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Grid tie with 100 amp service panel at house and 100 amp service panel at barn 300-350 ft away...

road glide

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Newbie here.......Reading about the rule of 120?? A little confused.....
I would like to install the 6 panel array 500 watt bifacial near the barn 40-50 ft away (clearest southern exposure). And wire a 20 amp double pole into the barn panel, maybe 30 if i could get away with that?
Our barn has a GE 100 amp panel but minimal breakers. We have heavy usage right now because we are running both 13500 BTU A/C units with 1250 wattage each....The barn panel is fed 350 feet away sched 40 underground #4 three conductors #8 grnd but I installed a 60 amp breaker at the main service panel for the barn. (minus 40 amps, just in case)
Main service GE 100 amp. 100amp main, 100amp house. The house uses minimal electric....I have captured specs on what we use.
Around 42 amps unless we use the dryer at 18 amps...That includes freezer 3 amps, refrig 6.5 amps, gas oven 5 amp for igniter, microwave 13.8 amps, room A/C 11k btu at 10 amps....We have on demand lp gas water heater, gas stove, wood burning FP for winter. Why I am reviewing this, our PG&E bill used to be 100 -200 (300 summer) monthly now with more folks here we are at $408, $528 and this month is tracking for $580 this next billing cycle...
Don't know whether to install a 2kw (and chip away at the monthly) or larger system 4-5kw (or shut it down during winter??)
I like the SMA grid tie inverter 3800 that has a dc disconnect at the inverter. Also studying micro inverters (somewhat confusing)
So configure it at the rack and then run a 40-50 ft #6 awg underground and tie into the barn sub panel. That would then feed the main panel and offset some of our utilization....
So after all that...My question is: can the PV array (3-4KW) tie into the barn sub panel via a 20-30 amp double pole 240 volt breaker? The energy would then travel back to main panel and back feed at the 60amp breaker. Max amps should be 20 amp or 30 amp correct? Would this be overloading my main house panel based on rule of 120?
 
....Reading about the rule of 120?? A little confused.....
NEC 705.12(D) (the 120% rule) through me for a loop at first too, but it's a good rule to follow and is explained nicely here. The gist of it is that the main might not trip when there's a short that exceeds the busbar's capacity - which can happen because both solar and grid are supplying power. Say the main is rated for 100 amps and has a 100 amp breaker and solar a 60 amp breaker. Solar is providing 60 amps and the grid 90 with a 150 amp short. The grid breaker won't trip as it's less than 100 and the solar breaker won't trip. But the backplane is carrying 150 amps.

... 6 panel array 500 watts...wire a 20 amp double pole...
At 240V that 6 x 500 / 240 = 12.5 amps. A 20 amp breaker should be fine.

....can a PV array (3-4KW) tie into the barn sub panel via a 20-30 amp double pole 240 volt breaker? The energy would then travel back to main panel and back feed at the 60amp breaker. Max amps should be 20 amp or 30 amp correct? Would this be overloading my main house panel based on rule of 120?
4000 watts / 240 v = 16 amps. Still good with a 20 amp breaker.

...could this be overloading my main house panel based on rule of 120?...
100 amp subpanel with 100 amp breaker
16 amps solar

Rule of 120 is 1.2 x 100 = 120 amps max.
20 amp solar breaker + 100 amp main panel breaker = 120 amps. ✅
30 amp solar breaker + 100 amp main panel breaker = 130 amps. ❌

But...
30 amp solar breaker + 90 amp main panel breaker = 120 amps. ✅*​

(*Assuming 90 amps on the main breaker can power everything when the sun is down).

Hope that helps!
 
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Thanks for that....I may change the main breaker to a 90 to get to a 30 amp solar...not sure...yet..probably won't
Also panels are 390 x 6 = 2340...any gain from backside would be gravy at max 105 each =630. total 2945??
So a 3kw grid tie inverter would be more inline. I read where it the PV's don't generate enough DC power, the inverter won't come on. So things need to be matched. Panel wattage could be slightly higher because of efficiency...
 
