Is My New System Undersized?

[jflorey2]

Keep in mind, the big wire will only reduce line voltage at the inverters a few volts, some fraction of the current
rise. The voltage at your meter will be just as high, might even rise a tad from the increased efficiency.

Given that he's right on the edge now (i.e. one inverter shutting down, then restarting later) it may be enough. Measurements will tell the story there.

From the inverter's perspective they don't care about efficiency. If the wires are dissipating ~100 watts (power which is being supplied by the inverter) the inverters are perfectly happy to drive that power; they can't tell where the power is being dissipated, just that the voltage at the output terminals is too high. Reducing the effective impedance between the inverters and the rest of the grid will reduce the voltage rise.

[jflorey2]

Any idea why it would recommend this sort of configuration?

It may simply be the default strategy it uses. Fill up the first tracker input (while ensuring that voltage does not exceed maximum) then fill the second, then the third etc.

[bcroe]

From the inverter's perspective they don't care about efficiency. If the wires are dissipating ~100 watts (power which is being supplied by the inverter) the inverters are perfectly happy to drive that power; they can't tell where the power is being dissipated, just that the voltage at the output terminals is too high. Reducing the effective impedance between the inverters and the rest of the grid will reduce the voltage rise.

Reduced rise in his wires yes. But with a slightly lower line voltage at the inverters, they will increase current
proportionately, and the voltage at the meter will rise a bit depending on how low Z the PoCo is. Bruce Roe

[tyab]

Hi PV4Me. I wanted to give you some info and links so you understand the voltage issue.

First having your inverters far away is introducing voltage loss issues that could have been avoided. One of advantages of string inverters is the ability to run the DC the long distance and locate the string inverters much closer to the service reducing the resistance losses. There are cost savings here since one is willing to accept higher losses on the DC side than on the AC side. Simply due to how inverters track the grid a design goal is to keep the AC losses from wire resistance as small as possible. You may want to talk to your installer about flipping this problem and see if the savings in wire justify the extra work.

But lets say we keep the current setup. Similar but longer than you I have a 400' run from my microinverters ground mount to my service panel and resistance losses were part of my design from day one.

One data point we must have (and already discussed in prior posts) is the minimum wire required by code (ignoring distance/voltage drop). Based on your zip code I get a historical high temp of a tad over 38C, and assuming I did my math correctly I come up with #1 CU is the minimum to meet code. Like others, I don't know how the installer thought #3 would be ok with the larger system. This really is basic code (310.15)

Now lets see how that #1 handles the distance. I used this tool (set to 240V single phase in the options).

http://www.nooutage.com/vdrop.htm

Using the actual generated output of 96 amps for 200' I get 2.5% losses. For the #3 that you have currently I get 4.1% loss. 4.1% is starting to get high and combined with a slightly off grid could cause your inverters to trip.

So now you have to decide what is the acceptable losses. Most inverter systems want to keep total AC losses under 2% - that is what I designed my system for. Assuming you have 0 losses anywhere else (not reality) for 96 amps over 200' using 4/0 CU gives you 1.2% - that sure is better than the 4% you currently have. Be aware that 4/0 CU is getting into some expensive wire and most would rather to go AL wire in those sizes. Given the high cost of CU wire in those sizes 4/0 CU may be paying more than is necessary. 4/0 AL would be 1.7% and is a lot less expensive.

I think I read that you have 2" conduit currently - that should be enough for up to 4/0.

I also saw that your guy is downsizing the grounded conductor (neutral). Code does allow that but there are a number of rules that must be followed. Your installer should know these rules and what the AHJ will expect.

Again I suggest talking to your guy about switching this problem and moving the inverters by the service entrance and running DC the long run. I'm sure folks here could help you figure out what is needed for that DC run.

Oh and as a PG&E customer - be aware that upsizing your system like you did will require an application with PG&E and that may kill your grandfather clause. If you don't do this - it will not take the PG&E computers long to figure out you are now generating a lot more and then you may be in violation of your current agreement. Better check into this before you get hit with fees.

Good luck!

[sensij]

Again I highly suggest talking to your guy about switching this problem and moving the inverters by the service entrance and running DC the long run. I'm sure folks here could help you figure out what is needed for that DC run.

Good luck!

+1 on the idea to evaluate the feasibility of relocating the inverters.

Edit... looks like the voltage drop calculator you linked suggests assuming the wire is at 75 deg C across its whole length. I'm not sure how valid that is, but for the sake of this discussion, there isn't much difference between the 3.1%-4.1% drop across the likely temperature range. It still feels low to me to explain the voltage problems reported by the OP... sure, the service could be high, but I'd still be looking for a bad connection somewhere or another source of impedance.

http://www.calculator.net/voltage-dr...es=96&x=86&y=5

[sensij]

Oh and as a PG&E customer - be aware that upsizing your system like you did will require an application with PG&E and that may kill your grandfather clause. If you don't do this - it will not take the PG&E computers long to figure out you are now generating a lot more and then you may be in violation of your current agreement. Better check into this before you get hit with fees.

Good luck!

