#4 battery cable

charlesduran
charlesduran Registered Users Posts: 1
in Solar Beginners Corner #1
Could someone explain the voltage drop per foot in #4 battery cable at 12 volts running to a 2000 watt inverter, in other words how long can I run a # 4 cable from a 12 battery to an inverter that could supply 2000 watts, with out losing too much power through cable wire resistance?
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  • Blackcherry04
    Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    #2
    Re: #4 battery cable
    charlesduran wrote: »
    Could someone explain the voltage drop per foot in #4 battery cable at 12 volts running to a 2000 watt inverter, in other words how long can I run a # 4 cable from a 12 battery to an inverter that could supply 2000 watts, with out losing too much power through cable wire resistance?
    The 2000 watts is AC current measurement , the inverter uses DC current @ 12 V. 2000 watts AC pulls over 200 + Amps DC @ full load, that would require at least 4/0 cable and a 250 amp fuse and a battery bank large enough to supply that amperage without a voltage sag. #4 cable would be like having a 1/4 " hose on a fire truck.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,642 admin
    #3
    Re: #4 battery cable

    With a 12 volt system... You can run the battery down to ~11.5 volts and the inverter will cut off at ~10.5 volts. Most inverters will supply about 2x surge current...

    So, if you have a 2,000 watt inverter and a heavy starting load (such as a well pump), then you can use a generic voltage drop calculator to figure out how long a wire at XX current will give you a a maximum of 0.5 volt drop (allowing for surge current).
    • 2,000 watts * 1/0.85 inverter efficiency * 1/10.5 inverter cutoff = 224 amp max continuous current

    Using a #4 cable, and a generic voltage drop calculator:
    224 amps @ 0.5 volt drop for #4 cable => 3.5 feet of one way cable length will give you ~0.47 volt drop.

    Now, there is another issue... If you use NEC, the cable and fuse/breaker rating should be:
    • 224 amps * 1.25 NEC derating factor = 280 amp circuit/breaker

    And using the NEC, the wire gauge should be around 250 cmills or larger (that is one size larger than 4/O cable--not a small diameter wire).

    If you use the ABYC chart, you could use around 1/O to 2/O cable.

    12 volt DC systems are difficult to wire at higher inverter power levels because of the very high DC currents and low allowable voltage drops.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #4
    Re: #4 battery cable

    Welcome to the forum.

    The issue is a combination of the current @ Voltage over distance through the wire. The current will vary for a a given output Wattage as the Voltage available to supply it goes down; a "12 Volt" system runs a range of Voltage that may be as high as 15 when Absorbing or as low as 10.5 when batteries are depleted.

    If you use the nominal Voltage and recognize the tolerances you can get a current of (2000/12) 167 Amps plus conversion losses and whatever the particular inverter uses itself. Without knowing the precise numbers it is best to count on the maximum draw being 200 Amps.

    If you check that against a chart listing the current carrying capacity of #4 wire you'll find it isn't capable of handling it. If you want to run a 2kW inverter from a 12 Volt battery bank you really should use the largest wire possible, typically 4/0 (aka 0000). It can handle over 300 Amps.

    Then you have the question of "acceptable" Voltage drop. As a rule it is wise to keep it under 3%. So in this case 4/0 wire on a 12 Volt system with 200 Amps is good up to about 18 feet.

    Keep in mind that Ampacity goes down if the wire needs to be encased in conduit.
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