Not going into your numbers but I have 9.6kw worth of grid tie micro inverters split (20amps each) onto two 30amp 10ga wired circuits, tied into a breaker box in the field (think your barn) and then 250 feet back to the house and a 50 amp breaker at the house panel. At peak performance I am seeing about 40-41 amps with about 2% (5v drop) over that 250foot (4ga wired) run. Just guessing you should be able to do 30amps or so on your 350 feet run. I didn't look it up, so I could be wrong. Interestingly, at the house the voltage is 245v and at the remote box 250v when things are going full force.
 
Thanks for that....I may change the main breaker to a 90 to get to a 30 amp solar...not sure...yet..probably won't
A 20A breaker is good for up to a 3840W inverter. 3840 / 240V = 16A x 1.25 = 20A. Don't oversize thinking more is better, it's not. Use the proper size breaker.

Edit: Unless you are saying you may get a 4000W inverter, in which case yes, you will need to downsize to a 90A breaker and use a 30A breaker for the inverter. Rereading your quote, I'm not sure which you meant.
 
A 20A breaker is good for up to a 3840W inverter. 3840 / 240V = 16A x 1.25 = 20A. Don't oversize thinking more is better, it's not. Use the proper size breaker.

Edit: Unless you are saying you may get a 4000W inverter, in which case yes, you will need to downsize to a 90A breaker and use a 30A breaker for the inverter. Rereading your quote, I'm not sure which you meant.
I would have to pull the PGE meter to change out the 100 amp main to 90 amp main....like I said probably won't...not enough gain for the hassle
I guess what I am asking would a 3000 watt array work with a 4000 watt GT inverter? Need to do more homework on where the GT inverter becomes active and not dormant.....
 
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Not going into your numbers but I have 9.6kw worth of grid tie micro inverters split (20amps each) onto two 30amp 10ga wired circuits, tied into a breaker box in the field (think your barn) and then 250 feet back to the house and a 50 amp breaker at the house panel. At peak performance I am seeing about 40-41 amps with about 2% (5v drop) over that 250foot (4ga wired) run. Just guessing you should be able to do 30amps or so on your 350 feet run. I didn't look it up, so I could be wrong. Interestingly, at the house the voltage is 245v and at the remote box 250v when things are going full force.
Do you have a 200 amp main service at the house?
 
I guess what I am asking would a 3000 watt array work with a 4000 watt GT inverter? Need to do more homework on where the GT inverter becomes active and not dormant.....
Most people recommend oversizing the array to the inverter. If you have 3000W, and don't plan on adding in the future, I'd go with a 3000W inverter and not mess with the main breaker. Most inverters have online calculators that will predict the output. See if the one you are interested in has one, may help you decide ideal size.
 
...any gain from backside would be gravy at max ...
Yes and no. It's gravy you have to pay for.

That is you might not get additional power out of the backside of your bifacial panels, but even if you never see any power out of the back you still have to wire and size everything as if you were getting full possible power because you might see it.

That said, you can hedge your bifacial bets by increasing the albedo around the panels and making sure they're at the proper elevation. SAM knows about bifacial panels, so it's a great tool to answer questions like: if 4 tons of snow-white pea gravel costs $x and needs $y yearly maintenance but increases the albedo by .5, is it worth it?
 
Yes and no. It's gravy you have to pay for.

That is you might not get additional power out of the backside of your bifacial panels, but even if you never see any power out of the back you still have to wire and size everything as if you were getting full possible power because you might see it.

