Wire Selection Formula, Charts, etc

Thread Starter


Joined Apr 10, 2018
Good Morning

Long time reader, First time poster.

I am a Mechanic for my Local fire department. Part of my job is fitting out our new and existing trucks with lighting, accessories, etc. I have a strong background in automotive electricity, but lack skills in proper design of circuits, mainly proper wire size. Iv always went overkill on wire size, which I know is not always a bad thing, but I would like to learn the PROPER way to select proper wire sizes for circuits. I would also like to know how to select the PROPER rated fuses for the circuit. Every item I read online seems to have a different opinion on the fuse rating selection (IE: one site says select wiring 25% above max circuit load, Other sites say 30)

I would appreciate any information, resources, Charts, Article's etc that anyone could provide me

Thank you so much for your time


Joined Nov 29, 2011
All that you want is not found in a chart that I know of. As you have found out, it is mostly "rule of thumb" stuff. Running a 2 foot wire is different from running a 50 foot wire, both with the same load. You have to treat the wire as a resistor, which for long runs can be significant. Here is a calculator for voltage drops over long runs. Let's say that you want to add a light feature that requires 5 amps at 10.5V at the end of a 50 foot run. You have available 12V at the source (engine compartment). Plugging in these requirements into the calculator shows that 14 gauge wire has a ~ 1.3V drop over the run, leaving you with 10.7V at the load end, so 14 gauge is the MINUMUM size you should use. For future expansion of additional features, you might want to string 12 gauge wire. As you are aware, it is better to err on the side of larger wire as opposed to smaller.

My opinion is that if you choose your wire so that the voltage drop is acceptable to your accessories, you'll be fine.

As for fuses, it depends a lot on what you are fusing. Most resistive loads, like lights are fairly straight forward and fuses can be selected to be 25-30% above the max current, but things like motors can be more difficult. If a motor has a start current that is 150% of its running current, you might want to use SLO-BLO type of fuses that will briefly allow current beyond their rating to pass without blowing.


Joined Nov 18, 2012
I agree with SLK001 that choosing your wire according to the load device is your first priority. When designing automotive circuits:
1 Test the load component for maximum current draw that is required. Make note of any surge current for startup as this will be necessary for circuit protection.
2 Refer to the American wire gauge charts available on the Internet for wire size. There are charts that outline amperage vs feet and gauge of wire required. You can also use a calculator as outlined above.
3 Layout where you want to run your wiring and keep in mind EMF factors. This may cause communication problems if not routed properly. Keep your connection points to a minimum.
4 choose your control devices according to amperage, voltage and wattage. Location is also important.
5 Choose your circuit protection as mentioned above. For high surge startup devices, we typically use circuit breakers in our industry. This allows surge and protection. Motors, heater circuits etc
6 Wire it up and keep environment in mind.
Good luck with your projects