Array Series / Parallel configuration dilemma

[samotlietuvis]

Hi guys, new to this forum, my first post. I am performing my first solar installation for my home – will draw the permit myself and will do the installation myself.

My PV specs are:

• Pmax=327W
• IMP= 5.98A
• VMP=54.7V
• VOC=64.9A
• ISC=6.46A

Total #of panels = 9 -> System DC Power = 9*327W = 2943W. The panels will be arranged in two rows one above another with 4 panels in first row and 5 panels in second row

I want to use SMA 3000TL-US string inverter that has following specs:

• MAX DC VOLT RATING=600V
• MPPT operating voltage range=125V to 500V
• MAX POWER @ 40C= 3200W
• MAX DC CURRENT=15A

My Temperature correction factor = 1.1292

If I wire all 9 panels in series on 1 string, I get DC VMAX = 64.9*9*1.1292 = 660V which is more than the inverters max DC Volt rating. That I cannot do.

If I have two strings: 7 panels in first string, and 2 panels in second string, I get DC VMAX = 64.9*7*1.1292 = 513V than is below Inverters max DC volt rating, but above MPPT operating max voltage of 500V.

Question: what will happen to MMPT if my system gets to 513V on a very cold but sunny day? Will MMPT go to maximum load resistor and that’s it, or will MPPT circuitry will be bypassed completely? Is it wise to exceed MMPT operating max voltage on paper?

My other option is to have 6 panels in first string, and 3 panels in second string, DC VMAX=440V, but that will delay my electricity production start time because my PV system will be operating at lower Voltage.

Thanks, Tomas

[PVAndy]

Hi guys, new to this forum, my first post. I am performing my first solar installation for my home – will draw the permit myself and will do the installation myself.

My PV specs are:

• Pmax=327W
• IMP= 5.98A
• VMP=54.7V
• VOC=64.9A
• ISC=6.46A

Total #of panels = 9 -> System DC Power = 9*327W = 2943W. The panels will be arranged in two rows one above another with 4 panels in first row and 5 panels in second row

I want to use SMA 3000TL-US string inverter that has following specs:

• MAX DC VOLT RATING=600V
• MPPT operating voltage range=125V to 500V
• MAX POWER @ 40C= 3200W
• MAX DC CURRENT=15A

My Temperature correction factor = 1.1292

If I wire all 9 panels in series on 1 string, I get DC VMAX = 64.9*9*1.1292 = 660V which is more than the inverters max DC Volt rating. That I cannot do.

If I have two strings: 7 panels in first string, and 2 panels in second string, I get DC VMAX = 64.9*7*1.1292 = 513V than is below Inverters max DC volt rating, but above MPPT operating max voltage of 500V.

Question: what will happen to MMPT if my system gets to 513V on a very cold but sunny day? Will MMPT go to maximum load resistor and that’s it, or will MPPT circuitry will be bypassed completely? Is it wise to exceed MMPT operating max voltage on paper?

My other option is to have 6 panels in first string, and 3 panels in second string, DC VMAX=440V, but that will delay my electricity production start time because my PV system will be operating at lower Voltage.

Thanks, Tomas

Strings of 7 and 2 or 6 and 3 won't work. The only string possibility with 9 panels with those characteristics are one of 5 and 1 of 4.

The problem with a string of 2 is that you won't reach the start voltage of the inverter which is 150 V DC. You need the Voc temp compensated for the hottest day in your local to be greater than the start voltage. The Vmp again compensated for temperature must stay above the 125 V minimum operating voltage. Your not going to have a problem with it not starting early as Vmp is primarily of function of temp and not irradiance.

Andy

NABCEP Certified Installation Professional
PV Design Engineer

[samotlietuvis]

Andy, I agree with you, but what if I wire 7 in series, and two in series, and on the roof I combine the two in parallel into the junction box; Only one set of wire would go down to the inverter. I understand by doing so I will be sending 2x current down to the inverter, but the inverter can handle 2x the current, and I will used thicker wire to account for the current. Is there any concern with doing 7+2 combined in parallel?

Would the current from the 7 in series flow to the 2 in series because the 7 in series is at higher Voltage?

Thanks, Tomas

Strings of 7 and 2 or 6 and 3 won't work. The only string possibility with 9 panels with those characteristics are one of 5 and 1 of 4.

