PV System Limits based on Service Panel Size

RacerChris

I received four quotes on a PV Solar GT system recently. Since i'm a newbie, I let each bidder provide whatever system they suggested. The sie ranged from 5kw to 8kw for my 3000 SF home with a Pool and a Pond. (I know, I should get rid of them to save real power...working on it)

Anyways, one instaler suggested that there is a limit to the size of the PV system based on my 200 Amp electric panel.

Can anyone confirm this and what the basis of this limit is?

Thanks all!

BB.

Re: PV System Limits based on Service Panel Size

Yea, you are only allowed to backfeed up to 120% of the service rating...

• 200 amp x 120% = 240 Amp maximum

So that would limit you to a pair of 40 amp (240 VAC) breakers derated by 80% or 32 Amps of solar.

• 32 amps * 240 volts = 7,680 watt rated inverter output (i.e., ~7k watt)

You can install a larger array--I use ~0.77 as the maximum "cost effective" derating for losses:

• 7,000 watts * 1/0.77 = 9,091 watts of solar array

You will get some clipping at 7kW mid day (by the gt inverter), especially in cool weather with a new/clean array... But it will not damage the inverter.

You can also "play games" with the main panel... you could, for example change to a 125 amp service breaker (if your normal home loads would allow it). Now:

• 240 amps - 125 amps = 115 amps of solar branch circuit

Anyway, there is lots to play with here... As you said in another thread--"the laws of diminishing return" applies here too.

-Bill

mike95490

Re: PV System Limits based on Service Panel Size

Yes, there is a limit depending on the size of the buss bars in the panel, and how heavily it is loaded already.

System2

Re: PV System Limits based on Service Panel Size

BB. wrote: »

Yea, you are only allowed to backfeed up to 120% of the service rating...

• 200 amp x 120% = 240 Amp maximum

So that would limit you to a pair of 40 amp (240 VAC) breakers derated by 80% or 32 Amps of solar.


-Bill

Hi Bill, et al,

sorry to revive this older thread. I have a older house with 100 amp panel. For this case I am assuming the quick answer would be to divide by 2 (i.e. 3840W max)?

Just for my sake, how do you get from 240 A to "a pair of 40A breakers", and is the dereating 80% of 40 to get down to 32A ? What about the second 40A?

thanks!

dave

niel

Re: PV System Limits based on Service Panel Size

dfigu,
for your panel that would be a correct assumption as a 100a panel has buses rated to 120a. i have no idea what the rest of your questioning is and i strongly suspect you are highly confused about some aspect of it. anyway, if you stay with the 100a main breaker that means that you can feed 20a of solar to it because the buses are rated for it. if the main breaker were to be say 75a then this adds extra capacity that the panel can be fed with solar. 120a-75a=45a in solar power. of course where the solar is inputted to the main panel it will do so with a breaker of the proper current rating the pvs may produce. does this clear it up for you?

BB.

Re: PV System Limits based on Service Panel Size

Not a problem... We are talking about North American 120/240 VAC split phase power...

Basically, a Center Tapped Transformer. 240 VAC between Line A and Line B; and 120 VAC between Line A to Neutral and Line B to Neutral.

So, when you have a 40 Amp breaker on Line A, and 40 Amp on Line B--The same 40 amps is flowing through both breakers (in series--and as a point of detail, there is no current flowing through the neutral if there is equal current in Line A and Line .

Nominally, you have 32 amps * 240 volts = 7,680 Watts for a 40 amp circuit and 1/2 that or 3,840 watts for a 20 amp 240 VAC circuit.

A Xantrex (Schneider) GT 3.8 kW rated unit is rated 16 amps maximum for a 240 VAC circuit. So that would be the largest unit you could fit to a 100 amp service (if I understand your question correctly).

-Bill

System2

Re: PV System Limits based on Service Panel Size

Ok I understand:

You can only feed back 40A (240 - 200) into 200A panel minus derating, 40*0.8 = 32A (7680 W).

And for 100 A service - 20*0.8= 16A (3480W).

Also per previous example, to maximize inverter utilization, the pv array should be sized 1/.77 or 1.298 times the max wattage (i.e 4519W in my case)? I suppose this takes into account the CEC ratings not the STC ratings, correct?

