Fog light wiring how much power can they take?

[cw40342]

I've got a 97 Ford Ranger XLT I'm wanting to put a set of lights on it but would love to use the existing wiring for the fog lights. What's the max wattage the wiring can take safely? I obviously don't want any electrical fires!


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[RonD]

You first need to convert Watts to Amps, here: http://www.rapidtables.com/calc/electric/Watt_to_Amp_Calculator.htm

Vehicles use 12volts DC

I think Ford used 55watt fog bulbs, so 110watts

i.e. 110watts at 12v = 9amps

200watts at 12v = 17amps

Then you can use this wire size for length and amps: http://www.hitmanhotrods.com/en/admin/sources/editor/assets/chartsGraphs/wire_sizing_chart2_titled_filled.jpg

Generally speaking in a vehicle 15ft would be longest wire you would encounter.

The Fuse used by Ford on current fog lights can tell you approx. gauge of wire that is there now.
If you use a 10amp fuse you have to use a wire size that can handle up to 15amps, otherwise the fuse is useless since wire would start to melt before fuse blew
If 20amp fuse then you use a wire size for 30amps, ect.....

16ga is found most of the time and 14ga for higher amp circuits

 

[cw40342]

You first need to convert Watts to Amps, here: http://www.rapidtables.com/calc/electric/Watt_to_Amp_Calculator.htm

Vehicles use 12volts DC

I think Ford used 55watt fog bulbs, so 110watts

i.e. 110watts at 12v = 9amps

200watts at 12v = 17amps

Then you can use this wire size for length and amps: http://www.hitmanhotrods.com/en/admin/sources/editor/assets/chartsGraphs/wire_sizing_chart2_titled_filled.jpg

Generally speaking in a vehicle 15ft would be longest wire you would encounter.

The Fuse used by Ford on current fog lights can tell you approx. gauge of wire that is there now.
If you use a 10amp fuse you have to use a wire size that can handle up to 15amps, otherwise the fuse is useless since wire would start to melt before fuse blew
If 20amp fuse then you use a wire size for 30amps, ect.....

16ga is found most of the time and 14ga for higher amp circuits

Awesome thanks for the reply!


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[cw40342]

Awesome thanks for the reply!


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I looked in my EVTM Manual and it says the fuse is under the hood and it is a 15amp Mini Fuse? Does it being a MINI fuse make a difference


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[ericbphoto]

Does it being a MINI fuse make a difference

No. In this case, amps are amps. 15A mini = 15A maxi = 15A.....

Generally speaking, the physical size of the fuse has more to do with what voltage it can handle. The voltage relates to the size of the arc that occurs when the fuse blows at a certain current. Higher voltage has a potential for a bigger arc. Also, fuses of higher current ratings have the potential for a larger arc when they blow.

But that's probably more than you want to know already. Fuse and circuit breaker ratings are complicated. When you consider ohm's law, you find that a true, direct short (0 ohms resistance) creates an infinite amount of current nearly instantaneously. Fuses have to be designed to blow fast enough to limit that current before it becomes infinite and big enough to quench the arc produced when they blow so that a fire does not start.

Ok, I'll shut up.

 

[cw40342]

As illiterate as I am when it comes to electrical that actually made sense to me. [emoji1303]

So far what I have is this-
The factory fog lights take a H3 bulb which from what I read is a 55 watt bulb.
Going by the equation RonD gave me that's
55x2=110 watts

110 watts to 12 volts is 9.16 amps
The thing I'm struggling with is the distance vs. wire size is it 72" of wire for the fog lights or 96" of wire I'm gonna say it's 72" and that would be a 16 gauge wire I think.


Ok I'm rambling that's where I am at though



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[ericbphoto]

The distance is the total length of the path that the 9.16 amps will travel. So you would add x inches from source to fuse plus you inches from fuse to relay or switch plus z inches from relay or switch to fog lights ( use the distance to the further light), plus w inches from light to ground.

Even though the wire will split somewhere and only half of the current will go to each light, you still use the same size wire. That is because the fuse is sized to protect the lights and the wire from overheating and melting. Also make sure your switch or relay contacts are rated for the same amount of current as your wire and fuse. A circuit is like a chain. Every link must be strong enough to handle the whole load.

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[ericbphoto]

I may have lost track of your original question. "How much load can the wire take?"

If your truck is similar to my '93, the fog lights probably have 16guage wiring. It most likely originates from a fuse in the engine bay fuse box and is controlled by a relay in that same box. This is just a few twisted feet away from the actual lights and the whole circuit is most likely good for 15 amps. 15 amps X 12volts equals 180 watts total or max 90 watts each for two lights. It is best to leave some margine for error and limit yourself to 80 per cent, maybe about 75 watts each. If you want more than that, you should rewire the circuit using the principles we outlined above.

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[cw40342]

I may have lost track of your original question. "How much load can the wire take?"

If your truck is similar to my '93, the fog lights probably have 16guage wiring. It most likely originates from a fuse in the engine bay fuse box and is controlled by a relay in that same box. This is just a few twisted feet away from the actual lights and the whole circuit is most likely good for 15 amps. 15 amps X 12volts equals 180 watts total or max 90 watts each for two lights. It is best to leave some margine for error and limit yourself to 80 per cent, maybe about 75 watts each. If you want more than that, you should rewire the circuit using the principles we outlined above.

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I am thinking of putting a set of LED round lights 55 watts each maybe?? I'd say my wiring is the same as your 93 mine is a 97.