8% voltage drop and running 80amps in 4 AWG for 300'

[ddxv]

I have panels that at a maximum would be max 67amps and 80amps including bifacial. The Ampacity chart shows that 80 is a bit close to 75c. Additionally, my calculation is that I would see 10.09v 8% voltage loss across the 300 feet from panels to Midnite Solar Classic 250 MPPT charge controller/batteries.

Setup:
8 old panels:
Watts: 295
Vmp: 36.5
Volts in Series: 146

16 new panels:
Watts: 440, bifacial 550
Vmp: 34.71
Volts in Series: 138.84

Name	Strings	Series	Panel Count	VMPP	Volts From Panels
old panels	2	4	8	36.5	146
new panels	4	4	16	34.71	138.84

The total at source:
Old panels amps: 16
New Panels amps: 50 (normal) 63 (+bifacial)
Total amps: 66, 80 (+bifacial)


New panels spec sheet if needed:
https://signaturesolar.com/aptos-44...to-550w-with-bifacial-gain-dna-120-bf10-440w/

 

[Lt.Dan]

Why not more in series and solve all your problems?

 

[EnergizerNate]

I have panels that at a maximum would be max 67amps and 80amps including bifacial. The Ampacity chart shows that 80 is a bit close to 75c. Additionally, my calculation is that I would see 10.09v 8% voltage loss across the 300 feet from panels to Midnite Solar Classic 250 MPPT charge controller/batteries.

Setup:
8 old panels:
Watts: 295
Vmp: 36.5
Volts in Series: 146

16 new panels:
Watts: 440, bifacial 550
Vmp: 34.71
Volts in Series: 138.84

The total at source:
Old panels amps: 16
New Panels amps: 50 (normal) 63 (+bifacial)
Total amps: 66, 80 (+bifacial)


New panels spec sheet if needed:
https://signaturesolar.com/aptos-44...to-550w-with-bifacial-gain-dna-120-bf10-440w/

How many panels in series in a string?

 

[ddxv]

How many panels in series in a string?

Thanks, didn't realized I missed adding that, will add it above as well.

Name	Strings	Series	Panel Count	VMPP	Volts From Panels
old panels	2	4	8	36.5	146
new panels	4	4	16	34.71	138.84

 

[ddxv]

Why not more in series and solve all your problems?

Right, currently both are in groups of 4 and I would also prefer to increase the voltage.

The first logical reason I had trouble with that 4 of the old panels are up on a tall pole mount, my uneducated assumption was that with 4 up in a group, I couldn't include those 4 in a series with other panels on a rack nearby since the panels (eg 4 on the pole, 2 somewhere nearby) would have an uneven gap in wiring going down the pole, underground, then back up into the main rack and thus would not work. What do you think?

The second less logical reason, ie, just ignore the pole, buy just new panels and put them in a longer series with 200v was that my mother (who's house is off grid) would not like to 'throw away' the old panels.

 

[hwy17]

Voltage drop doesn't hurt wild PV circuits, it's lost energy but the panels and controller won't mind. So as long as the wire is rated for it then it's acceptable.

What wire do you have out to the array right now?

If you up the voltage, don't do it by a little bit, get a 600v charge controller and run 8 in series. It's ok to have a long wire in the middle of a string, but a series string mostly doesn't benefit from part of the string being in better illumination, so not worth it to use the pole if you go with high voltage.

 

[ddxv]

Voltage drop doesn't hurt wild PV circuits, it's lost energy but the panels and controller won't mind. So as long as the wire is rated for it then it's acceptable.

What wire do you have out to the array right now?

If you up the voltage, don't do it by a little bit, get a 600v charge controller and run 8 in series. It's ok to have a long wire in the middle of a string, but a series string mostly doesn't benefit from part of the string being in better illumination, so not worth it to use the pole if you go with high voltage.

AWG 4, and I think THW or THWN-2/THHN. Since I wasn't sure which, I was assuming it's the lower THW for rating.

 

[hwy17]

AWG 4, and I think THW or THWN-2/THHN. Since I wasn't sure which, I was assuming it's the lower THW for rating.

You've got 4 in place already? Well then yeah you don't have to switch you could stay with 150v.

 

[EnergizerNate]

You've got 4 in place already? Well then yeah you don't have to switch you could stay with 150v.

Yes 4 AWG is good for 300 ft if stay under the current rating. I would do 2 strings of 8 to increase the voltage per string if the equipment will accept it and you can physically wire it that way.

 

[Zwy]

Right, currently both are in groups of 4 and I would also prefer to increase the voltage.

The first logical reason I had trouble with that 4 of the old panels are up on a tall pole mount, my uneducated assumption was that with 4 up in a group, I couldn't include those 4 in a series with other panels on a rack nearby since the panels (eg 4 on the pole, 2 somewhere nearby) would have an uneven gap in wiring going down the pole, underground, then back up into the main rack and thus would not work. What do you think?

