Solar on detached - wire size for feeding back?

[jpcjguy]

Hi all,

So I am about to start a 48x30 detached and was planning on have 150A in the garage. (largest breaker I can get in existing house panel) Probably more than I need but that is another discussion.
My question is that my detached roof is positioned so that theoretically each gable side would get morning/afternoon sun with zero trees around. appears to be a great setup for it.
Since I only want to run the wire to the garage once - wondering if upsizing would be smart for future proof if I ever decide to go that route? or would I need an entirely new line from the garage back to the house? Run from house panel to garage panel is about 225ish feet....
Just thinking out loud here....

 

[mm08822]

Hi all,

So I am about to start a 48x30 detached and was planning on have 150A in the garage. (largest breaker I can get in existing house panel) Probably more than I need but that is another discussion.
My question is that my detached roof is positioned so that theoretically each gable side would get morning/afternoon sun with zero trees around. appears to be a great setup for it.
Since I only want to run the wire to the garage once - wondering if upsizing would be smart for future proof if I ever decide to go that route? or would I need an entirely new line from the garage back to the house? Run from house panel to garage panel is about 225ish feet....
Just thinking out loud here....

Are you sure it is 150 and not 125A? The panel label will specify. Otherwise provide pic of panel label showing panel part #.

Answer depends on size of solar system and max outputs of both solar systems. (Each solar panel plane requires a separate string inverter unless micro-inverters used - then consider it one system.)

If your solar system will output more than 80% of the feeder rating then the feeder would need to increase so it does not exceed 80%.

Another point of concern is that the combined inputs of the feeder and the solar can not exceed 120% of the panel buss rating. So you may need to purchase a larger panel (200/225a) to accommodate this requirement.
(This also holds true for the main panel.)


At 225', the feeder should be upsized to limit voltage drop regardless of the solar.

 

[Git]

I am not an electrician, but when they installed my Solar, it required a breaker to be installed in the main panel (it had to be on the opposite end of the main breaker) and they also had to derate the main panel down from 225 to 200 amps

120% rule - The sum of their breakers (grid and solar) cannot equal more than 120% of the busbar rating of the breaker box

Plus, Solar is considered to be 'continuous load' so that breaker needs to be 125% of the output of the inverter

I would suggest you get some solar installers to come out and see what they have to say. You may be better off right now just adding an extra conduit to 'future proof' things, but you may have some problems with your main panel

 

[mm08822]

I just SWAG'd a 30 KW system assuming the entire roof is covered (worst case) and that is ~ 100 panels. At full output, the system could be outputting 100 - 125a.

Not sure of the size you have in mind but that's as much as the roof can fit.

 

[Git]

In California, most grid-tied, residential solar installs are limited to 120% of the last year's usage. You may want to check with the power company in your area to see if they have any limitations like that. Lastly, if you 'over-produce' the power companies usually only pays the wholesale rate so if you have too much capacity, you're just throwing money down the drain

 

[nadogail]

Writing from deep Southern Ca, My advice would be to call your local utility and learn what your options are.

IMHO, they hold all the cards when it comes to connecting to their system.

If you decide you don't like the way they are playing the game, your option, again IMHO, will be pretty much limited to cutting the wires and going off the grid.

 

[mm08822]

Op, look here for initial info:

https://www.solarpowerrocks.com/virginia/#netmeter

 

[dcg9381]

I just SWAG'd a 30 KW system assuming the entire roof is covered (worst case) and that is ~ 100 panels. At full output, the system could be outputting 100 - 125a.

Not sure of the size you have in mind but that's as much as the roof can fit.

That's a "big ***" system and would cost a ton. Off the top of my head, $40k in hardware.

People are right about the 125% rating.

Note, I'm an "installer" and unless you are burning through an absolute ton of power OR you have some crazy deal the the power company will cut you a check at near-retail prices (not going to happen) - a 30KW system will probably never pay back in your lifetime. And a 30KW system that is oriented two different directions will produce less than a south-facing 30KW system at the right tilt.

So I am about to start a 48x30 detached and was planning on have 150A in the garage. (largest breaker I can get in existing house panel) Probably more than I need but that is another discussion.