I assume this is a well know fudge factor - i.e. the city or power company inspector will allow this?

regards,

dave

solar_dave

Re: PV System Limits based on Service Panel Size

I have a pair of inverters each putting out 23 AMPS max combined in a sub panel with 30 amp 240 breakers each then a 60 amp line feed to my 200 main panel. To get to the max bus bar rating of 120% I had to derate the main breaker to 175 AMP. So now I have 175 AMP + 60 AMP = 235 AMP to get me under the 240 AMP max for the bus bar.

Even though the inverters combined will only output 46 amps max, this is the only way to get code approval. If I want to add more at some point (a crazy thought) I am not sure how that will happen. I have had electricians tell me they could add a 15 amp on the combiner panel. I am skeptical at the code approval for that. The code approval guys were pretty adamant at only a single solar back feed connection point in the main panel.

JB911

Re: PV System Limits based on Service Panel Size

This thread is what I was looking for....

I am getting a 11.5 KW PV system, with 48 panels, each with a micro inverter for my single resident home. I was told there is a max of 16 panels per 20A circuit, hence 60A total. I have a 100A box that was upgraded to a 125A box (150A bus?). The main breakers are going to still be 100 A (125A max wiring from street). Does that mean that I only have a maximum fo 50A to back feed from the solar (150-100= 50)? Initially the solar installers were trying to put in a 200A box with a 100 amp breaker to get a higher bus, but were nixed by the inspector (San Diego).

Is there a box upgrade that gives a higher rated bus but with a minimum (125A) breaker rating to pass code, and gives me more back feed availablility??

As a newbie, I hope I am not being too confusing!

solar_dave

Re: PV System Limits based on Service Panel Size

If you actually have a 150 buss bar with a 100 amp breaker main you should be OK. You get 150 amps X 120% = 180 amps total allowable feed into the panel. 180 amps - 100 amp main breaker = 80 amps allowable back feed. Now you still need your permitting authority to approve the design.

Now if the panel is rated at 125amps it is a different story. 125 Amps X 120% = 150 amps total allowable line in. 150 amps - 100 amp main breaker = 50 amp total back feed allowed. Most likely the AHJ won't allow you to derate the main breaker below 100 amps unless your electric load profile is pretty meager.

If the buss bar is actually less than 150 amps and you are installing 60 amps of back feed, you better plan on a main panel upgrade and a new heavier feed from the transformer as well.

Did the inspector say why they denied a 200 amp panel? Did the electrician not include the heavier feed from the street in the design? Did the utility deny the the heavier line addition to their transformer?

pS edit, just to be clear, the AHJ may disallow a 200 amp panel without a heavy enough line from the street. The thought process is that it is to easy to just put a larger breaker in the panel than the line to the transformer can handle. The whole system needs to be scaled accordingly, line, meter, panel.

ggunn

Re: PV System Limits based on Service Panel Size

mike95490 wrote: »

Yes, there is a limit depending on the size of the buss bars in the panel, and how heavily it is loaded already.

Just for clarity, "loading" implies to me breakers which supply loads, and load breakers do not figure into the 120% calculation, only breakers feeding the busbars matter.

JB911

Re: PV System Limits based on Service Panel Size

Thanks for the quick reply.

The new panel is a 125 amp, so the buss should be 150. The line from the street is now 125 amp rated and it would be a 250+ foot run from the street to trench a new 200 amp line ($$$), hence the gyrations to get the current system to work. I see that there are "Solar Ready" panels (Siemens) that have a separate buss that directly link to the meter feed, so that it seems to leave the total 150 amp buss free.....does that sound right?

solar_dave

Re: PV System Limits based on Service Panel Size

ggunn wrote: »

Just for clarity, "loading" implies to me breakers which supply loads, and load breakers do not figure into the 120% calculation, only breakers feeding the busbars matter.

True but they may figure in the derating of the main breaker if a lowering of the utility side breaker is needed to met the 120% rule. For me this was the case as the AHJ would not allow me to lower my main house breaker because of the load survey. This forced a complete upgrade of the service entrance all the way to the transformer. Like I said before I was really OK with that except for the expense at the time was not initially in the budget.

solar_dave

Re: PV System Limits based on Service Panel Size

JB911 wrote: »

Thanks for the quick reply.

The new panel is a 125 amp, so the buss should be 150. The line from the street is now 125 amp rated and it would be a 250+ foot run from the street to trench a new 200 amp line ($$$), hence the gyrations to get the current system to work. I see that there are "Solar Ready" panels (Siemens) that have a separate buss that directly link to the meter feed, so that it seems to leave the total 150 amp buss free.....does that sound right?

