Over-Paneling a 3s3p to 3s5p with Slightly Different Panels Advice

MrM1

I'm Here, But I'm Not All There
I designed my system 4 years ago and it has been up and running well since. I recently upgraded to a 24v 8s 230Ah LFP x 2 in parallel ~ 8s2p battery bank and I am finding I only have enough power coming in from the array for a full charge IF the conditions are optimal. Getting another Charge Controller is over my pay grade at the moment. But Over-Paneling is an option. I have not ordered anything yet, so wanting to run this by the group before I do

When I designed the system 4 years ago I used the Midnite Solar Classic Sizing Tool and built to the max number of panels (285w 3s3p) that the tool said I could use for a 24v system. I did not know about over-paneling at that time.

My array is built for 2565w but I generally see about a 15% derating on the best of days. I am in NE Florida.

Current setup
- Midnite Classic 150 set to 24v system
- 460Ah LFP ~ Eve 230Ah cells 8s2p 25.6v nominal
- Array = 2565w 3s3p: REC Twinpeak 2 285w 3s3p ~ VOC 38.6v (115.8 per string)
- Input Current ISC: 28.98 amps

Over-Panel Upgrade ??
- Array will = 4305w 3s5p: adding 6 New panels REC Twinpeak 2 290 ~ VOC 38.8v
- So I would have 3 old strings at 115.8v VOC and 2 new strings at 116.4v VOC
- The 2 new strings would have 0.6v more voltage per string
- New Input Current ISC would be: 48.4 amps from the 5p stings (3 at 28.98 and 2 new at 19.42)

I have reached out to Midnite and they say it is not problem as long as I stay within VOC (which I would be at with the new 3s5p array at 115.8v +/- VOC for the 5 strings). Midnite tells me the Classic 150 will simply not use the extra power beyond it's design and that it will not dispurse the extra as heat ... it simply will not use the over-panel wattage if it exceeds what the Classic 150 can use.

My Goal is simply to produce the max amount of current the Classic can out put during peak times, But more specifically to harvest more power on cloudy days, and in the morning / afternoon. I currently seen about a 11kwh - 12kwh Max daily production from the array over the past 4 years. Those 11 and 12kwh days are few and far between. My average is somewhere around 5-8kwh per day. Worse on really bad days. If I could get those numbers up say, 2-3kwhs per day I would be happy...er.

I realize on peak days/times this will cause the Classic 150 to be running and outputting at or near full capacity for maybe a few hours a day ~ 94 amps. I would keep an eye on my FETs and PCB temps, but again, Midnite says this would not be a problem

So, before I do anything, just checking one more time ... Is Over-Paneling a Midnite Classic 150 24v to an array of this size going to be a problem for the Classic 150 ??
Is that 0.6v VOC difference between strings 1-3 and the new strings 4-5 going to be a problem? Anything else I should know?

Thanks
Mike ~
 

MisterSandals

Participation Medalist
So, before I do anything, just checking one more time ... Is Over-Paneling a Midnite Classic 150 24v to an array of this size going to be a problem for the Classic 150 ??
Is that 0.6v VOC difference between strings 1-3 and the new strings 4-5 going to be a problem? Anything else I should know?
Shouldn’t be a problem, especially with a multi-directional array as specified.
.6Voc is insignificant
Make sure to wire/fuse for max possible amps of your array. It’s possible that this will vary across seasons (sun angle).
 
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MrM1

I'm Here, But I'm Not All There
Ok... So with that idea comes my next issue. The array output current will go from 28.9a to 48.4a. I am using 8awg THHN in conduit now, about 28 ft

I know the classic 150 will never demand more than 30 amps, but the ISC of the array will be 48.4a.

Do I need to upgrade the wire to 6awg?

I am seeing yes at 3% voltage drop, and maybe if a 10% drop is acceptable. (?)
 

MisterSandals

Participation Medalist
The array output current will go from 28.9a to 48.4a.
Are you pointing your 5 strings in the same direction?
If you are over panelled, pointing some strings east-ish and west-ish will spread your solar harvesting across the day: east facing strings will start earlier and west facing strings finish later.
Unless your panels are laying flat, the east and west should not be producing at the same time.
 

MrM1

I'm Here, But I'm Not All There
Im locked into having them all facing South because of a roof top mount.

This is my current 9. The new 6 or 9 will go on the right of this group in the same orientation.

Install3.jpg

Install1.jpg
 

fafrd

Solar Addict
I designed my system 4 years ago and it has been up and running well since. I recently upgraded to a 24v 8s 230Ah LFP x 2 in parallel ~ 8s2p battery bank and I am finding I only have enough power coming in from the array for a full charge IF the conditions are optimal. Getting another Charge Controller is over my pay grade at the moment. But Over-Paneling is an option. I have not ordered anything yet, so wanting to run this by the group before I do

When I designed the system 4 years ago I used the Midnite Solar Classic Sizing Tool and built to the max number of panels (285w 3s3p) that the tool said I could use for a 24v system. I did not know about over-paneling at that time.