I have 2400 sqft, air conditioned, fridge, kegerator, hot tub, water pump, multiple RV hookups. I weld. I'll never use the 90A that I have. It's very likely that running 150A to your garage will be the same wire as running 200A (off the top of my head). The economic point is right around 90-100 amps, after that wire starts getting more expensive. You have a long run, so you'll need to look at voltage drop - which means even bigger wire sometimes.

My question is that my detached roof is positioned so that theoretically each gable side would get morning/afternoon sun with zero trees around. appears to be a great setup for it.

You're 50% right. One side might be a great setup for evening, one side a great setup for morning. What is your actual compass orientation of your detatched roof(s)?
What is the pitch of your roof?

The ideal orientation is a single roof (slant roof) that faces south and can pickup sun morning through evening.

Note, I've got a 6KW system. It's oriented south. I removed the shade trees. It's on a 1/12 roof (not ideal pitch) - so it probably does 5.4 - 5.8 KW peak.

You would likely be much better off ground racking your panels, pointing them south at correct tilt. Again, I don't think the economics (payback) of a 30KW system will work, even correctly oriented, unless you're running something at super-large residential or almost commercial scale.


Here's the other thing - your solar power will essentially be "used" by your garage, if it is needed. If you ran 100A to the garage and had 30A worth of solar, you've really got 130A before you'd trip that main, at least under full sun full power conditions.


Around here, not saying this is true in your location:
I can tie solar in at a sub panel, including the garage panel.
Pre-wire requirements are that I have a physical disconnect outside of the building for both the grid and for the array. So you may want to pre-wire for that stuff.
Modern solar setups don't really require physical disconnects due to Rapid Shut-Down Technology that (I believe) is accepted by later NEC code books, but many jurisdictions don't keep up and may want a feel-good disconnects.

 

[jpcjguy]

Thanks everyone for the info! Great points brought up - I am not planning on going off the grid. Currently I have twin 200A panels in my house. The largest breaker I can get for the panel is 125A (thanks mm for making me double check that!)
I think maybe the best idea is to drop another conduit in and call it a day.....

 

[wyliesdiesels]

This is a lot more complicated than just picking the max breaker and wire size you can go with.

you need to start with a call to your PoCo.

Next we need to know your loads in the garage and how many people.

Also, need to know main panel size and loads.

 

[mm08822]

I just SWAG'd a 30 KW system assuming the entire roof is covered (worst case) and that is ~ 100 panels. At full output, the system could be outputting 100 - 125a.

Not sure of the size you have in mind but that's as much as the roof can fit.

That's a "big ***" system and would cost a ton. Off the top of my head, $40k in hardware.

People are right about the 125% rating.

Note, I'm an "installer" and unless you are burning through an absolute ton of power OR you have some crazy deal the the power company will cut you a check at near-retail prices (not going to happen) - a 30KW system will probably never pay back in your lifetime. And a 30KW system that is oriented two different directions will produce less than a south-facing 30KW system at the right tilt.


I have 2400 sqft, air conditioned, fridge, kegerator, hot tub, water pump, multiple RV hookups. I weld. I'll never use the 90A that I have. It's very likely that running 150A to your garage will be the same wire as running 200A (off the top of my head). The economic point is right around 90-100 amps, after that wire starts getting more expensive. You have a long run, so you'll need to look at voltage drop - which means even bigger wire sometimes.

You're 50% right. One side might be a great setup for evening, one side a great setup for morning. What is your actual compass orientation of your detatched roof(s)?
What is the pitch of your roof?

The ideal orientation is a single roof (slant roof) that faces south and can pickup sun morning through evening.

Note, I've got a 6KW system. It's oriented south. I removed the shade trees. It's on a 1/12 roof (not ideal pitch) - so it probably does 5.4 - 5.8 KW peak.

You would likely be much better off ground racking your panels, pointing them south at correct tilt. Again, I don't think the economics (payback) of a 30KW system will work, even correctly oriented, unless you're running something at super-large residential or almost commercial scale.


Here's the other thing - your solar power will essentially be "used" by your garage, if it is needed. If you ran 100A to the garage and had 30A worth of solar, you've really got 130A before you'd trip that main, at least under full sun full power conditions.

Around here, not saying this is true in your location:
I can tie solar in at a sub panel, including the garage panel.
Pre-wire requirements are that I have a physical disconnect outside of the building for both the grid and for the array. So you may want to pre-wire for that stuff.
Modern solar setups don't really require physical disconnects due to Rapid Shut-Down Technology that (I believe) is accepted by later NEC code books, but many jurisdictions don't keep up and may want a feel-good disconnects.

