There are 3 variables that determine how thick a battery wire should be:
- The maximum desired voltage drop, which should not exceed 3%. The thicker the cable, the more efficient it’s going to be.
- The maximum amount of current (in Amps) that’ll be flowing through the wire (from or to the battery). The higher the current, the thicker the cable needs to be.
- The distance between the battery and the load (inverter or appliance), or between the charge controller and the battery. The longer the distance, the thicker the cable needs to be.
In this quick article, and using these variables, I will provide a couple of tools that will help you size the cables running from and to your battery.
What is the cable size for a 12v 100ah battery?
Your 12V-100Ah battery will have 2 pairs of cables connected to it, a positive and a negative running from the battery to the inverter (or the appliance), and a positive and a negative running from the charge controller to the battery. And these 2 pairs of wires will not necessarily be of the same size.
At a certain desired voltage drop (3% for example), the size of each pair of wires will depend on the amount of current running through the wire, and the distance between the battery and the device (inverter or charge controller).
So, first, let’s see how you can determine the cable size that you need from your 12V-100Ah battery to the inverter (or load in general).
What size wire from the battery to the inverter?
For a maximum voltage drop of 3%, the size of the wire that you need to connect your 12V 100Ah to an inverter will depend on 2 factors:
- The Continuous Power rating of your inverter (in Watts).
- The distance between the battery and the inverter.
For example, a 12V-100Ah battery running a 500W inverter that is 5 feet away, would require a 6 AWG (13.3mm²) copper wire in order to not exceed an acceptable voltage drop of 3%.
If instead, the inverter was 10 feet away, the battery would require a 3 AWG (26.7mm²) copper wire to limit the voltage drop at 3%.
However, to comply with the National Electrical Codes (US), which dictates that the ampacity of the wire needs to be equal to or greater than 125% of your maximum current (Max. current x 1.25), you would need a 3 AWG (26.7mm²) copper wire for both distances.
To make things much easier, below, you’ll find our battery to inverter wire size calculator that will determine the cable size that you need based on the maximum amount of current that your inverter is going to use, the distance (in feet) between your battery and inverter, and the voltage of the battery (12V).
In the calculator, please enter the following:
- The Continuous Power rating of the inverter (in Watts): this is the main rating of the inverter (700W for example). Make sure not to confuse it with the Surge Power rating of the inverter.
- Battery Bank’s voltage (Volts): this is the voltage rating of the battery, which in this case is 12V.
- Distance between battery and inverter (feet): this is the one-way distance (in feet) between the terminals of your inverter and the terminals of your battery.
For examples and a more thorough explanation, please refer to this page: What size wire from battery to inverter?
Now let’s see the size of the wire that you need to connect your charge controller to your 12V-100Ah battery.
What size cable from charge controller to battery?
Again, at a maximum voltage drop of 3%, the size of the wire from the charge controller to the battery will depend on:
- The amount of current (in Amps) at which you’re going to charge the battery, which in this case, is the Output Current rating of the charge controller.
- The distance between the charge controller and the battery.
For example, if the 12V-100Ah battery is being charged by a 15 Amp solar charge controller, that is 3 feet away from the battery, we would need 6 feet of 14 AWG (2.1mm²) pure copper cable. 3 feet of 14 AWG wire from positive to positive, and 3 feet of wire from negative to negative.
Below, you’ll find our charge controller to battery wire size calculator. In the calculator, please enter the following:
- Output Current rating of the charge controller (Amps): this is the amperage of the charge controller (15 amps for example).
- Battery Bank’s voltage (Volts): If your battery is rated at 12 Volts, select 12V.
- Distance between the charge controller and the battery (feet): this is the one-way distance (in feet) between the terminals of your charge controller and the terminals of your battery.
For examples, feel free to click here.
After determining the sizes of cables that you need, you might want to determine the size of fuses that you need.
What size fuse do you need for a 12V 100Ah battery?
Fuses are rated in Amps, and the Amp rating of a fuse indicates the maximum amount of current that the fuse is capable of conducting without blowing.
The amp rating of the fuse that you need between your 12V 100Ah battery and the charge controller needs to be the same as the amp rating of the charge controller.
The amp rating of the fuse that you need between your 12V 100Ah battery and the inverter should be 25% higher than the maximum amount of current that your inverter is going to use.
The amp rating of both fuses should not exceed the Ampacity of the wires the fuses are connected to.
For example, let’s say you’re charging your 12V-100Ah battery with a 20 Amp solar charge controller, and you’re using an inverter that – at most – pulls 30 amps of current from your battery.
Let’s also say that:
- from your solar charge controller to your battery, you’re going to be using a 12 AWG (3.3mm²) wire that is rated for 167°F(75°C) and can handle up to 25 Amps of current.
- And from your battery to your inverter, you’re going to be using an 8 AWG (8.4mm²) wire that is rated for 167°F(75°C) and can handle up to 50 Amps of current.
A good fuse size between your solar charge controller and the battery would be 20 Amps. And a good fuse size between your battery and inverter would be 40 Amps.
Read more about this topic here: What size fuse for a 100Ah battery?
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