30' Run from BB to Stepdown Converter

Caneman
Caneman Solar Expert Posts: 71 ✭✭✭✭
in Solar Beginners Corner #1
I want to run a 30' wire from the 24v battery bank to a 24v/12v stepdown converter... I think I understand the voltage drop, wire size, and over current protection issues... is this 30' run unsafe and a bad idea?

I am doing this because it seems much safer, easier, and more practical to make the 30' run with 24v rather than 12v... the load is 300w, with a 600w startup surge... here are the preliminary calculations I get:

600w/24v = 25a
Voltage Drop Index, VDI with 5% drop = (25a * 30')/(24v * 5) = 6.3
Next Size Up Using VDI Table = 8 awg wire (VDI = 8 for #8 wire)
Converter input voltage range is 20-32 vdc, so with a 5% drop on the input voltage = 24 * 0.95 = 22.8v, within range

Seems like this might work OK, but I am not so sure... what do you think?
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Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: 30' Run from BB to Stepdown Converter

    Logical, and I came to the same wire size based on 25 Amps and 30' run; 8 AWG resulting in a 4% drop or about 1 Volt.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #3
    Re: 30' Run from BB to Stepdown Converter

    For a 30', 25 amp run of 8 awg wire, I get about 1.1 volt drop. It should work... But be aware you are losing about:
    • 1.1 volts * 25 amps = 27.5 watts of energy due to I2R heating of the wiring alone (12 volt run would be much worse).

    Will there be any surge current involved (starting motor load, etc.)? That could cost more energy/voltage drop.

    Install your (25 amp * 1.25 = 31.25 amp) next size larger fuse (or about 40-55 amps depending on wire insulation/conduit fill/etc.) to protect your wiring (install breaker/fuse near battery in positive lead).

    Also, there are a some 24/12 volt converters that are bidirectional--I.e., if you are charging a local 12 volt battery (from a car alternator or AC battery charger), it can "back feed" and start charging your 24 volt battery bank too (may be a good thing or not).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Caneman
    Caneman Solar Expert Posts: 71 ✭✭✭✭
    #4
    Re: 30' Run from BB to Stepdown Converter

    thanks guys for keeping me safe... BB, that 25a was the surge/startup load... i'll have to see if this particular converter is bidirectional
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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #5
    Re: 30' Run from BB to Stepdown Converter

    Is it a DC load, or AC load ?

    If AC, I'd park a 12V battery and 300W SureSine at the panels, and run 120V to the load. Maybe even do that anyway, and down-convert at the load to the 12V DC. Use the remote shut-off on the suresine when there is no load.....
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  • Caneman
    Caneman Solar Expert Posts: 71 ✭✭✭✭
    #6
    Re: 30' Run from BB to Stepdown Converter
    mike90045 wrote: »
    Is it a DC load, or AC load ?

    If AC, I'd park a 12V battery and 300W SureSine at the panels, and run 120V to the load. Maybe even do that anyway, and down-convert at the load to the 12V DC. Use the remote shut-off on the suresine when there is no load.....

    this is for DC load only...
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