I think you are going to have to get the panel manufacturer to actually state it in writing its rated at 150 amps, unless it states it on the panel label. That is all up to your permitting agency of course. My City is notorious for going over and above NEC code.

What you describe from Siemens is a line side tap, and not a panel add. Again it is all up to your AHJ to allow that, mine would not except in a commercial environment, a no no for residential but well described in NEC. What they may also be describing is a AC combiner that then feeds the main panel backfeed breaker. I have one of those in the pictures attached where the 2 central inverters are combined before the line to the main service entrance panel. They require this to allow for a single solar production meter and a single external AC disconnect for the utility and fire department as seen on the left part of the first attachment.

Yeah I figured the 200 amp panel was an issue for the AHJ because the line feeding it was only 125 amp rated, too easy for someone to add a big main breaker and overload it. I was lucky the new transformer feed "fit" into the existing underground conduit to the transformer. I say "fit" because at first the utility wanted it increased in size of the conduit, but they relented and only charge $300 to run the new 400 amp wiring in the existing conduit.

jaggedben

Re: PV System Limits based on Service Panel Size

This thread got off to something of a bad start by only mentioning the 120% rule limits on the load side of service disconnecting means.

The solution to that limit is to do a supply side connection (often called a 'line side tap', even though it needn't be a tap). A supply side connection allows the solar to be up to the limit of the service.

I'm aware that some AHJ's don't allow supply side connections. This is just bad behavior on the part of AHJs, and it's up to you whether you want to just put up with it. Sometimes it's just the inspector misunderstanding the code, and you can gently educate them and bring them around. If they are stubborn, find out whether your local jurisdiction actually has an ordinance that modifies the NEC or the applicable state code. If they don't have such an ordinance, appeal up the food chain to your elected officials and ask that the inspectors enforce code according to the law, and not according to their whim. If they do have such an ordinance, contact your elected officials and lobby to have it changed; ask them why the NEC is good enough for everyone else but not for you. If you're an upstanding citizen of a relatively small jurisdiction, it may take some time but should be doable to get things changed.

With regard to meter/main panels, some have space for more than one main breaker, and this is indeed useful (and the easiest way to do a supply side connection, at least if you already have one).

solar_dave

Re: PV System Limits based on Service Panel Size

We made several runs at the AHJ in my case, at least 4 that I know of, this caused a delay of over six weeks on my project and multiple line drawing revisions for the engineer. Sometimes complying is the better part of valor, at least it is all included in the 30% tax credit as part of the project. We probably could have prevail on the AHJ but lots of money was already tied up and multiple months of churn was in the cards. With an $75K project at stake, a time limit on the utility rebate running and interest accruing on a HELOC as the short term interim finance vehicle we just did the $4300 add.

jaggedben

Re: PV System Limits based on Service Panel Size

Dave, I get it. I don't expect most people to have the time or motivation to go political. However, some folks may get lucky enough to become aware of the issue before investing money. And folks can always lend support to others efforts even if it's too late to benefit themselves. Just trying to encourage people to consider taking action if they are able to. Most AHJ battles are too esoteric to have a political dimension, but in my opinion this one is not. AHJs who blanketly disallow supply-side connections are just being ridiculously arbitrary regarding a pretty simple issue that potentially affects a lot people. It's a solarphobic stance, not a pro-safety stance, and they deserve pushback.

solar_dave

Re: PV System Limits based on Service Panel Size

jaggedben wrote: »

Dave, I get it. I don't expect most people to have the time or motivation to go political. However, some folks may get lucky enough to become aware of the issue before investing money. And folks can always lend support to others efforts even if it's too late to benefit themselves. Just trying to encourage people to consider taking action if they are able to. Most AHJ battles are too esoteric to have a political dimension, but in my opinion this one is not. AHJs who blanketly disallow supply-side connections are just being ridiculously arbitrary regarding a pretty simple issue that potentially affects a lot people. It's a solarphobic stance, not a pro-safety stance, and they deserve pushback.