The second less logical reason, ie, just ignore the pole, buy just new panels and put them in a longer series with 200v was that my mother (who's house is off grid) would not like to 'throw away' the old panels.

No need for all new panels. You could simply add a charge controller for all the new panels and run wire for it, most likely much smaller AWG than 4AWG if you series the panels. Higher VOC rated charge controllers are fairly cheap.

What I would do? Leave that system in place, add a new mount and panels with that array having it's own charge controller.

One can never have too much array.

Get string voltage up, amps down is always my motto. I'm going to add that to my signature.

 

[ricardocello]

I also recommend getting the voltage up.

The power lost due to the resistance of the cable at maximum current of 80A is 960W.
Are you willing to give up the output of a couple panels when the sun is really beating down?
Just to heat up the wire in the ground?

Math
4 AWG copper: 0.25 milliohms per foot
600 feet total cable run (both + and -)
Power lost = I^2R = (80)(80)(0.00025)(600) = 960W

 

[Skypower]

I don’t see a way around it, at least 8 in series. More voltage and different charge controllers. Factor in what your cold weather open circuit voltage is to help select the new charge controller(s).

 

[ddxv]

I also recommend getting the voltage up.

The power lost due to the resistance of the cable at maximum current of 80A is 960W.
Are you willing to give up the output of a couple panels when the sun is really beating down?
Just to heat up the wire in the ground?

Math
4 AWG copper: 0.25 milliohms per foot
600 feet total cable run (both + and -)
Power lost = I^2R = (80)(80)(0.00025)(600) = 960W

I don’t see a way around it, at least 8 in series. More voltage and different charge controllers. Factor in what your cold weather open circuit voltage is to help select the new charge controller(s).

No need for all new panels. You could simply add a charge controller for all the new panels and run wire for it, most likely much smaller AWG than 4AWG if you series the panels. Higher VOC rated charge controllers are fairly cheap.

What I would do? Leave that system in place, add a new mount and panels with that array having it's own charge controller.

One can never have too much array.

Get string voltage up, amps down is always my motto. I'm going to add that to my signature.

All three of you have the same recommendation, voltage up and amperage down. Is it possible to do with 4 panels on the pole mount and the remaining 20 in 3 strings of 8 in series nearby? The issue is that one string would have 4 panels on the pole mount, 30' wires, then the rest of the 4 in series.

 

[Skypower]

All three of you have the same recommendation, voltage up and amperage down. Is it possible to do with 4 panels on the pole mount and the remaining 20 in 3 strings of 8 in series nearby? The issue is that one string would have 4 panels on the pole mount, 30' wires, then the rest of the 4 in series.

Yes if all the panels on the strings are of the same model. I don’t know about the disconnect or safety issues that adds.

 

[Skypower]

All three of you have the same recommendation, voltage up and amperage down. Is it possible to do with 4 panels on the pole mount and the remaining 20 in 3 strings of 8 in series nearby? The issue is that one string would have 4 panels on the pole mount, 30' wires, then the rest of the 4 in series.

You might want to sketch that out for us just to be clear.

 

[hwy17]

Is it possible to do with 4 panels on the pole mount and the remaining 20 in 3 strings of 8 in series nearby? The issue is that one string would have 4 panels on the pole mount, 30' wires, then the rest of the 4 in series.

It's possible but not worth it. Giving 4 out of 8 panels in an 8 panel string better illumination just creates a shading condition, it will be more work and barely any better than 8 on the ground.

If you want to keep the pole, leave the 4S system in place with their charge controller as is, and build your new 8S system separately with it's own charge controller.

But this does mean running new additional wire from battery to array for the new separate system.

 

[Farm Boy]

I started out with 4 panels per string with my first inverter which brought my voltage around 196v. After I changed over to the next inverter I tried 8 per string bringing my voltage up to around 300v. I noticed much better over all charging times as well as good starting voltage before the sun barred down onto the panels. Raise the voltage and bring the amps down is a smart move proved me.

 

[ddxv]

Apologies, I'm not currently at my parents home where this is going to be done, so can't get a more straight forward pic. Panels in field, dog for scale . Combiner box in middle of panels goes to where picture is taken from, buried 4 AWG.

This is the current setup. Currently there are 8 panels in 2 strings of 4. The 4 on the left pole and the 4 in the two very DIY racks. The upcoming plan will be to take apart the old racks, which I made 15 years ago, and this time go with a solar specific rack. The new one supports 30 panels, if we want to expand in the future.

The new rack will be 4 old panels + 16 new ones. Most likely in the same spot as the 2 DIY racks.



