How to Select the Correct Size Battery Cable

When selecting the correct size of a battery cable to power your electrical system or project, a few factors need to be considered.You should know what is the maximum amount of current your system will need, how long the cables need to be, and the cost of larger cables versus smaller cables.  When figuring out the length you need, you will add together the lengths of both the positive and negative cables to get your total length.

First let's talk about the importance of current when selecting your battery cable.  Current is measured in units called amps.  Each of your electrical devices will require a specific number of amps to operate.  For example, a light bulb might use 1/2 an amp of current, while an inverter might use 100 amps of current.  Add up the current requirements of all your devices and this will be your maximum amperage needed.  Sometimes, devices might be rated in watts instead of amps.  In this case, simply divide the number for watts by your system's voltage to get the amperage requirements. If a device says it uses 120 watts, then 120 watts / 12 volts = 10 amps.

The second thing to consider when selecting the correct size battery cable is the length of cable you need.  The length is important because as electricity flows through a battery cable, there is an resistance to the flow of that electricity which will generate heat in your battery cable and manifest itself in the form of what is called a voltage drop at the end of the cable. Voltage drop is simply the voltage seen at one end of the cable minus the voltage seen at the other end of the cable. This voltage drop is influenced by the diameter of the copper conductors inside the cable, or the gauge size, and the total length of the cable - The longer the battery cable is, the higher the voltage drop will be, and the larger the diameter, or gauge (AWG) of the battery cable, the less the voltage drop will be.

Why is voltage drop important? Let's say that your length of battery cable is connected to a 12 volt battery and has a voltage drop of 2%, which is considered adequate for almost all electrical systems. At the source, or battery, you will read 12 volts using a voltage meter, but at the other end of the cable your voltmeter will only read 11.76 volts (12 volts -2%). If your circuit needs 100 amps of current, the battery cable will be absorbing 2 amps (24 watts) of current because of it's resistance, leaving 98 amps available. Now lets say that you are using a smaller gauge cable for that same length and your voltage drop is 10%. Now your voltmeter at the other end of your battery cable will read 10.8 volts (12 volts - 10%) and that same 100 amp current draw will cause your battery cable to absorb 120 watts (10 amps) of power, leaving only 90 amps available! Your cable will definitely get warm to the touch.  This amount of voltage drop may begin to cause other devices to not work properly - light bulbs will be dimmer, fans and motors will be weaker, and if the voltage drop is high enough computer systems can fail, and even the cable itself can be destroyed leading to a catastrophic failure and even an electrical fire!

In summary, when selecting the correct size of battery cable, you need to take into account the total maximum amperage your electrical system requires, the voltage of your system (usually 12 volts), the total length that your cables need to be, and the amount of voltage drop you are willing to accept versus the amount you want to spend.  Also, be aware of imported cable versus American made cable.  Many imported brands are sized in millimeters instead of the gauge scale used in the USA.  Imported cable will often be smaller in diameter than American made cable, but be sold as it's gauge counterpart.  The copper used in factories in Asia and India have higher impurities, and the factories are not held to the higher manufacturing standards that American companies are held to.  All of the wire and cable that we sell is American made.

 Battery Cable Amperage Capacity Chart