Hi everyone. NEWB here. I've read Will's book and spent so many hours working on this, my head might explode. I have a very simple 12v system for our off grid cabin, I'm switching over to 24 and am having a heck of a time. I have a 60a controller, 3000w 24v inverter, 4- 12v 100ah lead acid batteries, and a large amount of 285w solar panels at my disposal. I'm having a heck of a time calculating how many solar panels will be in series to go to the 2- 24v battery series and figuring out the design. I have a 300amp fuse, a fuse block, battery monitor, and a 70amp circuit breaker, but still think I'm missing something (besides wires). For some reason, 12v is easy for my to figure out, but the calculations for 24v isnt. I've attached the label from the panels below.
NEWB going from 12v to 24v
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Steve_S
Offgrid Cabineer, N.E. Ontario, Canada
How many Volts, Amps & Watts is determined by your Solar Charge Controller (SCC) and what its limitations are.
You'll need to post the info from your SCC for anyone to be able to help you work it out.
Your panel specs are from the back.
Imp 8.06A
Vmp 35.4V
Isc 8.37A
Voc 44.9V
* Max series Fuse 20A
You'll need to post the info from your SCC for anyone to be able to help you work it out.
Your panel specs are from the back.
Imp 8.06A
Vmp 35.4V
Isc 8.37A
Voc 44.9V
* Max series Fuse 20A
FilterGuy
Solar Engineering Consultant - EG4 and Consumers
Do you have a solar charge controller?
FilterGuy
Solar Engineering Consultant - EG4 and Consumers
Make an model of the controller?
wiseacre
Solar Addict
Besides the amp rating they need the voltage rating to the charge controller also. That will be used to calculate the number of panels you can put in series. examples: 100v - 30a, 145v - 30a
Oh! Thank you!
It’s a Renology 60a Rover
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FilterGuy
Solar Engineering Consultant - EG4 and Consumers
Ok.... now we can run some numbers.
Your panels have a Voc of 44.9 V, so the absolute max you can run in series is 3. However, if they might be exposed to very cold conditions that might get reduced to 2. (To figure this out we would need to know the Voltage Temp Coefficient of the panels and the lowest temp the panels will ever be in.
The max wattage the controller will manage is 60*24 = 1440W for an empty battery or 60*28.8 = 1728 for a full battery.
If you have the panels and room I would try to get at least 1728/285 = 6 panels. Since panels rarely produce to spec, I would over pannel and try to get 7 or 8 Panels.
For 6 panels, you could do 2 parallel strings of 3 in series or 3 parallel strings of 2 in series.
The only way I can think of using 7 panels would be to put them all in parallel.
8 panels could be done with 4 parallel strings of 2 panels. (This is what I would do if I had the panels)
If you want to way over panel and the temp coefficient allows 3 in series, you could do 9 panels with 3 parallel strings of 3 in series.
Your batteries would be configured in 2 parallel strings of 2 batteries in series.
Thank you so much! I think I have 16 panels here so could easily do 8 or even 9. We maybe hit 30° a couple times a year for the low temp.
If I’m running wire about 70feet from the combiner box at the panels to the control box, is 8AWG overkill? I believe the panels say minimum of 12AWG.
Then I’ll have a 300a ANL fuse at the battery and 70a circuit breaker for the controller.
FilterGuy
Solar Engineering Consultant - EG4 and Consumers
Assuming a temp coefficient of .35, your total Voc for 3 in series would be ~147V at 30deg F. You are most likely OK running 3 in series, but you should check your temp coefficient.
With 4 parallel strings of 2 in series your Isc will be 4 x 8.37A = 32.2A. If you are trying to meet NEC requirements, the wire must be able to handle
32.2A x 1.55 = 49.4A. This means the *min* size wire is 6AWG before adjusting for voltage drop.
However, since you have the panels, doing 3 strings of 3 in series means your working current is only 3 x 8.37 = 25.11A. Putting NEC's safety multiplier on it you get: 38.9A. For this you would need a minimum of 8 AWG before adjusting for voltage drop.
Edit: I put the wrong length in the voltage drop calculator. Corrections in Purple.
Lets assume you are going with the 3x3 configuration. Your working voltage will be voltage will be 3 x 35.4 = 106.2V. Plugging the working voltage and current into a voltage drop calculator, you get 8AWG for only a 2.28% voltage drop or 10AWG for a 3.46% Voltage drop. Consequently, the NEC adjusted value of 8AWG is what should be used.
