voltage drop calculator confusion

muddomemuddome Registered Users Posts: 9
I'm confused on what value to enter for my voltage on the NAWS voltage drop calculator.
http://www.windsun.com/Hardware/Voltage_Calc.htm
Do I use my system voltage which is 24v or do I use the Voc from my panels prallel strings which is 133?
Also is the distance one way?

Comments

  • BB.BB. Super Moderators, Administrators Posts: 29,697 admin
    Re: voltage drop calculator confusion

    The distance appears to be one way (using another calculator with same numbers).

    You use the "nominal" or "minimum" operational voltage/maximum current expected at each point in your system.

    If is your battery bank--The worst case is ~21 volts for a low battery/high current/high voltage drop (assuming AC input to your inverter).

    For the solar array, it can be the Vmp-array-hot, or it can be a volt or two above Vbatt-charging--An MPPT controller would be Vmp-array. For a PWM (less expensive controller), the voltage would be ~Vbatt-charging.

    It just depends on how conservative you want to get--Hot copper wire has higher resistance, etc... You will get close enough if you use "nominal" numbers too.

    Voltage drop, for example, on a 12 volt battery bank. Assume battery minimum voltage ~11.5 volts and 10.5 volts minimum for inverter operation. The difference is ~1.0 volt drop.

    For a 1,200 watt AC inverter, the maximum worst case current would be:

    1,200 watts * 1/10.5 volts cutoff * 1/0.85 inverter efficiency = 134.5 amp maximum worst case current @ 1,200 watts

    And, most "good quality" AC inverters will power ~2x rated load for a few seconds (starting a pump for example)--So, you the worst case current could be as high as 269 amps... Therefore, you could use 0.5 volts as your maximum "nominal" voltage drop for a 12 volt battery bank (to allow 2x surge current). Or:

    0.5 volt/12 volts = ~0.04 = 4% maximum drop

    -Bill

    PS: To make it clear, Voc (voltage open circuit) would not be your base voltage--Since there is no current flow. You actual voltage drop would be Vmp-array (close enough).
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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