PG&E's net metering tariff sheds some light on this. The amount they are referencing is the greater of 1 kW or 10% of the system size. PGE NEM.JPG

[bcroe]

Relocating will help, but then several panel string combiner boxes would be needed to reduce down to 7 wires
for the 3 inverters. Probably has 3 or 4 wires now? And I would want to be sure this is a cure before spending
any more money. Accurate before and after numbers. The cheapest fix is getting the PoCo to lower their
voltage; mine is currently 120.5/241 at night, but before I complained it was 127.5/255, to which I added my
line rise. I think I'll see what the PoCo rise is when I hit them with 15KW. You might want to compare their
day loaded vs night values. Bruce Roe

[tyab]

Yes Sensji I was just taking the default of 75C.

For his existing setup, think about that #3 wire having to carry 96 amps and especially those sections above ground subject to ambient summer highs of 100F. Wow - that wire will be cooking and it may even exceed its 90C rating.and its true losses from resistance must be even higher than the 75C default value. The #1 wire at 75C may be a close to what he may see in summer at full output especially if part of the above ground conduit is subject to direct sunlight. And of course oversizing larger than #1 to get the resistance losses down then 75C is overkill and the real losses are lower. However if I was him I would not spend the $ for 4/0 CU - I would take the 4/0 AL.

Thanks for the PG&E snippet, I thought I saw that 10%. Given that he replaced all the panels it is not possible to meter just the new stuff and keep the old meter so he will lose his grandfather and thus have to switch to either TOUA or TOUB and be under NEM 2.0. Bummer for him but he has to let them know that the system is beyond its original application - I don't know what the penalties are or what PG&E can do with an out of compliance system but it can't be nice.

I also have to assume there are other wire issues so his real existing losses may be closer to 5%. Is that the reason his inverters are tripping?- seems to make sense given his reported error. But Bcroe is spot on - he needs to request the PoCo monitor his line at his service and see if there is a service condition that can be adjusted. Problem here is his installer seems to not fully understand this issue and the OP has limited experience so it may be a challenge to get PG&E to do that.

[sensij]

Yes Sensji I was just taking the default of 75C.

For his existing setup, think about that #3 wire having to carry 96 amps and especially those sections above ground subject to ambient summer highs of 100F. Wow - that wire will be cooking and it may even exceed its 90C rating.and its true losses from resistance must be even higher than the 75C default value. The #1 wire at 75C may be a close to what he may see in summer at full output especially if part of the above ground conduit is subject to direct sunlight. And of course oversizing larger than #1 to get the resistance losses down then 75C is overkill and the real losses are lower. However if I was him I would not spend the $ for 4/0 CU - I would take the 4/0 AL.

The inverters aren't tripping in summertime temps, they are tripping now. For his system to be producing anything close to the power he has reported, it is probably relatively cool. We don't know the OP's zip code, but at 215 deg azimuth, with daylight savings time the peak power is closer to 1:00 than 11:00, so it is probably tripping out at something less than 96 A. We don't know that THHN was used in the run, don't know if it is trenched underground or run through an attic or along a rooftop. We don't know if the tripping is occurring due to heavy backfeed, or if the production is fully self consumed and the grid is still delivering power. There are just too many unknowns to make recommendation on what the OP should do to fix the problem, except to bring in qualified help.

[J.P.M.]

The inverters aren't tripping in summertime temps, they are tripping now. For his system to be producing anything close to the power he has reported, it is probably relatively cool. We don't know the OP's zip code, but at 215 deg azimuth, with daylight savings time the peak power is closer to 1:00 than 11:00, so it is probably tripping out at something less than 96 A. We don't know that THHN was used in the run, don't know if it is trenched underground or run through an attic or along a rooftop. We don't know if the tripping is occurring due to heavy backfeed, or if the production is fully self consumed and the grid is still delivering power. There are just too many unknowns to make recommendation on what the OP should do to fix the problem, except to bring in qualified help.

See my 04/24/2017, 1624 hrs. post. Zip is 95672, I got a PVWatts max. hourly output ~ 20.1kWh at ~ 1300 - 1400 hrs. solar time this time of year w/10% system losses. I didn't do a SAM run.

[J.P.M.]

I'm getting a gnawing feeling in my brain that the OP and his vendor are one and the same. That is, the OP tried to upsize the array not knowing want was involved and added the vendor story for reasons of his own.

If I'm wrong,(and I'll probably never know), my sincere apologies to the OP. Just that, rereading all the OP's posts gives me the feeling that things aren't hanging together.

Whatever the case, I agree with sensij that it'll take more than the freely given and consequence free advice (including mine) the OP gets here to straighten this mess out.

[bcroe]

Earlier I bought this surplus for a song, from the state U. Just in case some temporary voltage adjustments had to be made.
Fortunately solutions were found without firing up this 15KW variac. Bruce Roe

PVviac.JPG

[PV4Me]

Gentlemen, thank you again for all the helpful replies. Sorry life caught up and I was not able to come back on here and post as soon as I would have liked to.

Current situation is that my installer is looking to redo the wiring from the arrays to the service panel. Currently the existing #3 CU is in a buried trench, in 2" conduit, and runs about 80% of the entire distance in this conduit. For the remaining 20% of the distance it is in a conduit but not buried, running through a part of the unfinished basement of the house, to the panel. The #3 CU is of course from the previous solar install that was done in 2006.