I tend to agree with taking the stance if it is not going to cost you, really the big worry was the expiration of utility rebate at $3 a watt which was 50% of the job and I was floating that. I wish the sales staff knew more about the installation process, maybe smaller companies do operate that way. At the point we hit this snag about $36K was already spent, in fact the panels were already mounted (with AHJ structural approval), one of the draw/payment conditions for the contract. The next draw phases of the contract were liven it up for testing and then AHJ/utility acceptance for final payment. I can understand the contractor wanting to just move forward at that point as he is in to it financially pretty heavy too.

dbrower

Re: PV System Limits based on Service Panel Size

I not sure I get the math here, or the terminology, or something -- please help my understanding.

I have 400A service, feeding two buildings. The main panel at the meter has one 200A breaker that goes to one building that has a 200A panel; that panel has a 40A solar feed being fed by 34 235w panels on enphase 215s on two 20A circuits.

On the main panel there is a 125A breaker feeding over to the second building, currently with a 125A panel. On that building, I'd *like* to install 44 265w panels w/enphase 215s (don't talk to me about clipping, please). This would be about 12kwDC total, or about 10kwAC, I'm guessing on 3 enphase strings of 20A each

My installer has suggested there is a newish 2013 code limit, and that there are limits of 34 panels to a 200A panel, and 17 to a 100A panel. So far he's been vague about where the restriction is from. Panel count makes no sense, so I think he's simplifying net amperage somewhere.

If I understand correctly, to feed the 60A, I'd need (a) a 200A panel (b) 125(a) breaker to the local load; (c) feed the solar to the supply side of the 125A breakers, which has potential AHJ/Inspector issues. I'm not sure why connecting to the supply side matters -- why not connect to the load-side buss and have the amps cancel there instead of on the supply side of the 125A?

I'm fine with changing the panel.


Have I more or less understood the problems?

thanks!

-dB

BB.

Re: PV System Limits based on Service Panel Size

Welcome to the forum db. And I will take a shot at answering the basics.

dbrower wrote: »

I not sure I get the math here, or the terminology, or something -- please help my understanding.

Been there, done that. And still learning (problem is I am probably forgetting faster than I am learning).

I have 400A service, feeding two buildings. The main panel at the meter has one 200A breaker that goes to one building that has a 200A panel; that panel has a 40A solar feed being fed by 34 235w panels on enphase 215s on two 20A circuits.

The limits, as I understand, apply to any panel/sub panel. The only variation would be if the sub panel was dedicated to GT solar connections only (then it can carry 100% of the rated GT loads back to the up stream panel).

The 200 amp breaker may be the rating of the bus bars, or the panel+bus bars may be rated to (for example) 225 amps, but a 200 amp breaker was installed (for some reason).

So, if that is a 200 amp panel/bus bar--Then it would be rated for 120% of the 200 amps or 240 amps total (40 amps for GT circuit).

If that was a 225 amp panel with 200 amp main breaker... Then it would be:

• 20% * 225 amps = 45 amps "over" rating
• 225 amp panel rating + 45 amps = 270 amps maximum+GT
• 270 amps max - 200 amp main breaker (derated panel) = 70 amps available for GT solar

On the main panel there is a 125A breaker feeding over to the second building, currently with a 125A panel. On that building, I'd *like* to install 44 265w panels w/enphase 215s (don't talk to me about clipping, please). This would be about 12kwDC total, or about 10kwAC, I'm guessing on 3 enphase strings of 20A each

My guess... The 215's are rated:

• 20 amp circuit * 80% = 16 amps max continuous
• 16 amps / 17 units per string = 0.94 amps per unit (maximum, or a bit less)
• 2x 17 panels per string = 34 units, leaving 10 more panels
• 10 units * 0.94 amps = 9.4 amps on string
• 9.4 amps * 1.25 NEC up rating = 11.75 amps, round up to 15 amp circuit
• 2x 20 amp circuits + 1x 15 amp circuit = 55 amps total GT

To put this on a 125 amp panel:

• 125 amps * 1.20 NEC GT up rating = 155 amps
• 155 amp panel max - 55 amps GT = 100 Amp main breaker

So, in theory, you should be able to derate the 125 amp to a 100 amp main breaker and run 2x20 amp + 1x15 amp breaker and meet your 44 panel requirement.

My installer has suggested there is a newish 2013 code limit, and that there are limits of 34 panels to a 200A panel, and 17 to a 100A panel. So far he's been vague about where the restriction is from. Panel count makes no sense, so I think he's simplifying net amperage somewhere.

I think you can do better--Especially if you can derate the main breakers down one value (125 amp to 100 amp; 200 amp to 175 amp, etc.)bit and still meet your requirements.

