Wire sizing

sridharTSsridharTS Registered Users Posts: 33
How to choose the wire guage for solar sytem in accordance with volts,amps and feet

Comments

  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    There are two issues.  Ampacity and voltage drop.  Ampacity is the maximum amperage that a wire can safely handle.  It varies with the insulation and ambient temperature.  There are plenty of ampacity charts on the internet.  Voltage drop depends on both the gauge and length of the wire.  There are plenty of voltage drop calculators on the internet.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • oil pan 4oil pan 4 Solar Expert Posts: 759 ✭✭✭✭
    edited November 2015 #3

    What the voltage and amps you think it will run?

    Remember a 1 volt drop across wires in a 12 volt system is double the power loss of a 24 volt system and 4 times the loss in a 48 volt setup.

    A 1 volt drop is about the absolute max you want to see in a 12 volt system. If you have a 1 volt drop on your 12 volt system you just wasted 6% to 8% of your power on heating up the wires.

    Solar hybrid gasoline generator, 7kw gas, 180 watts of solar, Morningstar 15 amp MPPT, group 31 AGM, 900 watt kisae inverter.

    Solar roof top GMC suburban, a normal 3/4 ton suburban with 180 watts of panels on the roof and 10 amp genasun MPPT, 2000w samlex pure sine wave inverter, 12v gast and ARB air compressors.

  • sridharTSsridharTS Registered Users Posts: 33
    Give me a example.

  • BB.BB. Super Moderators, Administrators Posts: 28,186 admin
    I don't know what standard India uses (AWG, mm diameter, mm^2 cross sectional area, or what--So, I will use AWG (American Wire Gauge) because it is easy for me.

    Say you have a 1,200 Watt solar array with Vmp~17.5 volts per panel. You can put them in parallel to charge a 12 volt battery bank with a PWM solar charge controller, or you can put them in series/parallel with a MPPT charge controller (higher end controllers are around 100 Volts max for Vmp):
    • 1,200 Watts / 17.5 volt Vmp per panel = 69 Amps Imp-array
    Using a generic voltage drop calculator like this:

    http://www.calculator.net/voltage-drop-calculator.html

    If we want a maximum of 1-3% voltage drop (typical for solar) and want to run the wiring ~33 feet (~10 meters) from panels to charge controller):
    • 17.5 volts * 0.03 volt drop = 0.525 volt drop maximum, 69 amps, and 33 feet--Voltage drop calculator would tell us:
    1/0 copper cable (would give numbers below)
    Voltage drop: 0.45
    Voltage drop percentage: 2.57%
    Voltage at the end: 17.05
    http://www.rapidtables.com/calc/wire/awg-to-mm.htm (AWG converter)
    8.2515 mm
    53.4751 mm^2

    Now, lets say we were using 140 Watt Vmp~17.5 volt panel, and we wanted around 100 Volts Vmp maximum for our MPPT controller:
    • 1,200 Watts / 140 Watt per panel = 8.6 panels
    • 100 VDC / 17.5 volts = 5.7 panels in series
    So, if I wanted 8 panels, I could put:
    4 in series * 2 parallel strings
    9 = 3s*2p
    10=5s*2p
    Lets do 10 panels (5sx2p) for a 1,400 Watt array (just to show the wire size reduction.
    • 5s * 17.5 volts = 87.5 volts
    • 140 Watts / 17.5 Vmp = 8 amps Imp per string
    • Vmp-array=87.5 volts; Imp-array=16 amps (2x parallel strings)
    • 87.5 volts * 0.03 max drop = 2.625 volt drop, 16 amps, and 33 volts--Voltage drop calculator gives:
    14 AWG @ 33 feet:
    Voltage drop: 2.67
    Voltage drop percentage: 3.05%
    Voltage at the end: 84.83
    1.6277 mm
    2.0809 mm^2

    Or for a slightly larger array (and higher working voltage), we can use about 1/26 times less copper wire vs running at a low "17.5 volt" working voltage.

    Does this sort of make sense?

    If you give us a "real" system design, and we go through the steps--It will make more sense. Just pure "what if" questions get a bit confusing.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • sridharTSsridharTS Registered Users Posts: 33
    Thank you guy.

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