Feeding Solar Into Garage Sub-Panel, Back to Main at House Amperage

[NP Carling26]

Hi all. In the very early stages of planning a small solar system. I would like to put a 4kW Micro Inverter Based Grid Tie System on top of the roof of my new garage. Right now the garage has a 100A sub panel wired up with 2-2-2-4, large enough to be fed by a 90A breaker, however, currently it is only being fed by a 60A breaker, back to the house main panel, which is a 100A main panel. My first question is, being 4kW, will a 20A breaker connection at the subpanel be large enough, or too small? And my next question. I am aware of the NEC 120% rule. Being that it is a 100A breaker on a 100A bus bar, would the 60A (or in the future 90A) breaker violate this since it technically exceeds the rating,. even though at the garage it would be protected by a 20A breaker, and therefore no more than 20A would ever be backfed through the sub panel breaker?

Any help is greatly appreciated. If push comes to shove and we have to upgrade the main service to 200A, then it might come to that, but we are really trying to avoid having to take that route if possible.

[sensij]

The size of the PV breaker is carried forward as the OCPD size for all panels. For your 100 A main panel, that means that a 20 A breaker is the max that is supported (even when installed in a subpanel). As long as your inverter is something like 3800 W or smaller (with a rated output current of 16 A or less), you should be OK.

[NP Carling26]

The size of the PV breaker is carried forward as the OCPD size for all panels. For your 100 A main panel, that means that a 20 A breaker is the max that is supported (even when installed in a subpanel). As long as your inverter is something like 3800 W or smaller (with a rated output current of 16 A or less), you should be OK.

Well each Microinverter outputs 1.05A, and I planned on 16, which would output 16.8A, probably not Kosher with code. If need be, I could knock it down one panel and Microinverter. But from what it looks like you're saying is that as long as I have a 20A breaker protecting it in the sub-panel, I am good with running a larger breaker at the main panel for the entire garage?

[sensij]

What microinverter are you using? Yes, with a 20 A breaker in the sub-panel, you can have a larger breaker on the subpanel feed from the main. Note that the 120% rule only applies if the PV breaker is opposite the main supply breaker, so that means your sub-panel feed would need to be at the opposite end of the bus from the main supply. If your 100 A panel is centerfed, you can't use the 120% rule so simply.

[NP Carling26]

What microinverter are you using? Yes, with a 20 A breaker in the sub-panel, you can have a larger breaker on the subpanel feed from the main. Note that the 120% rule only applies if the PV breaker is opposite the main supply breaker, so that means your sub-panel feed would need to be at the opposite end of the bus from the main supply. If your 100 A panel is centerfed, you can't use the 120% rule so simply.

I'm looking into an Ubiquiti SunMax system. The specs for their MicroInverters can be found here on Page 9: https://dl.ubnt.com/datasheets/sunma..._Datasheet.pdf

And my main is top fed, and the breaker feeding the garage can be moved to the bottom if that's the case.

[sensij]

The data sheet explicitly says up to 16 can be put on a 20 A circuit, so whether they are rounding to 1.0 or perhaps using the 250 W rated power / 240 V = 1.04 A, somehow they seem to think it will comply with NEC. It might be worth a call into them to ask, before you buy that 16th panel.

[solarix]

Does your POCO require an AC disconnect safety switch (maybe a dedicated solar kwh meter as well) and does it need to be located by the main panel? If so, you might as well bite the bullet and run the solar circuit directly to the main. However, sometimes they will allow locating the AC disconnect by your garage and just require a permanent map at the main panel directing emergency personnel where to find the disconnect. (I know the AC disconnect is just superfluous as the inverters naturally shut down with loss of grid - but its an OSHA rule that causes a whole bunch of grief sometimes and for very little gain...)
Is this a DIY project? Your POCO is going to require a licensed electrician to do the interconnection, and they ought ot know what is the practice in your area...

[NP Carling26]

Does your POCO require an AC disconnect safety switch (maybe a dedicated solar kwh meter as well) and does it need to be located by the main panel? If so, you might as well bite the bullet and run the solar circuit directly to the main. However, sometimes they will allow locating the AC disconnect by your garage and just require a permanent map at the main panel directing emergency personnel where to find the disconnect. (I know the AC disconnect is just superfluous as the inverters naturally shut down with loss of grid - but its an OSHA rule that causes a whole bunch of grief sometimes and for very little gain...)
Is this a DIY project? Your POCO is going to require a licensed electrician to do the interconnection, and they ought ot know what is the practice in your area...

It is a personal project. The POCO does require an AC disconnect, however, it can be located on the garage as long as, like you said, it is noted at the main service entrance that there is a solar disconnect on the garage. I have to look into the solar meter aspect. I plan on doing as much of the work as possible myself, and may have an electrician friend come over here for the "final connection" to the grid to satisfy the POCO, as my AHJ is pretty lenient as long as it's up to code.

EDIT: After carefully reviewing the contract with my power company (PP&L) for customer generation, it appears as though I can do all work myself. I have to get my AHJ to inspect and sign off, and can be inspected by the POCO if they choose so, but there is no specified requirement within the contract that it be an electrician, I can simply check that the equipment is "owner-installed" on the contract/form.