Still need clarification on were the 125 amp and 200 amp panels join to the main meter panel (not sure--but I would expect that you need to "carry" the GT solar breakers up to the main panel "do the calculations for it" too.

Of course, you could bring the Enphase strings to a sub panel and bring them directly to the main panel (bury more cable from out buildings to main panel)--Then you would only have to up grade the main panel (if needed).

If I understand correctly, to feed the 60A, I'd need (a) a 200A panel (b) 125(a) breaker to the local load; (c) feed the solar to the supply side of the 125A breakers, which has potential AHJ/Inspector issues. I'm not sure why connecting to the supply side matters -- why not connect to the load-side buss and have the amps cancel there instead of on the supply side of the 125A?

If I understand your question--If you put the GT connections "up stream" of the sub panel breaker--then you have to rate the cable to carry both the 200 amp from the main panel + the XX amps of the GT system too (there are now two AC power supplies on that interconnect cable.

I'm fine with changing the panel.
Have I more or less understood the problems?

I think so--However, I am not sure why you would derate a 200 amp box to 125 amp sub panel breaker instead of 175 amps (assuming you can get the intermediate value breaker for you panels). And the smaller breakers work for your loads.

-Bill

dbrower

Re: PV System Limits based on Service Panel Size

BB. wrote: »

Welcome to the forum db. And I will take a shot at answering the basics.



Been there, done that. And still learning (problem is I am probably forgetting faster than I am learning).


My guess... The 215's are rated:

• 20 amp circuit * 80% = 16 amps max continuous
• 16 amps / 17 units per string = 0.94 amps per unit (maximum, or a bit less)
• 2x 17 panels per string = 34 units, leaving 10 more panels
• 10 units * 0.94 amps = 9.4 amps on string
• 9.4 amps * 1.25 NEC up rating = 11.75 amps, round up to 15 amp circuit
• 2x 20 amp circuits + 1x 15 amp circuit = 55 amps total GT

To put this on a 125 amp panel:

• 125 amps * 1.20 NEC GT up rating = 155 amps
• 155 amp panel max - 55 amps GT = 100 Amp main breaker

So, in theory, you should be able to derate the 125 amp to a 100 amp main breaker and run 2x20 amp + 1x15 amp breaker and meet your 44 panel requirement.



I think you can do better--Especially if you can derate the main breakers down one value (125 amp to 100 amp; 200 amp to 175 amp, etc.)bit and still meet your requirements.

Still need clarification on were the 125 amp and 200 amp panels join to the main meter panel (not sure--but I would expect that you need to "carry" the GT solar breakers up to the main panel "do the calculations for it" too.

Of course, you could bring the Enphase strings to a sub panel and bring them directly to the main panel (bury more cable from out buildings to main panel)--Then you would only have to up grade the main panel (if needed).



If I understand your question--If you put the GT connections "up stream" of the sub panel breaker--then you have to rate the cable to carry both the 200 amp from the main panel + the XX amps of the GT system too (there are now two AC power supplies on that interconnect cable.



I think so--However, I am not sure why you would derate a 200 amp box to 125 amp sub panel breaker instead of 175 amps (assuming you can get the intermediate value breaker for you panels). And the smaller breakers work for your loads.

-Bill

Not sure I can downrate the breaker to 100A given the load of the building, but that is interesting to check.

I think I may have given too much data that is confusing. Let's say the meter panel is 400A, and there is a 200A off that; then forget the 200A completely. There's a 125A breaker, and 300' of underground conduit to another building. It has a 125 amp panel, and that's where I want to connect, lets say 55A on 20, 20 and 15A backfeeds.

It seemed to me that a 200A panel with the same 125A main breaker gives 75A more backfeed than the 125 panel, except that there's something I'm not getting. Something seems to be getting added up in the worst possible way for reasons I don't follow, maybe because something doesn't cancel out the way I think it does. Let's say the load on the panel is an actual 70A, and the PV backfeed is 55A. It seems to me the current on the busses is still 70A, and the current over the 125A circuit is 15A. Yet it sounds like the calculations are being done saying it's 70 + 55 = 125, which doesn't yet make sense to me.

Also, I'm not sure which side of the 125A breaker is "better", or "worse", or why. It seems like feeding it into the load side of the 125 means there will never be more than 125 through that breaker, hence safe for the underground wiring. But the underground line has a 125A breaker on both ends, so I don't see how it matters which side you connect the PV to at the far end.