Thanks everyone for the info! Great points brought up - I am not planning on going off the grid. Currently I have twin 200A panels in my house. The largest breaker I can get for the panel is 125A (thanks mm for making me double check that!)
I think maybe the best idea is to drop another conduit in and call it a day.....

Keep in mind the OP's purpose for the post "Since I only want to run the wire to the garage once - wondering if upsizing would be smart for future proof if I ever decide to go that route? or would I need an entirely new line from the garage back to the house?"

Since no details other than roof size and feeder size were given, it became an upper limit check to see what size system a 125/150a feeder could support and how much hardware can be thrown on the roof. System cost, economics, local state/poco regs not a concern for today's question.

This is not system design nor justification of a size but merely answering the OP's q about running a new feeder and installing a new panel - what should he do to prepare? All other existing hardware remains as is until a solar system is spec'd, then he can deal with existing conditions as part of that new project.

There isn't even justification for the feeder size. He never states building purpose or loads.

Best advice I can give is throw a 200a main cb panel in the garage and base the feeder ampacity on worst case load calcs completely supplied by the main panel. Compensate for voltage drop. 90/100a feeder I'm sure is enough.
Maybe the 125% requirement for a solar feedback ups it, but I doubt it. A 100a feeder can support an 80a inverter output which is ~ 20kw system.

The feeder will only carry the net amount of current provided by the main panel minus the solar inverter output. Figure worst case conditions and that is the feeder size needed. Also for worst case assume garage draw is zero.

Not sure why some are saying add another conduit - not needed b/c of the solar possibility. Just size the one feeder properly.

 

[SkeeterZX200]

I am going through all of this right now. I just built a 30x60 shop and installed a 100 amp panel. I thought I would be able to interconnect my solar to my shop, but was introduced to the 120% rule that has been mentioned.

Also, our local utility wanted the solar disconnect adjacent to my main meter. This was the second reason I could not tie into my sub panel.

If I were to do it all again. I would put another conduit in the ground and call it a day. I went thought extreme measures to make sure I was coordinated with the solar installer and utility, but in the end they trenched my yard and put in another conduit.

 

[Git]

It makes a lot of sense having the disconnect next to the main panel. Can't really expect the Fire Department running all over you property looking for disconnects. In my area, there is actually a placard attached to the main panel with a diagram showing where the panels are located, where the disconnects are, where they can get up on the roof etc.

I don't think you can ever go wrong running extra conduit when you already have a trench open

 

[mm08822]

I am going through all of this right now. I just built a 30x60 shop and installed a 100 amp panel. I thought I would be able to interconnect my solar to my shop, but was introduced to the 120% rule that has been mentioned.

Also, our local utility wanted the solar disconnect adjacent to my main meter. This was the second reason I could not tie into my sub panel.

If I were to do it all again. I would put another conduit in the ground and call it a day. I went thought extreme measures to make sure I was coordinated with the solar installer and utility, but in the end they trenched my yard and put in another conduit.

It makes a lot of sense having the disconnect next to the main panel. Can't really expect the Fire Department running all over you property looking for disconnects. In my area, there is actually a placard attached to the main panel with a diagram showing where the panels are located, where the disconnects are, where they can get up on the roof etc.

I don't think you can ever go wrong running extra conduit when you already have a trench open

Just looked back at my interconnect agreement with the POCO and they did strongly recommend the installation of an exterior AC disconnect for the solar inverter output near the meter. If installed, placards were required to state the purpose of the disconnect and depict equipment locations.

POCO specifically stated it was not required in the state net metering regulations. Your state requirements may vary.

If the OP were to install an exterior AC disconnect switch for the garage feeder near the POCO meter it would perform the same inverter output isolation purpose as if installed in a dedicated inverter output line back to the house. The POCO wants assurance that the inverter is isolated from the network.

Apparently there is still little comfort in the anti-islanding feature that inverters for sale in the US are required to have in order to pass UL1741 requirement.

OP, if you have the time, I would check this out if it is an acceptable method. If you get pinched for time and faced with closing the trench, then drop in the conduit.

 

[jpcjguy]

Great feedback everyone - a lot more to consider and who knows if solar will ever be a reality for me - so i will probably go with the advice of dropping in another 2" conduit and call it a day....