrenches] Enphase grid tie question

Jason Szumlanski jason at fafcosolar.com
Tue Jun 26 05:27:39 PDT 2012


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


On Tue, Jun 26, 2012 at 7:25 AM, Chris Mason
<cometenergysystems at gmail.com>wrote:

> I would use a 200A sub panel with 8 x 25A breakers to accumulate the PV
> output, as I doubt you could install 8 3Ph breakers in your main panel, and
> why would you want to?
> You shouldn't need to observe the 120% rule for the main breaker on the
> main panel as there are no loads. Same for the panel rating.
> The main panel would need a 200A breaker for the PV input and 120% comes
> into play. Be careful that breaker is rated as bi-directional.
>
> Chris
>
>
> On Mon, Jun 25, 2012 at 10:39 PM, Jesse Dahl <dahlsolar at gmail.com> wrote:
>
>>
>> All,
>>
>> I'm looking for ideas on the electrical integration of the following
>> system:
>>
>> 800A service
>> PV system employing enphase 215W inverters broken down into 8 output
>> circuits.  Each circuit will be 208V 3 phase 20A.
>>
>> The question is on how to grid tie this.  My first thought is a sub panel
>> for PV intertie only.  I saw a similar diagram on the enphase website,
>> using 5 circuits and a 125A panel....
>>
>> What are people doing in situations like this?
>>
>> Thanks as always.
>>
>>
>> Jesse
>> Sent from my iPad!!!
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>
>
> --
> Chris Mason
> President, Comet Systems Ltd
> www.cometenergysystems.com
> Cell: 264.235.5670
> Skype: netconcepts
>
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