The problem with a string of 2 is that you won't reach the start voltage of the inverter which is 150 V DC. You need the Voc temp compensated for the hottest day in your local to be greater than the start voltage. The Vmp again compensated for temperature must stay above the 125 V minimum operating voltage. Your not going to have a problem with it not starting early as Vmp is primarily of function of temp and not irradiance.

Andy

NABCEP Certified Installation Professional
PV Design Engineer

[Mike90250]

As Sunking says, Odd or Prime numbers of panels won't work 90% of the time.

You need one more panel, or hold one back as a spare.

unless your inverter has 2 separate MPPT inputs, you can only parallel the same number of panels:
2 parallel strings of 4 or 5 panels. You can't parallel mis-matched strings.

[PVAndy]

Andy, I agree with you, but what if I wire 7 in series, and two in series, and on the roof I combine the two in parallel into the junction box; Only one set of wire would go down to the inverter. I understand by doing so I will be sending 2x current down to the inverter, but the inverter can handle 2x the current, and I will used thicker wire to account for the current. Is there any concern with doing 7+2 combined in parallel?

Would the current from the 7 in series flow to the 2 in series because the 7 in series is at higher Voltage?

Thanks, Tomas

You would effectively only have 7 panels. The 2 in the short string would not contribute at all. Basic series parallel network analysis.

The only combination that will work with those panels and inverter are strings of 4 and 5 connected to separate MPPT's. With that inverter with it's low start voltage and very low operating voltage you'll be fine. Remember to turn on Global Optitrac which which by default is off when you receive the inverter. It requires you purchase a SMA Speedwire/Webconnect module and use a laptop to program.

Andy

[Sunking]

With 9 panels you only have 3 options. Al in parallel, all in series or 3 x 3. Take your pick. As you have discovered you cannot use any of those 3 options. Trust me you have a lot more expensive surprises just waiting for you to discover.

[inetdog]

If you get the current model, TL-3000-US-22, which has two independent MPPT inputs, you could use a 5 and 4 combination with no problems.
If you have the older, non-22, model, then as Sunking stated, you are screwed.

[PVAndy]

If you get the current model, TL-3000-US-22, which has two independent MPPT inputs, you could use a 5 and 4 combination with no problems.
If you have the older, non-22, model, then as Sunking stated, you are screwed.

Since it is a 3000TL I believe it has to be a -22 with 2 MPPT. With inverters wide MPPT Range the OP's concerns about string length are unwarranteed

Andy

[samotlietuvis]

Thanks for all the replies guys - very good points. Did more research to find out that:
1) only solar panels of exact or similar current should be wired together in series.
2) only solar panels of exact or similar voltage should be wired together in parallel.

I am leaning towards using 8 panels only that will give me DC VMAX = 64.9*8*1.1292 = 586V < 600V Max Inverter DC Voltage, and keeping 1 panel as a spare, as Mike90250 recommended.

The SMA-3000-TL-US does have two MPPT inputs, and 5 in series + 4 in series will work as well.

The size of the panels, 61" x 41" makes them so hard to handle, and I need to take them outside before installing to test each one. I haven't even figured out how I am going to haul them on the roof.

With 9 panels you only have 3 options. Al in parallel, all in series or 3 x 3. Take your pick. As you have discovered you cannot use any of those 3 options. Trust me you have a lot more expensive surprises just waiting for you to discover.

Sunking, I hope there will be very few surprises, but you are probably right - this is my first solar job, and the first permit I am drawing in my lifetime

I purchased my 2nd home that is 2X bigger than the one I am doing solar for, has pool, and has Spanish Tile Architectural Roof. Closing in a month, moving in after Christmas. Now that will be pain to do, but I already bought solar panels for the home as well. My current home - single story shingle roof south facing space over garage enough for 9x jumbo panels I got is a primer for the job I will do next year.

As Sunking says, Odd or Prime numbers of panels won't work 90% of the time.

You need one more panel, or hold one back as a spare.

unless your inverter has 2 separate MPPT inputs, you can only parallel the same number of panels:
2 parallel strings of 4 or 5 panels. You can't parallel mis-matched strings.

I like the spare option. I do have an extra panels, but no room for it.

If you get the current model, TL-3000-US-22, which has two independent MPPT inputs, you could use a 5 and 4 combination with no problems.
If you have the older, non-22, model, then as Sunking stated, you are screwed.

It is the new model that has 2x MPPT inputs, and 5x String with 4x String is something I am considering.

[sensij]

I am leaning towards using 8 panels only that will give me DC VMAX = 64.9*8*1.1292 = 586V < 600V Max Inverter DC Voltage, and keeping 1 panel as a spare, as Mike90250 recommended.