My array is built for 2565w but I generally see about a 15% derating on the best of days. I am in NE Florida.

Current setup
- Midnite Classic 150 set to 24v system
- 460Ah LFP ~ Eve 230Ah cells 8s2p 25.6v nominal
- Array = 2565w 3s3p: REC Twinpeak 2 285w 3s3p ~ VOC 38.6v (115.8 per string)
- Input Current ISC: 28.98 amps

Over-Panel Upgrade ??
- Array will = 4305w 3s5p: adding 6 New panels REC Twinpeak 2 290 ~ VOC 38.8v
- So I would have 3 old strings at 115.8v VOC and 2 new strings at 116.4v VOC
- The 2 new strings would have 0.6v more voltage per string
- New Input Current ISC would be: 48.4 amps from the 5p stings (3 at 28.98 and 2 new at 19.42)

I have reached out to Midnite and they say it is not problem as long as I stay within VOC (which I would be at with the new 3s5p array at 115.8v +/- VOC for the 5 strings). Midnite tells me the Classic 150 will simply not use the extra power beyond it's design and that it will not dispurse the extra as heat ... it simply will not use the over-panel wattage if it exceeds what the Classic 150 can use.

My Goal is simply to produce the max amount of current the Classic can out put during peak times, But more specifically to harvest more power on cloudy days, and in the morning / afternoon. I currently seen about a 11kwh - 12kwh Max daily production from the array over the past 4 years. Those 11 and 12kwh days are few and far between. My average is somewhere around 5-8kwh per day. Worse on really bad days. If I could get those numbers up say, 2-3kwhs per day I would be happy...er.

I realize on peak days/times this will cause the Classic 150 to be running and outputting at or near full capacity for maybe a few hours a day ~ 94 amps. I would keep an eye on my FETs and PCB temps, but again, Midnite says this would not be a problem

So, before I do anything, just checking one more time ... Is Over-Paneling a Midnite Classic 150 24v to an array of this size going to be a problem for the Classic 150 ??
Is that 0.6v VOC difference between strings 1-3 and the new strings 4-5 going to be a problem? Anything else I should know?

Thanks
Mike ~
You have not stated the max output rating of your SCC. Is it 100A on the output? Meaning 2.5kW peak power charging a 24V LiFePO4 battery?

You have not provided data on Vmp but assuming it is 31.9V, your maximum current under normal operation is unlikely to ever exceed 26.2A (2500W / 95.7V).

The only exception to that could be if partial shading causes multiple strings to operate at Imp at voltages below Vmp because bypass diodes have been activated. At 2/3Vmp (single bypass diode activated), current can be as high as 39.2A before the SCC will start to throttle it back, meaning you should have a 50A breaker to protect the wire and the wire should be rated for at least 62.5A (beyond what 8AWG is generally rated for).

2 of your 5 strings being ~1/3 blocked by shade will cause your MPPT to prefer ~67% power from 5/5 x 67% = 67% versus 3/5 x 100% = 60%.

Scenarios involving 1/3 Vmp (involving activation of 2 bypass diodes) are harder to engineer but would result in even higher currents of over 5 x Imp = 44.75A (which will require a 60A breaker and a wire rated for at least 75A after temperature derating).

With the shading I’ve seen in the picture you posted, you really ought to move to 6 AWG wire or better yet, put in an additional 8AWG string for the new panels (to be combined before entering the SCC).
 

MrM1

I'm Here, But I'm Not All There
You have not stated the max output rating of your SCC. Is it 100A on the output? Meaning 2.5kW peak power charging a 24V LiFePO4 battery?
My apologies. The Midnite Solar Classic 150 is a 150vdc input / 94 amps output with a 24vdc nominal system. I am using LFP Batteries in an 8s2p configuration 460Ah

You have not provided data on Vmp but assuming it is 31.9V, your maximum current under normal operation is unlikely to ever exceed 26.2A (2500W / 95.7V).
REC TwinPeak2 285w and 290w (STC Data)
Vmp = 31.9v
Imp = 8.95a
Voc = 38.6v
ISC = 9.49a

With the shading I’ve seen in the picture you posted, you really ought to move to 6 AWG wire or better yet, put in an additional 8AWG string for the new panels (to be combined before entering the SCC).
Yeah I was kinda thinking the same thing ... just not in the fantastic (over my head) way you described it. But I get it. 🤪 Thanks so much for the details. And I was thinking essentially the same thing ... Keep my initial 3 strings (in the picture) and the existing 8awg wire in the existing combiner box running to the Epanel busbar and the Classic 150 ... then just duplicate it and run the 2 (I might just go up to 3) new parallel strings in a new combiner with new 8awg wire to the Epanel busbar and on to the Classic 150.