Puzzled!

thanks
-dB

inetdog

Re: PV System Limits based on Service Panel Size

dbrower wrote: »

It seemed to me that a 200A panel with the same 125A main breaker gives 75A more backfeed than the 125 panel, except that there's something I'm not getting. Something seems to be getting added up in the worst possible way for reasons I don't follow, maybe because something doesn't cancel out the way I think it does. Let's say the load on the panel is an actual 70A, and the PV backfeed is 55A. It seems to me the current on the busses is still 70A, and the current over the 125A circuit is 15A. Yet it sounds like the calculations are being done saying it's 70 + 55 = 125, which doesn't yet make sense to me.

Also, I'm not sure which side of the 125A breaker is "better", or "worse", or why. It seems like feeding it into the load side of the 125 means there will never be more than 125 through that breaker, hence safe for the underground wiring. But the underground line has a 125A breaker on both ends, so I don't see how it matters which side you connect the PV to at the far end.

Puzzled!

thanks
-dB

It is a hard one to wrap your head around at first (and maybe at second.)
The idea is that since there is no limit under the code on how many and what size load breakers you put in the panel, you may end up pulling current until the supply breaker(s) open. But if you have 125 from POCO and 55 from PV, your loads on that panel could end up pulling 180. That will overload a 125A bus bar set in the panel. If you reduce the feed breaker to 100A, then you can still pull 100A at night if you need it, and the worst possible load on the bus will now be only 155. And if you have a 125A bus bar in the panel and put the PV at the opposite end from the main, no part of the bus will have to carry more than 125A or so. (That is the reason for 120% instead of just 100% in the rule. With a 125A bus bar, that allows up to 150, but 155 is still higher than that, so even that would not be allowed. A 75A main breaker in the panel or a 75A breaker feeding the panel would be OK.

BB.

Re: PV System Limits based on Service Panel Size

dbrower wrote: »

I think I may have given too much data that is confusing. Let's say the meter panel is 400A, and there is a 200A off that; then forget the 200A completely. There's a 125A breaker, and 300' of underground conduit to another building. It has a 125 amp panel, and that's where I want to connect, lets say 55A on 20, 20 and 15A backfeeds.

It seemed to me that a 200A panel with the same 125A main breaker gives 75A more backfeed than the 125 panel, except that there's something I'm not getting. Something seems to be getting added up in the worst possible way for reasons I don't follow, maybe because something doesn't cancel out the way I think it does. Let's say the load on the panel is an actual 70A, and the PV backfeed is 55A. It seems to me the current on the buses is still 70A, and the current over the 125A circuit is 15A. Yet it sounds like the calculations are being done saying it's 70 + 55 = 125, which doesn't yet make sense to me.

If I am understanding you--Remember for residential systems you are allowed 120% of bus bar/enclosure worth of "power sources" in the box. So, for the two types of enclosures:

• 125 amps * 1.2 = 150 Amps total of supply current
• 150 amps - 125 amp utility breaker = 25 amps of GT solar
• 200 amps * 1.2 = 240 Amps total of supply current
• 240 amps - 200 amp utility breaker = 40 amps of GT solar

Now, you can derate the breaker from the utility power into the sub panel... Say 125 amp to 100 amp, and 2oo amp to 175 amp:

• 150 amp - 100 amp utility breaker = 50 amp GT solar
• 240 amp - 175 amp utility breaker = 65 amp GT solar
• 240 amp - 100 amp utility breaker = 140 amp GT solar (of course, you cannot feed 140 amp of GT into a 100 amp utility breaker)

Also, I'm not sure which side of the 125A breaker is "better", or "worse", or why. It seems like feeding it into the load side of the 125 means there will never be more than 125 through that breaker, hence safe for the underground wiring. But the underground line has a 125A breaker on both ends, so I don't see how it matters which side you connect the PV to at the far end.

I think you are looking at a "line side tap" at the sub panel--Or connection up (for example) connect that 125 amp branch circuit to two 125 amp sub panels (as an example). I am not sure code would allow that (but it could). Then you could, for example, have your 125 amp loads for the building supply from the utility and 125 amps worth of GT solar. Two boxes, so neither will over heat. And, the 125 amp branch circuit will not have more than 125 amps flowing in it (assuming there are no loads in the GT sub panel). If you put the 125 amp cable to the load panel and then jump it to the 125 amp GT panel--And there was a short to ground anywhere in that 125 amp cable, there will be a maximum of 125 amps available from each end--so cable will not overheat and 2x125 amp=250 Amps available to the short.