I have not yet given the installer the green light on this and am interested in the suggestions made here regarding running the DC leg the longer distance vs AC. Is there a technical benefit to this or is it just cost-effective since we can save on copper wiring costs? Currently the installer placed the 3 inverters underneath the array themselves (attached to the ground mount structure). This was where the previous installation had the inverters placed so he simply replaced those and placed the new ones there. Having them inside my garage or basement area would definitely protect them better from the elements, not to mention keep them cooler in the summer.

For switching the long run from AC to DC, I will ask my installer about it today and see what he says. I am sure there will be added expense to having him relocate the inverters but will see all of what is required. Someone suggested that there will be the need for panel string combiner boxes. Any other ideas of what would all be involved in doing something like this?

Thank you btw to everyone who pointed out the PG&E compliance issue in regards to system size. I have yet to ask my installer about this but intend to as he claims since we are just doing an upgrade, and not a "new system", nothing is required in terms of paperwork with PG&E. Looking at the information posted here, I disagree with his conclusion. I suppose I will have to kiss my ideas of being grandfathered in goodbye.

Zip code for if anyone missed it is 95672. I did look into the web monitoring for my system and am including a screenshot of the kind of behavior I see whenever there is sun out and one of the inverters trips. It then comes back online, all on its own, a few hours later, but only when the daylight hours are close to ending.

J.P.M., I assure you that the installer and myself are not one and the same. I would have zero issue with admitting that I screwed up! I screw up very regularly and this would be no different, but I take absolutely zero offense to any cynicism or skepticism regarding my story. I would feel the same way if I was on the other end reading it all.

It is indeed a complicated relationship as the installer did come highly recommended and has done other jobs and I have heard zero complaints about him. On top of that, he is a very easy going and easy to talk to person, but does seem to have huge gaps in his understanding of how stuff should work and kept insisting that the #3 CU would work. I independently verified that he had a C10 electrician's license and a general contractor's license before I signed anything and everything seemed like it would work out well. Well hindsight is 20/20 but I am hoping I can salvage this job at this point and get the system within code and performing correctly. I know some of you have warned me about continuing to have him work on the project and I have taken that feedback into consideration but I am hoping that perhaps with the oversight of the county inspector, and the recommendations and advice I am receiving from all of the helpful people here, I may be able to end up OK after all. Overly optimistic or consistently foolish, I am not sure.

I am going to bring up the issue about the PoCo line voltage with PG&E and the installer also -- perhaps that is where the majority of the problem lies and even though the wire needs to be replaced, maybe as others have suggested here, the wire is not the sole or main culprit behind the tripping inverters.


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[J.P.M.]

J.P.M., I assure you that the installer and myself are not one and the same. I would have zero issue with admitting that I screwed up! I screw up very regularly and this would be no different, but I take absolutely zero offense to any cynicism or skepticism regarding my story. I would feel the same way if I was on the other end reading it all.

Understood.

J.P.M.

[sensij]

Calculations on the DC side are a bit more complicated.

The 325 W panels have an Isc of 9.21 A, which is the Isc for the string. Each string has its own mppt, so you don't need a combiner, just a junction box to transition from the PV wire to the wire you are running through the conduit.

The wire needs to have an ampacity of 156% of the Isc, or 14.4 A. That would be satisfied by 14 AWG, but there are conditions of use problems because so many current carrying conductors (9 strings * 2 conductors = 18) will be in the same conduit.

The 310.15(B)(3)(a) adjustment for 18 current carrying conductors is 0.5
The 310.15(B)(2)(a) adjustment for 38 deg max ambient temp is 0.91

10 AWG THWN-2 has a 40 A base ampacity, in this case corrected down to 18.2 A. It has a termination rating (at 75 deg C) of 35 A, also safe. Voltage drop of 9.21 A over 200 ft is around 2%... not awesome, but since it is on the DC side, it won't affect the AC line voltage and cause the tripping problems, it is just loss.

Those 18 #10 conductors (+ a #10 ground conductor for each inverter, maybe) would only fill the 2" conduit 13%, 40% is max so that is fine. You could bump up to #8 AWG and cut the voltage drop down to 1.5% or so, it should still fit in the conduit. 6 AWG is starting to get tight, but would further reduce the voltage drop. (Grounding is a whole separate topic to consider, as eleceng1979 brought up earlier in the thread).

Technical benefits of running the DC the long length are that you need less copper to carry the same power, because the voltage (on average) is higher than the AC side which means the current is lower. Also, as mentioned, any voltage drop won't cause your AC circuit voltage to rise, taking that off the table completely as a potential contributor to your tripping problems.

Note that the tripping occurred at aroud 5.75 kW * 3 = 17.25 kW. That is only 72 A, within the rating of the #3 copper (assuming THWN-2) and probably around 3.5% voltage drop (even assuming the wires are hot) if the terminations are good.

If these calculations are right (hopefully others will chime in if they see errors or omissions), and you have the space for the inverters in your basement, I can see no reason at all that you would leave them out at the array.