That said, you can hedge your bifacial bets by increasing the albedo around the panels and making sure they're at the proper elevation. SAM knows about bifacial panels, so it's a great tool to answer questions like: if 4 tons of snow-white pea gravel costs $x and needs $y yearly maintenance but increases the albedo by .5, is it worth it?
Thank you for the link SAM......
Stupid me...actually bifacials have to be wired twice? Because actually they are 2 panels.....Now I am not seeing the gain.
I am viewing ebay New ZNSHINE Solar Bifacial 500's, 72 split cell ($199 each +ship)....With 3 diodes, Junction box IP68...I'll have to inquire if this is wired as one panel and not two....that would make a big difference.

I saw a pole mount that holds 6 panels (maybe 8) ....Really don't want to buy 2 pole mounts ($3k) 12 panels @300 watts =3600 (or 16 panels @300 watts = 4800)
The bifacials at 390w on the front side (10 panels @390w = 3900w (not including any gain from the backsides..maybe 300w-500w extra ?)

I'm coming to the conclusion that 20 amp double at my sub panel in the barn is max. From input here on the forum I can install a 4000w GTI, 17amp AC output or a 3800 GTI with 16.5amp AC output. So, if I have an array say at 4800 and a 4000 or 3800 GTI I should be okay?
Also I read, from a 3800w GTI, once the DC goes over the rating of 3800...that's all you are going to get...Does say 4200 watts incoming from the panels hurt or overload the GTI...I wouldn't think so..
Back to drawing board and research available PV panels to see if I can get over 4k wattage. And talk to the vendor on those 390w (500w) bifacialss-l500.png
 
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Oversizing the array by about 25% or more is very common. It will not harm the inverter (as long as you don't wire too many in series and have too high of a voltage. Remember that it will be very rare for you to get full rated output from the array. That's at 77F (25C), noon sun, perfect angle, no bird poop, the list goes on. So on the rare occasion that you do get almost full rating, the output of the inverter will just clip to output 3800W. But during the rest of the day, you'll have increased your output having a bigger array that is putting out less than its rating, so even with occasional clipping, your daily output is higher.

Notice the inverter is 3800W, not 4000W, that's so you can use the 20A breaker in a 100A box.
 
Oversizing the array by about 25% or more is very common. It will not harm the inverter (as long as you don't wire too many in series and have too high of a voltage. Remember that it will be very rare for you to get full rated output from the array. That's at 77F (25C), noon sun, perfect angle, no bird poop, the list goes on. So on the rare occasion that you do get almost full rating, the output of the inverter will just clip to output 3800W. But during the rest of the day, you'll have increased your output having a bigger array that is putting out less than its rating, so even with occasional clipping, your daily output is higher.

Notice the inverter is 3800W, not 4000W, that's so you can use the 20A breaker in a 100A box.
I'm beginning a spread sheet today on estimated cost of system....I am getting to that point now....Because 20a breaker is my max. So the solar array and GTI, racking, hardware, etc are still in limbo....
 
... bifacials have to be wired twice? Because actually they are 2 panels.....

Apologies to have left you with that impression! No, it's like any other panel with just a plus/minus connector. I didn't see it in the datasheet for the product you mentions, so I grabbed some data from an LG datasheet as an example:

1598027561298.png

This LG panel is rated for a maximum of 395 watts and a temperature correction of -0.36 %/C.

If it were a normal panel, you're all set. Say you live in Montana and the lowest recorded low temperature is -30F (-34C). If you had 10 panels it would be 395 x 10 = 3,950 watts. The temperature correction for your coldest day is -.36 %/C x (25 - (-34)) = +21.24%.

So, the most power that could be generated would be 3950x1.214 = 4,789 watts (~20 amps @ 240V). That would be the maximum you need to size your gear on if it were a normal panel.

But, with bifacial panels you need to consider the albedo which gives the % bifacial increase. For example, sunshine on snow (our Montana case) has a very high albedo, so you might see +30% power. So, in that case you'd have to size your gear for 4789 x 1.30 = 6226 Watts (~26 amps @ 240).

