First PV System! Can you check this math?

Hi!

I was wondering if someone more technical than I could check my math, for my first PV system!

I recently purchased 2x Grape Solar GS-S-250-Fab5 (specs: http://www.grapesolar.com/files/3113/3789/6931/GS-S-250-Fab5_Rev-2.pdf)

And 2x SunExtender AGM batteries (PVX-1080T).

And a Blue Sky 3024iL (specs: http://www.blueskyenergyinc.com/uploads/pdf/SB3024iL_datasheet.pdf).

For a 17' cable run between the panels and the 3024iL, I am calculating that I should use 12AWG wire. Also, I will wire the panels in parallel.

For a 3' cable run between the 3024iL and the batteries, I am calculating that I should use 8AWG wire. Also, I will wire the batteries in parallel.

One final question: For the long cable run between the panels and the 3024iL, what would the ramifications be if I used a 20' cable, instead of 17' (since those are easier to find)?

This installation is for the top of my RV, which I'll be driving around!

These are just my first time calculations and anything in here could be wrong. Feel free to critique, where necessary.

Cheers!

Comments

  • BB.BB. Super Moderators, Administrators Posts: 30,645 admin
    Re: First PV System! Can you check this math?

    Welcome to the forum "krazy".

    You have jumped in with both feet here. I will see if I can help.
    krazy wrote: »

    OK--First guesses are the batteries are in parallel for a 12 volt system, and the two solar panels are also in parallel.
    For a 17' cable run between the panels and the 3024iL, I am calculating that I should use 12AWG wire. Also, I will wire the panels in parallel.

    The panels are Imp=8.15 amps and Vmp=30.7 volts; Isc=8.72 amps.

    In parallel, the two panels have ~17.4 amps maximum Isc. We usually take 1.25x (NEC Derating) Isc which would be:
    • 2 * 8.72 amps * 1.25 NEC derating = 21.8 Amps

    Technically, the NEC would require 10 AWG wiring--But it is "OK" at 12 AWG (your choice).

    We should look also at the voltage drop for your 17' cable run using a generic voltage drop calculator:
    • 8.15 amps Imp * 2 panels (16.3 amps) with 3% max drop of 30.7 volts (0.9 volts) => 10 AWG @ 0.7 volt drop

    So, you should be thinking about 10 AWG wiring anyway (12 AWG will work with 1.1 volt drop too--just more losses).
    For a 3' cable run between the 3024iL and the batteries, I am calculating that I should use 8AWG wire. Also, I will wire the batteries in parallel.

    So, the calculations from your array would be:
    • Worst Case = 2*250 watt panels * 1/12 volts charging * 1.25 NEC derating * 1.25 2nd NEC derating = 65 amps branch circuit wiring

    Your MPPT controller is rated at 40 amps @ 12 volts -- So the NEC branch wiring/breaker/fuse would be rated at:
    • 40 amps * 1.25 NEC derating = 50 amps

    So, for your controller, you should have a 50 amp minimum rated wire/breaker/fuse. Using NEC Table, that would be 8 awg to 6 awg cable (depending on insulation type, mounted in conduit, etc.).

    You should also double check the voltage drop... For a typical charge controller on a 12 volt battery bank, I would recommend a maximum of 0.05 to 0.10 volt maximum drop. Using the voltage drop calculator.
    • 40 amps at 3 feet and 0.05 drop => 2 awg gives 0.05 volt drop
    • 40 amps at 3 feet and 0.1 drop => 6 awg gives 0.11 volt drop
    One final question: For the long cable run between the panels and the 3024iL, what would the ramifications be if I used a 20' cable, instead of 17' (since those are easier to find)?

    Not much--That is a relatively small difference in overall length (you can use the voltage drop calculator to confirm).
    This installation is for the top of my RV, which I'll be driving around!

    These are just my first time calculations and anything in here could be wrong. Feel free to critique, where necessary.

    Cheers!

    If you are planning of "weeks+ of dry camping at a time" and want to minimize generator usage--I would also look at the array size to battery bank size. We suggest a 5% to 13% rate of charge for the battery bank (pure off grid system--For RVs, many times you cannot fit enough panels on the roof and need to use genset/AC mains to fully charge the battery bank):
    • 2 * 108 AH batteries * 14.5 volts charging * 1/0.77 panel+controller deratigns * 0.05 rate of charge = 203 Watt array minimum
    • 2 * 108 AH batteries * 14.5 volts charging * 1/0.77 panel+controller deratigns * 0.10 rate of charge = 407 Watt array nominal
    • 2 * 108 AH batteries * 14.5 volts charging * 1/0.77 panel+controller deratigns * 0.13 rate of charge = 529 Watt array "cost effective maximum"

    So, your 500 Watt array is just about perfect for your present battery bank.

    Lastly, if you are camping in the northern US (and/or during winter), you might want to make the panels so they can tilt up for better power collection outside of the summer months (solid mounts so the panels don't blow off when you drive).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • krazykrazy Registered Users Posts: 2
    Re: First PV System! Can you check this math?

    Bill, that was an outstanding and hugely helpful reply!! Thank you! That helps tie together lots of things I was still trying to figure out.

    Let me digest all of this and I'll come back if I have any questions (but, I suspect I won't!).

    Cheers!
  • BB.BB. Super Moderators, Administrators Posts: 30,645 admin
    Re: First PV System! Can you check this math?

    And let us know how it all works out for you... That is how we learn from our mistakes (and you can help answer the next RV questions).

    -Bill :D
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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