noob seeks help wiring bed lights in to truck.

[mikepike1125]

So my basic plan is to tap into the wire that provides power to the 7 pin trailer harness plug located at the rear bumper. From that tap I will run wire up and into an on/off switch that I will locate in my truck bed and then on to the lighting kit which will then, if all goes as planned, light up my truck bed. I will be using LED's so the power draw will be minimal. So my questions are these....

1. Though the trailer plug 12 volt power wire is already fused at the fuse box with a 40 amp fuse, I would like to add another fuse between the tap location and bed lights to specifically target the lights should anything go wrong. Where should I position this fuse link? Between the tap location and the on off switch? Between the on/off switch and the first light? And what amp fuse do I use?

2. Is connecting different gauge wire dangerous?

3. Can I connect the negative wire from the lights to the negative wire on the trailer plug? Or would it be better to just run it into an existing ground on the truck?

Sorry for the long write up but this is all new to me and I love my truck and don't want to see her burn to the ground.....

Thanks in advance

 

[Diver300]

You should put a fuse where the wire gets smaller. If you are running LED lights, they might only take 1 Amp or less. I suggest you use some thin wire, as it only has to take 1A, and thin wire is easier to run. Fit a fuse with a lower rating than the wire. You should use the same gauge of wire that the 12 Volt power is wired with from where you tap off to the fuse. After the fuse, use thin wire, to the switch and the lights.

The negative can go to which ground is easiest to wire up to. It makes no difference.

The large gauge wire that the 12 V power is wired with is protected with the 40 A fuse. If that wire shorts to the chassis, the 40 A fuse will blow and the large wire will not get hot before the 40 A fuse has blown.

If you are using thin wire, it must be capable of taking enough current to blow the smallest fuse that it is connected to. So if the cable is rated to 5A, a 5A or 3A fuse will blow before the cable is hot.

I hope this explains things.

 

[mikepike1125]

You should put a fuse where the wire gets smaller. If you are running LED lights, they might only take 1 Amp or less. I suggest you use some thin wire, as it only has to take 1A, and thin wire is easier to run. Fit a fuse with a lower rating than the wire. You should use the same gauge of wire that the 12 Volt power is wired with from where you tap off to the fuse. After the fuse, use thin wire, to the switch and the lights.

The negative can go to which ground is easiest to wire up to. It makes no difference.

The large gauge wire that the 12 V power is wired with is protected with the 40 A fuse. If that wire shorts to the chassis, the 40 A fuse will blow and the large wire will not get hot before the 40 A fuse has blown.

If you are using thin wire, it must be capable of taking enough current to blow the smallest fuse that it is connected to. So if the cable is rated to 5A, a 5A or 3A fuse will blow before the cable is hot.

I hope this explains things.

Ok I see what you're saying. So same gauge wire from tap to the fuse and then smaller gauge from the fuse to the on/off switch. So, to be sure, I bought something like this.... https://southerntier.us/products/ATO-Fuse-Holder-With-Splashproof-Cover-and-12-AWG-Wire.html ..... will this work for what I want to do?? I could only find one of these with a 12 gauge wire and I am not sure on the size of the trailer plug wire. but its should be either 12 gauge or 14....

 

[mikepike1125]

after doing some research the power wire for trailer plug might be a 10 gauge wire. Should I get a 10 gauge fuse holder instead? Or would a 12 gauge still work ok?

 

[Diver300]

Well you should really put a tap in the same gauge of wire, and it would be very difficult to get 10 gauge wire into a 12 gauge fuse holder.

However, a short length of smaller wire is most unlikely to cause a problem. If you have smaller wire with a large fuse, there are two possible ways that there can be a problem.

1. There is a short circuit and the current isn't large enough to blow the fuse quickly, so the heating continues for some time and damages the wire.
2. Too much load is taken from the circuit.

Well if you have a few inches of 12 gauge wire tapped off the 40 A circuit, and then a 5 A or less fuse, neither of those conditions can happen. The 5 A fuse is plenty small enough to protect the 12 gauge wire. If the 12 gauge wire is shorted to earth, there is only a few inches of it, so it has very little resistance and the current will be nearly as large as if the short were in the 10 gauge wire. 10 gauge wire is only about 1mΩ per foot, so if you have a 20 ft of that, it will only have a resistance of 20 mΩ, so if there is a short, there will be about 500 A flowing, and a 40 A fuse will blow very quickly, before any damage is done.

If you do use wire smaller than 10 gauge between the tap off and the fuse, keep it as short as possible.

 

[mikepike1125]

I could solve all this by ordering the 10 gauge in line fuse holder but I would have to order that on line and wait for it to be shipped.... I have the 12 gauge in line fuse holder in hand and it would only be a finger lengths run from the 10 gauge wire to the actual fuse holder. Basically the only 12 gauge section is the section of wire that comes out of the fuse holder. So I could shorten this even further to a minimal working length of 2 inches or so to tie it all in.



Next question.... If I were, in the future, to tow a trailer that would then draw power through the trailer plug wire that I tapped off of, would this then send a potential 40 amp maximum charge (while towing the trailer) up and into the 5 amp fuse in place for the bed lights? I.E. would this then cause the 5 amp fuse to blow every time I tow?


and again, I thank you for your patience

 

[Diver300]

Next question.... If I were, in the future, to tow a trailer that would then draw power through the trailer plug wire that I tapped off of, would this then send a potential 40 amp maximum charge (while towing the trailer) up and into the 5 amp fuse in place for the bed lights? I.E. would this then cause the 5 amp fuse to blow every time I tow?

No, there would be no problem. The voltage is 12 V everywhere. (14 when the alternator is running). The lamps will take nothing when turned off, maybe 1 A when turned on. Nothing that the trailer can do will change the voltage much, so it will have nearly no effect on the current the lamp takes.

Current has to flow around a circuit. When your trailer takes some current, current flows from the battery, along the 10 gauge wire, to the trailer socket, through whatever load in the trailer and then down the ground wire, back though the ground connection on the trailer socket, through the chassis and back up the battery negative lead.

No other wires have the trailer current flowing along them if the trailer is the only thing turned on.

In your truck, the starter motor probably takes 300 - 500 A. Your 40 A trailer supply is tiny compared to that, but it taps off the battery and the 0 gauge or bigger wire that can, and does, provide 300 - 500 A. Well that 40 A fuse (or any of the fifty or so fuses in the truck) don't blow just because you start the engine.

The only possible issue is that if the trailer shorted, that would blow the 40 A fuse that feeds it, so the truck bed lights would go out, until you removed the fault in the trailer and changed the fuse.

Separate circuits are used where you need to protect against faults in one circuit from blowing fuses that stop other circuits. Generally trailer circuits have fuses so that a faulty trailer can't affect the truck too much, but as it is only the truck bed lights, it wouldn't be dangerous if they turned off unexpectedly because of a faulty trailer.

 

[mikepike1125]

roger that. I think I am good to go.

Thanks again!

 

[tinkerunique]

I missed some info........ where were the lights to be mounted AND what were they being used for. Clearance lights/tail lights, stop lights, bed illumination lights for cargo or mood lights for 'tailgaiting' ?? If they were to be mounted up front, and 'Mood lights, I'd run a new wire, separate fuse and switch from the battery and come up between the cab and bed. A mercury switch can be mounted on the tail gate to light the bed when the 'gate is dropped and will go off when the gate is closed. <like under-hood or car-trunk lights]

 

[mikepike1125]

I was actually able to complete this project with success. Lights are working great. They are light pods I installed under the bed rails to illuminate the bed.