Adding Solar to garage

frofro19

Member
Location
VA.
Occupation
Master Electrician
Looking at wiring a detached garage approximately 260 feet from house. There's not much in the garage, I believe about 6 or 8 receptacles and 2 garage door openers and some LED lights. A 10/3 uf would probably surfice but with the price of copper these days, decided to go with #2 AL 4 conductors.
Was going to install a 100 amp sub-panel In the garage. Can the #2 AL be on a 100 amp breaker if the load is way under 90 amps? Load will probably never see 20 amps.
Also he is planning on installing, I believe 5 kw of solar at the garage to back-feed to the house. That's another reason for the upsize of the wire feeding from the house to the garage. The house has a 400 amp meterbase with 2-200 amp panels in the basement. The solar company told me all they needed was a 100 amp disconnect at the garage for them to attach their wires to. Does this sound correct and does the 2 AL work for this application?
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
#2 Al is only good for 90A.

#2 Al would be perfectly acceptable to have a 100A panel in the garage if the supply breaker at the house is 90A or smaller.

Double check that the EGC is sized correctly if the breaker at the house is smaller than usual for the conductors.

There are requirements for bus size vs breaker vs solar backfeed which I've not checked.

Jon
 

frofro19

Member
Location
VA.
Occupation
Master Electrician
#2 Al is only good for 90A.

#2 Al would be perfectly acceptable to have a 100A panel in the garage if the supply breaker at the house is 90A or smaller.

Double check that the EGC is sized correctly if the breaker at the house is smaller than usual for the conductors.

There are requirements for bus size vs breaker vs solar backfeed which I've not checked.

Jon
Yes, I was planning on installing a 90 amp breaker in the house panel and using #2,2,2,4 AL for the conductors from house to garage. Are the requirements for bus size vs breaker size vs solar backfeed something the solar company could tell me?
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
Are the requirements for bus size vs breaker size vs solar backfeed something the solar company could tell me?
Yes, but for a single PV inverter on site, it goes like this:

Take the PV inverter maximum continuous rating, convert it to amps, and multiply by 125%. PV inverters are often sized so that this comes out to a standard breaker size. For example, a 5 kW DC array might be on a 3.84kW AC inverter, which is 16A continuous at 240V, so the computation gives 20A. For any panel, add that value to the rating of the upstream (grid-wards) breaker protecting the panel (e.g. 90A for the panel in the garage). Then every panel between the PV inverter and the grid (including the service panel) has to satisfy one of these rules:

- The sum is no more than the busbar rating.
- The breaker leading towards the PV inverter is at the opposite end of the bus as the breaker protecting the panel, and the sum is no more than 120% of the busbar rating
- The sum of all breakers in the panel (per leg) excluding the breaker protecting the panel is no more than the busbar rating.

Cheers, Wayne
 

Carultch

Senior Member
Location
Massachusetts
Take the PV inverter maximum continuous rating, convert it to amps, and multiply by 125%.

Not all inverters have a current rating that directly relates to their power rating. Some inverters have headroom on their current value in amps for one reason or another (kVA vs kW and compensating for lower-than-nominal grid voltages as examples). The amp value on the inverter datasheet matters more for this calculation, than the amp value that you would infer from I=P/V or I=P/V/sqrt(3) for three phase.

It will usually be called "Rated output current" or "maximum continuous output current", or some other term that has the same meaning.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
I'd install a 125 rated subpanel at the garage, not a 100amp rated. (The former are more common anyway.)
Put the AL #4 on a 90A breaker, that will allow a 60A solar breaker as far as the garage subpanel is concerned, which is roughly 10kW of solar inverter, much more than you say you need.
The main panel will still limit the size of the inverter(s). I presume you'll still be backfeeding one of the 200A panels. That will limit you to 7.6kW (32A) of inverter power, also more than you say you need. That could be increased, if needed, by downsizing the 200A breaker to 175A (if load calc checks out, but it probably would).

The solar company's advice to you was not precise in my opinion.
 

frofro19

Member
Location
VA.
Occupation
Master Electrician
I'd install a 125 rated subpanel at the garage, not a 100amp rated. (The former are more common anyway.)
Put the AL #4 on a 90A breaker, that will allow a 60A solar breaker as far as the garage subpanel is concerned, which is roughly 10kW of solar inverter, much more than you say you need.
The main panel will still limit the size of the inverter(s). I presume you'll still be backfeeding one of the 200A panels. That will limit you to 7.6kW (32A) of inverter power, also more than you say you need. That could be increased, if needed, by downsizing the 200A breaker to 175A (if load calc checks out, but it probably would).

The solar company's advice to you was not precise in my
 

frofro19

Member
Location
VA.
Occupation
Master Electrician
Why #4 instead of #2AL? #2AL is more accessible I'm my area and give room for future expansion. Yes they are planning on back feeding into one of the 200 amp panels.
 
Top