This type of wiring is actually proper code for the UK and known as "ring wiring". And if you have a 125 amp main breaker in each sub panel, the panels will not every "pass" more than 125 amps (in UK Ring Wiring, each plug has a 13 amp fuse because the ring circuit has 2x13 available as the ring is fed 13 amps from each end).

So--This is at the cost of joining two sub panel, each with an "extra" main sub panel breaker and a "line side" connection. But not sure if AHJ/Code would allow this or not.

One other thing to check. With "normal" utility power systems, you only deal with "one way" voltage drop (long wire runs with heavy loads give you low line voltage at the out buildings--your problem). If you have (for example) high line voltage in your neighborhood (say 255 volts), and add a 50 amps worth of GT solar--You could have "voltage rise" from the GT inverter through the 300' of wiring (lots of solar power, no local loads at the moment), through the main panel, through the feed from the house to the pole transformer--So if you get more than 5-10 volts of "voltage rise" due to 50 amp of GT solar, you will get >260 volts (260-264 volts is typical GT solar "line fault" voltage) at the GT inverter and it will shut down (after 5 minutes, the GT Inverter will check the line voltage/frequency and attempt to reconnect). The qualification voltage can be adjusted in some GT inverters with the permission of the utility by an installer using a password/communications equipment supplied by the Mfg. I am not sure that all GT Inverters/installers can adjust the maximum voltage--And even if the utility allows, you may not want 264+ volts in your out building.

This has been an occasional problem for smaller utilities and remote locations (say a farm at the end of a long power run from the substation). Sometimes you can get the utility to admit it is their problem and they will adjust the line voltage a little bit. And sometimes they will not, or cannot adjust the voltage (in Puerto Rico, there is a military base with a huge GT Solar system and it raises the local grid voltage--causing issues with everyone trying to GT Solar in the region).

I am not sure if I answered your question(s) or not...

-Bill

jaggedben

Re: PV System Limits based on Service Panel Size

dbrower wrote: »

My installer has suggested there is a newish 2013 code limit, and that there are limits of 34 panels to a 200A panel, and 17 to a 100A panel. So far he's been vague about where the restriction is from. Panel count makes no sense, so I think he's simplifying net amperage somewhere.

He is correct as long as we are talking about Enphase inverters , and as long as it's assumed that the utility breakers feeding said panels are the same as the panel rating. This is because 17 Enphase inverters are the max on a 20A breaker, and the utility+solar can equal up to 120% of the panel rating.

This isn't new to the 2013 California code, which in any case doesn't take effect here until next year.

If I understand correctly, to feed the 60A, I'd need (a) a 200A panel (b) 125(a) breaker to the local load; (c) feed the solar to the supply side of the 125A breakers, which has potential AHJ/Inspector issues.

Option 1 is a+b. Except that you may also have to upgrade the conductors from the main to the new panel. They'd have to be rated for 148A. That's (125+55)/1.2.
Option 2 is c.
It's one or the other.

If you'd have to upgrade conductors, it might be cheaper to run a new line for a line side tap. There are a few considerations there, such as whether you'd have to upsize the existing conduit anyway, or not. You'd also have some voltage drop issues to consider.

I think Contra Costa County will allow you to do a line side tap; I haven't heard of any issues with that with Bay Area AHJs. I will say that in my only direct experience submitting plans with them so far they asked me to upsize an inverter output conductor, for no reason I could discern, and it wasn't worth my time to argue with them over a handful of feet of wire. So don't expect them to necessarily understand the rules entirely correctly.

YehoshuaAgapao

Re: PV System Limits based on Service Panel Size

I did not have to touch my main service panel to do my system. It is a 200A panel with a 200A main. There is a 60A inverter breaker in there, but a single inverter will only sell up to 27A and they were happy with that. 200A panel * 1.2 = 240A. (240A-200A)*.8 = 32A of sell capacity

For expansion, doubling the panels, and getting a 2nd inverter, I will have to do a panel upgrade because my sell capacity will go up to 54A. 225A panel * 1.2 = 270A. (270A-200A)*.8 = 56A of sell capacity. 2 inverters will require a 125A backfeed breaker, but only the sell amps matter. Bi-Modal inverters need more draw amps than sell amps because of the possibility of passthru servicing subpanel load equal to its capacity plus battery charging at capacity. That is why the PDP uses 60A breakers for each inverter.