So, when you suggested 3 strings of 8, it would work pretty well, as long as those 4 panels on the pole + 4 panels on a new rack immediately to the right could still be put together in series OK despite the gap, which since it would go down underground and across then back up, I guess is like 30 feet?


For reference, here is the comparison:

6 strings, 4 in series​

Name	Strings	Series	Count	VMPP	Volts From Panels	Watts	Max Watts	Amps by Lowest Voltage
old panels	2	4	8	36.5	146	295	2360	16.99
new panels	4	4	16	34.71	138.84	440	7040	50.70

Total Watts	9400
Total Amps	33.85
Volts to House	277.68
Total Amps	33.85
Dropped Volts to house	5.05
Actual Watts at House?	9229
Cost per Watt	$0.80

3 strings, 8 in series​

Name	Strings	Series	Count	VMPP	Volts From Panels	Watts	Max Watts	Amps by Lowest Voltage
old panels	1	8	8	36.5	292	295	2360	8.50
new panels	2	8	16	34.71	277.68	440	7040	25.35

Total Watts	9400
Total Amps	67.70
Volts to House	138.84
Total Amps	67.70
Dropped Volts to house	10.09
Actual Watts at House?	8717
Cost per Watt	$0.85

 

[ddxv]

Actually, I went a bit quick above the 3 series of 8 is in fact too high of voltage for the current Midnite Classic 250 MPPT Charge Controller, though I could do several strings of 6:

Name	Strings	Series	Count	VMPP	Volts From Panels	Watts	Max Watts	Amps by Lowest Voltage
old panels	1	6	6	36.5	219	295	1770	8.50
new panels	3	6	18	34.71	208.26	440	7920	38.03

 

[Skypower]

View attachment 220650


Apologies, I'm not currently at my parents home where this is going to be done, so can't get a more straight forward pic. Panels in field, dog for scale . Combiner box in middle of panels goes to where picture is taken from, buried 4 AWG.

This is the current setup. Currently there are 8 panels in 2 strings of 4. The 4 on the left pole and the 4 in the two very DIY racks. The upcoming plan will be to take apart the old racks, which I made 15 years ago, and this time go with a solar specific rack. The new one supports 30 panels, if we want to expand in the future.

The new rack will be 4 old panels + 16 new ones. Most likely in the same spot as the 2 DIY racks.
View attachment 220660


So, when you suggested 3 strings of 8, it would work pretty well, as long as those 4 panels on the pole + 4 panels on a new rack immediately to the right could still be put together in series OK despite the gap, which since it would go down underground and across then back up, I guess is like 30 feet?


For reference, here is the comparison:

6 strings, 4 in series​

Name	Strings	Series	Count	VMPP	Volts From Panels	Watts	Max Watts	Amps by Lowest Voltage
old panels	2	4	8	36.5	146	295	2360	16.99
new panels	4	4	16	34.71	138.84	440	7040	50.70

Total Watts	9400
Total Amps	33.85
Volts to House	277.68
Total Amps	33.85
Dropped Volts to house	5.05
Actual Watts at House?	9229
Cost per Watt	$0.80

3 strings, 8 in series​

Name	Strings	Series	Count	VMPP	Volts From Panels	Watts	Max Watts	Amps by Lowest Voltage
old panels	1	8	8	36.5	292	295	2360	8.50
new panels	2	8	16	34.71	277.68	440	7040	25.35

Total Watts	9400
Total Amps	67.70
Volts to House	138.84
Total Amps	67.70
Dropped Volts to house	10.09
Actual Watts at House?	8717
Cost per Watt	$0.85

Lucky SOB to wake up to a view like that every morning! My dreams. I even like the dog

 

[time2roll]

I would be looking at a 600v controller to put all 16 in series. or as many as possible.

 

[hwy17]

You can't have 3 series strings of mixed panel types and run them all over 2 4 awg wires.

My advice is just to scrap your old panels and go all new 8S2P over the existing 4 awg.

Otherwise, easiest to leave your old panels as they are on your old CC then run conduit and new wire to your new array and new CC.

 

[Skypower]

Actually, I went a bit quick above the 3 series of 8 is in fact too high of voltage for the current Midnite Classic 250 MPPT Charge Controller, though I could do several strings of 6:

Name	Strings	Series	Count	VMPP	Volts From Panels	Watts	Max Watts	Amps by Lowest Voltage
old panels	1	6	6	36.5	219	295	1770	8.50
new panels	3	6	18	34.71	208.26	440	7920	38.03

Your VMPP is important, but also look at your OCV. If you add that value up in series, factor in cold panel voltage increase and if it’s higher than your max of your charge controller OCV rating, it’ll get expensive. The first day that those conditions are exceeded and your battery is fully charged with nothing to hold the voltage down, poof! The charge controller makes magic smoke.

