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Old 07-23-2015, 08:00 PM   #1
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Wiring resistance

My used Damon Daybreak had non-working fog lights when I acquired it. I traced the wiring to a melted in-line fuse connection.

Initially, a large wire (possibly 12 ga.) came from the fuse box under the dash into the female connector, droped size (maybe 20 ga.?) before going through a 20 amp in-line fuse, and resuming the same size to an in-dash switch, then to the fog lights. The melt down happen AFTER the fuse.

I installed a new in-line fuse, the lights powered on, but the wire after the fuse starts to heat up, as well as all of the smaller gauge wire leading to the dash switch. Initially I had a 40 amp fuse, but the condition remains present when I drop to a 20 or 15 amp fuse. I know the heat is due to resistance, and I've thought about running large gauge wire to and through the switch, but I'm concerned about resistance on the smaller gauge wire at the lamps.

Any thoughts?
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Old 07-23-2015, 08:31 PM   #2
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First things first. Are the fog lamps factory or aftermarket? If they are factory you'd have to assume that the wires at the load are the correct size for the current they have to carry.

If it's an after market light set up or wiring that has been changed for what ever reason I'd rewire everything and match the wire size at the (load) lights. Also make sure the switch is capable of handling the current. Again if it's after market you don't know what was changed. If you're not sure on the wire size go with bigger for added current capacity.

From your comments you know what you are talking about. Yes the larger gauge wire (smaller numbers) will carry more current and the smaller wires will get hot if the device they are feeding requires more current than they were meant to carry.

Go to a parts house and look at some wire on spools and check the diameter as a reference. Most auto wiring is going to be 12, 14,16,18 gauge. Check on line for a chart to match amperage draw with wire sizes so you can get wire large enough to handle the current.

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Old 07-23-2015, 10:07 PM   #3
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First things first. Are the fog lamps factory or aftermarket? If they are factory you'd have to assume that the wires at the load are the correct size for the current they have to carry.

If it's an after market light set up or wiring that has been changed for what ever reason I'd rewire everything and match the wire size at the (load) lights. Also make sure the switch is capable of handling the current. Again if it's after market you don't know what was changed. If you're not sure on the wire size go with bigger for added current capacity.

From your comments you know what you are talking about. Yes the larger gauge wire (smaller numbers) will carry more current and the smaller wires will get hot if the device they are feeding requires more current than they were meant to carry.
Go to a parts house and look at some wire on spools and check the diameter as a reference. Most auto wiring is going to be 12, 14,16,18 gauge. Check on line for a chart to match amperage draw with wire sizes so you can get wire large enough to handle the current

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Hey TeJay,
How is ya Partner? Your evaluation of "factory wiring" might be a tad bit off here. I say that because, there's quite a few documented cases here and on other RV forum sites where, folks have re-wired headlights on many coaches, using a larger gauge wire. The did so because after testing the output voltage at the headlights, they've found less than 12V in many cases. So, they simply installed a relay, some larger gauge wire and, using the stock headlight wiring to trigger the relay, they gained much brighter headlights.

So, as for the OPs fog lights, they may or, may not be factory wired. But, what he found was an overloaded circuit. Now, at this point, what I'd do is, test each light, with a jumper of larger gauge wire and make sure that they both work as expected. Then, once it's proved they both work well, then decide what gauge wire will be needed to supply the correct load, from the point of power supply, through the switch, through a fuse and, to the load. Good luck.
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Old 07-24-2015, 06:08 AM   #4
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Fog lights unless they are are very low outpost should run through a relay. And have 12 gauge wire through out the circuit.
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Old 07-24-2015, 06:25 AM   #5
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Fog lights unless they are are very low outpost should run through a relay. And have 12 gauge wire through out the circuit.
Those things are pretty small. If aftermarket, what is the power draw of these?
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Old 07-24-2015, 06:34 AM   #6
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Changing fuse sizes has no effect on resistance. Putting a large fuse in is really dangerous. The wire will not handle 40 amps and if it melts and shorts you could have a real disaster.

Fog lamps can require a lot of amps. Insufficient wire diameter will cause heat build up. Either run a heavier guage wire from the new fuse all the way through the switch and to the lights or install a relay and run heavy wires to them.

I suspect you are close to 20 amps with those halogens. Put a 10 amp fuse in the line and see if it pops. If it does you know they are drawing a lot. Disconnect one lamp and put a new 10 amp in. If it does not pop then each light is a bit less than ten amps each.
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Old 07-24-2015, 07:29 AM   #7
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Most fog lights are rated at 55 watts. Each light will draw a little over 4 amps. An 18 gauge wire is sufficient to power each light and a 16 gauge to power the pair. IMHO, "fog" lights are useless for providing any meaningful light. "Driving" lights would provide a longer-range light, but should only come on when the high beams are on (and shut off with the high beam switch).
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Old 08-01-2015, 08:17 PM   #8
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I knew the fuse wasn't going to provide resistance, I was trying to figure out where the blowing point (didn't have a multimeter at the time). FIRE UP, after reading your post, I was secretly hoping to have intensity gain, and it worked! I ran 14 ga. wire through a relay (before your tips, I never knew how they worked), and instantly I see a difference. These lights came with the rig, so spending >$10 to get them working and learn something new in the process seemed like a better option. It looks like the next project will be rewiring the headlights! I greatly appreciate all your input.
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Old 08-02-2015, 07:08 AM   #9
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If you measure the voltage on the fog lamps now you will have a good reference to measure the voltage on the headlights. Be sure to measure the hot side and the ground side of the headlamps. If you have any voltage at all on the ground side then the ground needs to be repaired.
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