Wire Amperage Capacity Chart
In order for your automotive electrical system to perform properly, your wire must be sized properly based on the amount of current (amperage) the wire must carry, and the length that the wire needs to be. For example, you want to add some flood lights to your vehicle. The floodlights draw 15 amps of current. The length the wire needs to be to connect to the floodlights is 9 ft. Based on the chart above, you would want to use 12 gauge automotive wire for your floodlights.
Why is the size of an automotive wire so important? When electricity passes through a wire, The wire resists the electricity and some of it is converted into heat that is absorbed by the wire. This lost energy is realized in the form of a voltage drop at the end of the wire. In its simplest terms, you can think of 12 Volts as during 12 units of work. If you’re wire has a 2% voltage drop, then the wire is using up 0.24 volts, or units of work. That leaves 11.76 volts, or units of work left. So if you put a volt meter on your battery and read 12 volts, when you put your volt meter on the end of the wire, you will read 11.76 volts. If you are running 20 amps through your wire then your wire is absorbing 4.8 watts of power. Now let’s say your wire has a 20% voltage drop. The voltage reading on your volt meter at the end of the wire will only be 9.6 volts, and the wire is absorbing a whopping 48 watts of power!
Is it better to solder or to crimp connectors to automotive wire? Soldering is superior since it creates a maximum amount of surface contact between the wire and the terminal or connector. However, in 95% and more cases, a good crimp will work just as well. The key is to make sure that there is good electrical contact between the copper strands of the wire, and the barrel of the terminal or connector. Basically, if you crimp your wire and terminal together, and you cannot pull them apart, the connection will most likely suffice. It is always a good idea to cover the terminated ends of your automotive wire with some heat shrink tubing.
When choosing the size of a battery cable to power your electrical system or project, it is important that it is sized appropriately. As electricity flows through a cable, there is an inherent 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. 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, 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? for example, let’s say that your length of battery cable is connected to a 12 volt source 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 absorb 24 watts of power because of it’s resistance. Now lets say that 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 of power! Your cable will definitely get warm to the touch! This amount of voltage drop will 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!
When determining the length of cable you need for your circuit, both the positive cable, and the negative cable need to be considered, especially if you are wiring a vehicle that does not have a chassis ground such as a boat or other vehicle with a fberglass body. So if you have a 10 foot run from your battery, you actually have a total of a 20 foot run because the negative cable will also be 10 feet back to the battery.