Properly fusing a Circuit

Hi everybody!

Have been having a lot of fun wiring various cars here and there, but I recently took on the project of both fuel injecting and re-wiring an entire 1955 Thunderbird. Not particularly worried, but I guess my biggest concern comes down to how to properly size the wires and fuses within a circuit like an OEM would do in a new vehicle.

For instance, I'm narrowing down my wiring system between things that need constant 12v, and things that need switched 12v. Among the things that need constant 12v is the brake light system. I'm trying to limit the amount of fuses I need to account for in this car, so for this instance, we are converting these lights to LED's, and accounting for voltage drop, etc. I was planning to run 20ga wire to the lights on a 7.5a fuse through a Relay, but run much smaller gauge wire (maybe 22/24) to the switch in the cabin. My fear is that if for some reason the smaller gauge circuit for the switch fails, the wire could theoretically cause a fire (or any number of issues) long before the fuse itself blows.

I'm running into a similar dilemma with all of the switches in the car actually. (ignition, hi/lo beams, wiper, blower motor, turn signals, power windows, etc.) I would really like to protect several circuits together to cut down on the size of fuse box I need, but I would like to avoid having to run mondo wiring to all of the switches. What is the proper (professional) way to go about this? Same for circuits like me deciding to run the clock, radio, and maybe the courtesy lights on one circuit? Thanks!

My rule of thumb, and i've done a complete re-wire of a car, plus much much wiring on 4wd's and other vehicles. is that any circuit needs to be considered on its own, and that every circuit should be fused, and that fuse should be lower than the lowest rated item in that circuit (wire, switches, connectors, pins, and the thing its powering etc). And the fuse should be as close as possible to the circuits power source, either battery or a fuse box (in which case the fuse box would have a fuse near the battery)

Eg if you have a 5A rated switch, to a 3A max rated light running, with 7A rated connectors on a 2.4A rated wire, and that light typically draws 1.5A. Then the fuse should be 2A that is providing power to the switch.

If you say had 4 of these lights and you want to have these as a single circuit, each light with its own wire running back to say the switch, then the switch needs a 6A power feed, and is now 1A over its rating, the connectors will be fine, and the wire to each light is fine, assuming they are damaged, grounded or the light decides to draw its max due to an issue.

So if one wire for one of the lights got damaged or the light was faulty, then in theory that wire would draw up to 6A before the fuse blows, nearly 2.5 times the wires rating.

You've now set fire to the car, which is bad.

So lets assume you want to combine these, which many OEM's do then you'd want to ensure that the wire running to each light is capable of handing the entire load of the circuit on its own plus some margin for error and spikes (like the light first starting) and also the length of the wire run. So lets say to be safe you use a 10A rated wire. And also swap the switch out for a 10A rated switch.

Now, if the same issue was to occur and one of those wires was to be damaged, same fuse. This time, the fuse will blow. And, best thing. Car isn't on fire.

Using this theory, you can combine related items where it makes sense to do so, eg 1 circuit (1 fuse) to run all the parkers lights, one circuit and fuse for headlights, one for high beams, one for left indicator, one for right.

An your example of "clock, radio, and maybe the courtesy lights" also is fine. And if you aren't sure on what any of these items draw, use a multimeter or clamp meter to check, add up all the current of the loads, pause a think of anything that might cause that load to change, eg turning up the volume, wiper speeds etc. Then pick the switch, fuse, and wire to suit the total potential load of the circuit plus a safety factor

And keep in mind, current changes relative to voltage. Eg if you set your fuses around the current for when the engine is on, the alternator will provide 13 or 14V. But turn the car off and it will drop closer to 12. So if your lights normally draw 10A when the engine is on, if you turn the engine off, it may draw 11 or 12A because the voltage is lower yet the power draw is the same. This is why you always want that safety factor, i usually run with around 30% for things that have fairly stable current draw, like lights, but more like 30->50% for fluctuating things, like electric motors