You may want to review this resource:
Fusing & Wire Sizing guidelines for solar panels
This resource is intended to help the user understand when they need to add Over Current Protection Devices (OCPDs - Fuses or Breakers) to the solar panel array and what size the OCPDs need to be. There is also guidelines for wire sizing for the...
diysolarforum.com
Last edited:
Thank you! That's a great resource. It looks like I would be OK with the breakers in the combiner box per the fuse guide.Assuming a temp coefficient of .35, your total Voc for 3 in series would be ~147V at 30deg F. You are most likely OK running 3 in series, but you should check your temp coefficient.
With 4 parallel strings of 2 in series your Isc will be 4 x 8.37A = 32.2A. If you are trying to meet NEC requirements, the wire must be able to handle
32.2A x 1.55 = 49.4A. This means the *min* size wire is 6AWG before adjusting for voltage drop.
However, since you have the panels, doing 3 strings of 3 in series means your working current is only 3 x 8.37 = 25.11A. Putting NEC's safety multiplier on it you get: 38.9A. For this you would need a minimum of 8 AWG before adjusting for voltage drop.
Edit: I put the wrong length in the voltage drop calculator. Corrections in Purple.
Lets assume you are going with the 3x3 configuration. Your working voltage will be voltage will be 3 x 35.4 = 106.2V. Plugging the working voltage and current into a voltage drop calculator, you get 8AWG for only a 2.28% voltage drop or 10AWG for a 3.46% Voltage drop. Consequently, the NEC adjusted value of 8AWG is what should be used.
You may want to review this resource:
Fusing & Wire Sizing guidelines for solar panels
This resource is intended to help the user understand when they need to add Over Current Protection Devices (OCPDs - Fuses or Breakers) to the solar panel array and what size the OCPDs need to be. There is also guidelines for wire sizing for the...
The temp coefficient for the specific panels I have is -0.44 (%). We have more issues with 110*F temps here than we do cold. If the calculator is giving me 10AWG for a 2.97% voltage drop for 3 strings of 3 in series and 6AWG for 4 strings of 2 in series, it looks like it would be more cost effective (wire wise) to run the 3 strings of 3 in series if the coefficient allows?
FilterGuy
Solar Engineering Consultant - EG4 and Consumers
Yes
That is higher than most but not unheard of. Plugging that in, I get 150.7V for -27C (-16F). You should be fine on the voltage.
It is nearly always true that if you run higher voltage/lower current your copper costs will be lower. However, keep in mind that you should use the larger of the NEC calculations for safety and the Voltage Drop calculations for functionality. In your case the NEC calculations is the larger sized wire at 8AWG.
FilterGuy
Solar Engineering Consultant - EG4 and Consumers
Good point..... I am so used to thinking about LiFePO4 I did not catch this.
OK perfect! Thank you. This solves a lot of my headache as where to go from here. Having the components but not making sense of how it'll go together has been frustrating.
So I'll run the 3 strings of 3 in series to the combiner box with the breakers, 8AWG wire from there to the charge controller. From the 24v (2 strings of 2 in series 100ah 12v) battery, I have a 300A ANL fuse to the inverter then from there a 70A circuit breaker to the charge controller. Bus bar for the negatives. Will only be using the inverter to run the freezer for now but have a fuse block to later run the wires for the 12v lights, etc.
I currently have a converter to charge the batteries off the generator, and understand the calculation to figure this out for 12v, but I'm not sure how to calculate for 24v in case I now need a larger converter to charge the batteries in the winter with periods of no sun? For these12v, the battery data sheet says 30A charge rate, and in parallel it would be 30Ax4=120A but would it only be 60A for the 2 strings of batteries of 2 in series of 100ah 12v?
Oh no. Ok, I havent switched to the LiFePO4 yet but am planning on it. Sounds like I should just do it all at once with the new system.
Would keeping the 30amp charger I am currently using work until I get the LiFePO4 set up instead? ?
Thank you all for your help. These calculations are not my area of expertise (clearly). I'm more of a practical application type of gal but this is all necessary evil (for me) to get it right.
chrisski
Solar Boondocker
- Joined
- Aug 14, 2020
- Messages
- 5,192
THere’s some beefit to having the 24 volts set up prior to dropping in the lithium’s. That is what I’m doing with my batteries. For me its no small job putting in the new inverter and the 24 volt DC to DC converter to run my leveling jacks. By the time that’s done, my lithiums will arrive.
Oh that's great to hear. Makes sense. Once they arrive, it'll be ready to go. Would dropping the number of panels but keeping the 60A charge controller work with the FLA?