My service panel was ruled very unfriendly o line-side tap (in between the meter and the main breaker) so load side and line side will require a new main panel either way. Also my service entrance is not to modern code and will have to be at least partially replaced if anything is done to the main panel. This part of code can't be skimped around because while Mesa doesn't require permits for grid-tie or bi-modal solar, but it does require permits for main electrical panel upgrades or line-side taps.

Mesa AZ is very solar friendly. CA? well they advertise they like solar but the bureaucracy don't like anything over there.

Cariboocoot

Re: PV System Limits based on Service Panel Size

Trying out this simplified rating formula for explaining GT allowance here. See what you all think:

Service panel rating * 1.20 = total current allowed - main breaker rating = allowed breaker rating for GTI.

Thus a 200 Amp service rating * 1.20 = 240 - 200 Amp main breaker = 40 Amp for GTI (this is the most common install from what I can see).

Reduce the main breaker size and you increase the GTI allowance.

(Ironic that some of the "rules-of-thumb" we create to simplify explaining complex questions are themselves become complex, or at least voluminous.)

inetdog

Re: PV System Limits based on Service Panel Size

Cariboocoot wrote: »

Trying out this simplified rating formula for explaining GT allowance here. See what you all think:

Service panel rating ...

This part is a little tricky. The service panel rating could be considered either the rating on the panel box or the largest main breaker that is sold for it. But neither of those is what counts in this case. You need to find out the manufacturer's rating of the busbars in the panel. Even for 200A, some manufacturers use a 225A bus bar set.
And for a 100A panel it is often a 125A bus. One way to discover this is if the same bus part number is used in a 125A panel or if the same 175A bus is used in 150A and 175A panels. (It will never be less than the largest main breaker size that is offered for that panel though.)
If you are lucky, this will give you a little bit more of an edge IF you can document that bus rating to the inspector.

Cariboocoot

Re: PV System Limits based on Service Panel Size

inetdog wrote: »

This part is a little tricky. The service panel rating could be considered either the rating on the panel box or the largest main breaker that is sold for it. But neither of those is what counts in this case. You need to find out the manufacturer's rating of the busbars in the panel. Even for 200A, some manufacturers use a 225A bus bar set.
And for a 100A panel it is often a 125A bus. One way to discover this is if the same bus part number is used in a 125A panel or if the same 175A bus is used in 150A and 175A panels. (It will never be less than the largest main breaker size that is offered for that panel though.)
If you are lucky, this will give you a little bit more of an edge IF you can document that bus rating to the inspector.

Right: technically the rating is for the bars inside, not the main breaker. I think this one gets lots of people into trouble. Especially if that rating is not in evidence on the box. This is why I said "service panel rating" to start with, but perhaps that is not clear enough.

inetdog

Re: PV System Limits based on Service Panel Size

Cariboocoot wrote: »

Right: technically the rating is for the bars inside, not the main breaker. I think this one gets lots of people into trouble. Especially if that rating is not in evidence on the box. This is why I said "service panel rating" to start with, but perhaps that is not clear enough.

I think that what causes some people a problem is that there is no way provided that the 225A bus can actually be used at 225A without some sort of backfeed. The manufacturer Just beefs it up to be conservative on the 200A panel. but for some other panels it is a matter of common parts between two panel sizes. :-)

dbrower

Re: PV System Limits based on Service Panel Size

Well, the electrician, the solar and I got together this morning and figured out that panel -- the 125a with a 90A will meet the load and allow all the backfeed needed for the desired system.

BUT, another limit pops up -- there is a 400A main panel, and apparently the 2011 NEC doesn't allow more than 80A (120% again) backfeed total, and since I have 40A on my main building, only 40A allowed on the other building, even though I'd like to do 55A. Apparently this isn't amenable to down-rating some of the feeds off the panel, either.
(There's two 200A branches off of it, and one could easily be reduced to 150 or 175).

Any ideas on getting around that?

thanks,
-dB


Upgrading the main panel to 600A, including upgraded service is probably not cost effective.

Cariboocoot

Re: PV System Limits based on Service Panel Size

*No comment as I've recently been chastised for mentioning this double-derating dragon which I am assured is a mythical beast therefor what you are experiencing here does not exist.*