grid-tie on a detached garage?

System

Hello all - nice forum you've got here.

I'm looking at putting a new roof on my detached garage and I'm thinking about doing a grid-tie solar system there while I'm at it.

The garage is currently wired for power with a 240v single phase circuit from the 200 amp main panel going to a small 2-breaker panel in the garage.

My question is - can I do the grid-tie into the garage circuit(without tearing up my yard for new wiring), or do I have to run new wires into the main panel, or does the grid tie have to live between the main panel and the meter?

Thanks for any input.

mike95490

Re: grid-tie on a detached garage?

hi PORTLAND, Or INTERNETWORKS (If you don't tell us where you are, we have to track you down - Europe does use a 240 single phase, 50hz)

How do you get your 240 V single phase ? Most of the US uses a 120V split phase, and between the 2 split phases, you can get 240V

If your sub panel has capacity to add 2 more 15 A breakers ( one on each phase ), and your main panel can handle it, you should be OK.

BUT the Electric Co. may want an accessible cut-off at the main meter, they don't want to have to dig around looking for the solar breakers.

BB.

Re: grid-tie on a detached garage?

It depends on the size of your branch circuit to your garage... For example, If I recall correctly, for a 125 amp service to your garage, you will be able to run a 20 amp breaker to your solar system--or a maximum of about 240vac*20a*0.8=3,840watts...

For a residential system, you can only run a maximum of ~20% (I am sorry, I don't recall the exact number) of your utility service/sub-panel breaker for your solar system. And you can only run a circuit at a maximum of 80% of its rated capacity. For a commercial building--IIRC, basically, add up all the breakers and subtract from rated capacity of the panel.

So, depending on your sub-panel size in the garage will define how large a system you can have before you have to rewire and dig up your yard.

Also, in most places, the utility requires a easy to get to solar disconnect switch--typically somewhere near your meter is the best place. If you have dogs/etc. that limit access--that will be an issue.

Lastly, if, for example, your utility feed is on your house, and the garage is some distance away and you don't have a 125+ amp panel in the garage (you would have to rewire the panel)--you also have the option of placing the panels on the garage and trenching the DC solar cables to your home and installing the inverter/disconnect there and wiring to your main distribution breaker box (or put the panels/inverter on the garage and run the 240 VAC circuit back to the home)... This would probably be the better solution--you will save a lot on copper wire / installation since you will not be running a large branch circuit/sub-panel to your garage... Just enough copper to support the solar system.

-Bill

System2

Re: grid-tie on a detached garage?

mike90045 wrote: »

hi PORTLAND, Or INTERNETWORKS (If you don't tell us where you are, we have to track you down - Europe does use a 240 single phase, 50hz)

How do you get your 240 V single phase ? Most of the US uses a 120V split phase, and between the 2 split phases, you can get 240V

If your sub panel has capacity to add 2 more 15 A breakers ( one on each phase ), and your main panel can handle it, you should be OK.

BUT the Electric Co. may want an accessible cut-off at the main meter, they don't want to have to dig around looking for the solar breakers.

Sorry, Portland Oregon, PGE customer.

Exactly what you said, split phase. I believe the 240v breaker on the subpanel is 50A, so if I'm calculating correctly that's 960w? Which is actually a fair sized setup if I'm not looking to do battery-based off grid operation - correct me if I'm wrong?

My subpanel may not have space for two more breakers, however I could expand it.

Now what's this about a cutoff? Does it have to cut the solar specifically or couldn't the cutoff work on the entire main panel if the need arises?

I realize I should probably start a different thread for this, but I was thinking about doing a metal roof and the thin-film solar material that comes in long rolls? Sorry I'm not totally up to speed on the terminology yet... any comments on this idea?

Thanks again.

BB.

Re: grid-tie on a detached garage?

PG&E is one of the (few?) utilities that may not require you install a lineman accessible disconnect.

Regarding the sizing of the system, 1kW is on the small size for a grid tied system... 3kW seems to be a sweat spot for the best $$$/watt pricing.