If I understand you correctly, that might be the easiest / safest way to set it all up ... AND ... then I would be ready if I ever added a second Charge Controller to really make use of the whole array.

BTW ... From the Busbar in the Midnite Solor Epanel (Conext SW version) to the CC I have 4awg wire so that is ready. And the breaker at the load center is actually an 80amp (yeah thats too big for 8awg and would trip probably just as my house was burning down), But if memory serves me, the flow coming into the load center (E-Panel) for PV is: PV wire to Busbar, the 4awg to breaker and 4 awg from breaker to CC. So I think that is better. I'll have to look. Its been 4 years ;)

Thanks
 

fafrd

Solar Addict
My apologies. The Midnite Solar Classic 150 is a 150vdc input / 94 amps output with a 24vdc nominal system. I am using LFP Batteries in an 8s2p configuration 460Ah


REC TwinPeak2 285w and 290w (STC Data)
Vmp = 31.9v
Imp = 8.95a
Voc = 38.6v
ISC = 9.49a


Yeah I was kinda thinking the same thing ... just not in the fantastic (over my head) way you described it. But I get it. 🤪 Thanks so much for the details. And I was thinking essentially the same thing ... Keep my initial 3 strings (in the picture) and the existing 8awg wire in the existing combiner box running to the Epanel busbar and the Classic 150 ... then just duplicate it and run the 2 (I might just go up to 3) new parallel strings in a new combiner with new 8awg wire to the Epanel busbar and on to the Classic 150.

If I understand you correctly, that might be the easiest / safest way to set it all up ... AND ... then I would be ready if I ever added a second Charge Controller to really make use of the whole array.

BTW ... From the Busbar in the Midnite Solor Epanel (Conext SW version) to the CC I have 4awg wire so that is ready. And the breaker at the load center is actually an 80amp (yeah thats too big for 8awg and would trip probably just as my house was burning down), But if memory serves me, the flow coming into the load center (E-Panel) for PV is: PV wire to Busbar, the 4awg to breaker and 4 awg from breaker to CC. So I think that is better. I'll have to look. Its been 4 years ;)

Thanks
Yes, two ‘home runs’ combined close to where the SCC is located gives you much more flexibility (including using two SCCs to have a partially-shaded new array not impact output from an unshaded old array).

As far as fusing, I assume the existing 3P array already has fuses or breakers somewhere (in the MC4/PV wiring or in a combiner box before combining to the 8AWG home run.

If the two or three new panels being combined into a second home run are being fused the same way, I don’t believe any further fusing or breskering is needed before you combine and run into the SCC. It’s always a good idea to have a switch on the hone run and at that point, you might as well make it a breaker/switch, so if you’ve already got a box of some kind near the SCC for breakers (and the existing home run is already running through one), adding a second breaker for the second home run and then combining both strings into a single 6AWG or better 4AWG wire before running into the SCC is how I would do it.

The seperate second breaker means you’re essentially fully pre-wired for a second SCC if you ever decide to add one…

I’m using exactly this configuration for 3 independent home runs in my current array…
 

MrM1

I'm Here, But I'm Not All There
Thanks. Yes each string has its own Midnite 15 amp DC breaker in the combiner box. The combiners is upstairs in the attic near the array, the 80 amp PV input breaker acts more like a switch at the inverter / CC location
 

MrM1

I'm Here, But I'm Not All There
Looking at my old pics, I'll really have to check. It looks like 8awg coming in from the combiner to the busbar, but I have not sure what goes to the breaker that disconnects the array at the main load center. it might also be 8awg. And it looks maybe like I ran 8awg to the CC from the PV breaker and came back with either 6awg or 4 awg to the battery connections. I'll need to pop the top and see

Attic Combiner
Combiner.jpg

Down in the Garage
Epanel.jpg
FinalLook.jpg
 

fafrd

Solar Addict
Looking at my old pics, I'll really have to check. It looks like 8awg coming in from the combiner to the busbar, but I have not sure what goes to the breaker that disconnects the array at the main load center. it might also be 8awg. And it looks maybe like I ran 8awg to the CC from the PV breaker and came back with either 6awg or 4 awg to the battery connections. I'll need to pop the top and see

Attic Combiner
View attachment 68858

Down in the Garage
View attachment 68859
View attachment 68860
Nice setup.

You should probably use a second combiner box for the new array if you want to leave yourself the flexibility of adding a 3rd parallel string to that new array, but with only 2P, you can combine with MC4 Y connectors and just rely on the new switch / breaker to protect the new 8AWG home run wire.

You’ll need to combine the output wires from the two array breakers to a suitably-sized wire to enter the SCC (at least 6AWG, probably 4AWG) and, obviously, the output wire from the SCC needs to be sized to comfortably handle it’s full 95A max output meaning a 120A breaker on a wire sized for 150A (ie: at least 3AWG).

But looks like you’ve got more than enough space to pull it off ;).
 
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