 

[Skypower]

I would be looking at a 600v controller to put all 16 in series. or as many as possible.

I’m thinking that 16 in series might be a tad over OCV

 

[DougfromdaUP]

You just need more wires. I don’t think many mppt can handle 10,000 watts.

You need to be able to bring multiple strings back to the multiple mppt you need for a higher power installation

 

[DougfromdaUP]

The Midnite Classic 250 appears to be limited to 55 amps output to the battery for a 48 volt nominal system, or about 2860 watts (52*55).

That would be about 15 amps input at 200 volts. So good news- your losses aren’t that high.

 

[ddxv]

You can't have 3 series strings of mixed panel types and run them all over 2 4 awg wires.

My advice is just to scrap your old panels and go all new 8S2P over the existing 4 awg.

Otherwise, easiest to leave your old panels as they are on your old CC then run conduit and new wire to your new array and new CC.

Curious why you said "you can't have 3 series strings of mixed panel types" from what I've seen I can mix panel types, as long as the resulting voltages are near enough. The voltage will drop to the lower of the strings' voltage though, which I accounted for in my calculations by assuming amps at source is based on the lowest voltage of various strings.

Does that sound correct to you?

 

[hwy17]

Curious why you said "you can't have 3 series strings of mixed panel types" from what I've seen I can mix panel types, as long as the resulting voltages are near enough. The voltage will drop to the lower of the strings' voltage though, which I accounted for in my calculations by assuming amps at source is based on the lowest voltage of various strings.

Does that sound correct to you?

Oh I assumed they'd be different cell counts, looks like they are actually all 72 cell type panels (144 for bifacial) so nevermind, yeah I guess it could work.

 

[ddxv]

s

Oh I assumed they'd be different cell counts, looks like they are actually all 72 cell type panels (144 for bifacial) so nevermind, yeah I guess it could work.

Well then that is just luck, I never knew about that being an issue, I thought the only thing I needed to line up was the total Voltage for each string type.

 

[hwy17]

Well then that is just luck, I never knew about that being an issue, I thought the only thing I needed to line up was the total Voltage for each string type.

There could be more flexibility than I'm respecting in paralleling like that. For me equal cell count would be the bare minimum.

You got any charge controllers in mind yet?

 

[ddxv]

Your VMPP is important, but also look at your OCV. If you add that value up in series, factor in cold panel voltage increase and if it’s higher than your max of your charge controller OCV rating, it’ll get expensive. The first day that those conditions are exceeded and your battery is fully charged with nothing to hold the voltage down, poof! The charge controller makes magic smoke.

Thanks, You were correct that 8 in series VOC would go over the max VOC for Midnite Classic 250. Likely will stick with the inefficient 4 panels in series for now until other parts of the setup can be fixed.

 

[ddxv]

There could be more flexibility than I'm respecting in paralleling like that. For me equal cell count would be the bare minimum.

You got any charge controllers in mind yet?

Just keeping their existing Midnite Classic 250. It's pretty beat up at this point, hums like crazy and the screen craps out all the time, but I think I'd like make that a separate project and avoid upgrading multiple things at once.

 

[hwy17]

Just keeping their existing Midnite Classic 250. It's pretty beat up at this point, hums like crazy and the screen craps out all the time, but I think I'd like make that a separate project and avoid upgrading multiple things at once.

Oh got it, sticking with 4S strings.

The good thing about voltage drop is it's only that bad at full output, which is when the solar is more plentiful anyway. In winter and on cloudy days when you really need all it can get it'll be pulling less amps and the losses are smaller.

 

[Zwy]

I don't know why you want to run higher amperage instead of higher voltage. I'd look for a charge controller that is higher than 250VOC rated and run new wire for it.

I run 16 panels with 8S and 2 strings. The run is 420 feet from array to house. My Vmp is around 350V. I run 10AWG wire. If I had run 8AWG it would have cost $2600, moving to higher voltage allowed 10AWG to be used and cost $600. I don't use a combiner, each string has it's own MPPT and circuits.

Running higher amperage creates power loss which is an exponential as amps go up. The heating of the wire is what causes the loss.

Are the current array wires in conduit or direct burial?

 

[ddxv]

Are the current array wires in conduit or direct burial?

The wires are buried in a large maybe 3" flexible PVC, like the ones used for water irrigation, the parts above ground are in electrical conduit.

 

[time2roll]

https://www.morningstarcorp.com/products/tristar-mppt-600v/

close to $900. Shop around. Probably half price or less from China.

 

[Zwy]

The wires are buried in a large maybe 3" flexible PVC, like the ones used for water irrigation, the parts above ground are in electrical conduit.

That should be easy enough to string more wire.