Also, check your metering laws... In California, we have to use E6 Time of Use metering... A small grid tied system on a home where people use a lot of power (especially afternoon/evening)--adding a small grid tie system can actually cause your electric bill to be higher (because of the rate plan penalties for afternoon/evening use).

-Bill

newenergy

Re: grid-tie on a detached garage?

690.64 (B)(2)

The sum of the ampere ratings of overcurrent devices in circuits supplying power to a busbar or conductor shall not exceed 120 percent of the rating of the busbar or conductor.

---

My interpretation - which has been agreed on by one inspector anyway

All breakers that supply current must add up to 120% or less of the busbar/enclosure rating of the panel. Generally, this will just be the main and the breaker for the PV. If the panel/busbar are rated for 125 amp then the total of main + PV breakers must be less than or equal to 150 amps.

This could mean a 125amp main and a 25 amp PV breaker or it could be a 100 amp main and a 50 amp PV breaker.



Also, BB, when you calculated the max power - 240Vac*20amp*.8 - why * .8?
Your max AC power should be 240Vac*20amp(in that example) = 4800W AC
and 4800W AC / .8 = 6000W DC
no?

BB.

Re: grid-tie on a detached garage?

NewEnergy,

Your explination sounds like the correct interpretation--the intent is to keep everything safe--and your method, as far as I can tell, does meet the intent.

Also, if there are sub-panels involved--those same questions need to be asked/answered for upstream breaker panels too...

newenergy wrote: »

690.64 (B)(2)
Also, BB, when you calculated the max power - 240Vac*20amp*.8 - why * .8?
Your max AC power should be 240Vac*20amp(in that example) = 4800W AC and 4800W AC / .8 = 6000W DC
no?

The assumption is that NEC will only let you install permanent loads (fixed installations vs plugs like wall outlets) up to 80% of the branch circuit rating...

For example, a 15 amp branch can only have 15a*0.8=12 amp wired load...
So, my assumption is that would be true going back the other way--the solar generation source can only drive 80% of the branch circuit's current rating (one could argue 240 vs 220 VAC in the formula--Grid Tied inverters are constant power devices... As the AC Mains voltage falls, the output current rises--to keep P=I*V=constant).

The issue is that you have two devices that limit current/power... One is the solar panels themselves--but they are not very accurate and the sun/weather does not always perform as expected... So while 3,500 watts of solar panels are PTC rated around 3,000 watts (including inverter losses)--on cold bright days, with a little cloud/snow reflections, the panels can exceed their ratings... However, in my case, my inverter is also limited to 3,000 watts maximum by design--so even if I have larger panels, the inverter will not pass more energy.

Jim/Crewzer is the expert at the NEC ratings/deratings... It works out that the max current the circuit has to be designed for is something like 1.25 (1/0.80 for NEC branch circuit rating) times 1.25 (allow for solar panel safety factor) = 1.5625 wiring up-rating over solar panel expected output.

There are other discussions that we have gotten into here before about NEC's sometimes a bit crazy rules for solar charge controllers / Grid Tied inverters that some of us don't agree with--but that is for another post/thread.

-Bill

newenergy

Re: grid-tie on a detached garage?

Oh yeah, Continuous load, forgot about that. I just thought the .8 was a deration of the PV output.

Roderick

Re: grid-tie on a detached garage?

Just so there's no confusion, the original poster seems to be on PGE (Portland General Electric), not PG&E (Pacific Gas and Electric).

If there's already a service panel in the garage, I see no reason why the solar couldn't be grid-tied at that point.

Be sure to check the insolation maps for your area. I lived in Beaverton for a while, and seem to recall there was a lot of cloudy and foggy weather. Maybe climate has changed, that was decades ago.

mike95490

Re: grid-tie on a detached garage?

Roderick wrote: »

Maybe climate has changed, that was decades ago.

How could the climate change ?

--
global warming - ever since the last ice age