I like the spare option. I do have an extra panels, but no room for it.

It is the new model that has 2x MPPT inputs, and 5x String with 4x String is something I am considering.

5 and 4 on the -22 inverter is fine. Have you gotten any professional quotes? For your next job, doing an installation on those tiles without breaking them or damaging the roof might be challenging. A 3kW system should run between $10k - $12k installed, although the 327 W panels are expensive. Maybe you'll find someone willing to cut you a deal if you agree for them to do the work on both houses.

[samotlietuvis]

5 and 4 on that inverter is fine. Have you gotten any professional quotes? For your next job, doing an installation on those tiles without breaking them or damaging the roof might be challenging. A 3kW system should run between $10k - $12k installed, although the 327 W panels are expensive. Maybe you'll find someone willing to cut you a deal if you agree for them to do the work on both houses.

sensij, I agree, the Spanish tiles look very dreadful. The Spanish tile roof I am taking possession is only 9 years old, so hopefully I can find replacement tiles of exactly the same type for the ones that the roofer will break. That 3200sqft home with pool screams for Solar, especially me being in the desert.

As far as price goes, I will get done for 1/2 of that before tax rebates, and I will gain solar experience that is on my list of things to know how to do. I already know how to do, do and have done HVAC. If my current job ever goes away, I become PE and do both - Solar and HVAC. Both have very good profit margins.

I already bought all the 327 panels for both installs, got lucky on that one. I already have everything I need for my 1st install - SnapNRack System, Inverter / AC Disconnect, Wires, Connectors, Conduit, Professional Crimper, built MC4 connector solar panel tester, thinking about buying irradiance meter. I am drawing permit and performing systems level engineering myself. I am hiring licensed roofer for both jobs - he has have done solar panels before. My neighbor and friend who will be renting my current home from me is an electrical contractor. He is very interested in solar and will be helping me along electrical work during install. According to him Romex 10/2 wire for AC wire run inside garage is per code, and it does not need to be in conduit since it is in garage. I need to confirm that.

Tomas

[sensij]

sensij, I agree, the Spanish tiles look very dreadful. The Spanish tile roof I am taking possession is only 9 years old, so hopefully I can find replacement tiles of exactly the same type for the ones that the roofer will break. That 3200sqft home with pool screams for Solar, especially me being in the desert.

As far as price goes, I will get done for 1/2 of that before tax rebates, and I will gain solar experience that is on my list of things to know how to do. I already know how to do, do and have done HVAC. If my current job ever goes away, I become PE and do both - Solar and HVAC. Both have very good profit margins.

I already bought all the 327 panels for both installs, got lucky on that one. I already have everything I need for my 1st install - SnapNRack System, Inverter / AC Disconnect, Wires, Connectors, Conduit, Professional Crimper, built MC4 connector solar panel tester, thinking about buying irradiance meter. I am drawing permit and performing systems level engineering myself. I am hiring licensed roofer for both jobs - he has have done solar panels before. My neighbor and friend who will be renting my current home from me is an electrical contractor. He is very interested in solar and will be helping me along electrical work during install. According to him Romex 10/2 wire for AC wire run inside garage is per code, and it does not need to be in conduit since it is in garage. I need to confirm that.

Tomas

You might find the Mike Holt forums to be a good source of education on the code-compliant electrical design. Do a search for any of your questions, almost everything you can think to ask has been answered there before.

[samotlietuvis]

You might find the Mike Holt forums to be a good source of education on the code-compliant electrical design. Do a search for any of your questions, almost everything you can think to ask has been answered there before.

Thanks. I read through Mike Holt's "WIRING METHODS AND MATERIALS" publication, and I have all the solar code applicable to where I live compiled in one folder, and I reference every calculation and tables I am using in my permit so that Permit Approver understands that I understand. I do use search quite a bit. Trying to bounce a few ideas to see what's per code, and what's not. If I were not to do the permit myself, them I would not gain the experience I am looking for.

[sensij]

Thanks. I read through Mike Holt's "WIRING METHODS AND MATERIALS" publication, and I have all the solar code applicable to where I live compiled in one folder, and I reference every calculation and tables I am using in my permit so that Permit Approver understands that I understand. I do use search quite a bit. Trying to bounce a few ideas to see what's per code, and what's not. If I were not to do the permit myself, them I would not gain the experience I am looking for.

Ideas for more problems you might run into can be found here found here. Good luck!