So, even though you might never see 6,226 watts... you might still get that on those rare days at solar noon when it's very cold if the panels are tilted right (all other times you'll get less) and should size appropriately for it. As @SolarQueen says, it's common for inverters to clip the peaks off - but you need to make sure the volts/amps won't fry the wires or gear when it does hit the peak.

That's what I meant by "you pay for it", it'll cost a bit more up front for gear that handles more volts/amps. The good news is with SAM you can figure out if it's really worth it. SAM is pretty complex. I recommend the video tutorials and built in help system.
 
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These peaked my curiosity....dangerous to view Google or eBay...LOL because I could imagine viewer panels to get to 4kw plus 800-1000 more when conditions allowed it...
My personal dilemma is I know 4kw system is where I am at (20amp breaker). Trying to get an array to cover that +-25% (800-1000watts) on ONE pole mount instead of buying 2 pole mounts...$1500 vs $3000. Really don't want to do a ground rack...but maybe forced to...
Update...just called the vendor on the pole mount...they only offer the 6 panel array....so I would need 2 pole mounts with 12 panels :unsure:
I searched....Canadian Solar offers a 400watt panel $227...that might work for 12 panels....
 
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Oversizing the array by about 25% or more is very common. It will not harm the inverter (as long as you don't wire too many in series and have too high of a voltage. Remember that it will be very rare for you to get full rated output from the array. That's at 77F (25C), noon sun, perfect angle, no bird poop, the list goes on. So on the rare occasion that you do get almost full rating, the output of the inverter will just clip to output 3800W. But during the rest of the day, you'll have increased your output having a bigger array that is putting out less than its rating, so even with occasional clipping, your daily output is higher.

Notice the inverter is 3800W, not 4000W, that's so you can use the 20A breaker in a 100A box.

Do you know if oversizing will work with the Sol-Ark 5K? I just had my old Delta Solivia 3.8TL replaced today with the 5K with the intention of increasing my small 3kw array to something much larger. The Sol-Ark 5K indicates that it maxes out at 6.5kw of solar, but I would like to catch as much of that on the side of the roof that faces the west as the side that faces the east, so i'm wondering if oversizing will work with the 5K. Thoughts?

P.S. Love your videos - i've learned a lot from them :)
 
If you do want to go over, they have the 8k and 12k models.

As I'm sure you're aware you'll only see maximum array rated power when the sun angle is perfectly aligned to the panel angle and it's solar noon and even then only for a few hours and probably only on cold days. But, going over the max power for any length of time can fry the unit, so not worth it IMO.

If Sol-Ark limits over-paneling to 6.5 kW, I'd say don't go over unless you've very carefully analyzed the power from your existing system and know your configuration never exceeds 5 kW.
 
If you do want to go over, they have the 8k and 12k models.

As I'm sure you're aware you'll only see maximum array rated power when the sun angle is perfectly aligned to the panel angle and it's solar noon and even then only for a few hours and probably only on cold days. But, going over the max power for any length of time can fry the unit, so not worth it IMO.

If Sol-Ark limits over-paneling to 6.5 kW, I'd say don't go over unless you've very carefully analyzed the power from your existing system and know your configuration never exceeds 5 kW.

I'll contact Sol-Ark support to get the details on this i.e. whether the 5k clips to 6.5kw if the panel input goes above that or not. I already have the 5k installed and tbh I was aiming for 6.5-7kw, so 6.5kw max is no problem.
 
If you do want to go over, they have the 8k and 12k models.

As I'm sure you're aware you'll only see maximum array rated power when the sun angle is perfectly aligned to the panel angle and it's solar noon and even then only for a few hours and probably only on cold days. But, going over the max power for any length of time can fry the unit, so not worth it IMO.

If Sol-Ark limits over-paneling to 6.5 kW, I'd say don't go over unless you've very carefully analyzed the power from your existing system and know your configuration never exceeds 5 kW.
It seems hard to imagine that someone designing a solar charger would not put current/power limit protection in the design. The EPEver Tracer has it.
 
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