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Wire gauge questions

Discussion in 'Lighting' started by brow, Jun 9, 2010.

  1. brow

    brow [OP] Well-Known Member

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    I was wondering if anyone knew what gauge wire I should be using to wire from my switches to my lights I'm installing in my truck bed. Theire are two 55 watt lights, plus one LED indicator light.

    I'm using a 15 amp inline fuse, 12 gauge from the battery to my first switch. From this point on is where I am not sure what gauge to use, I have two switches, the two 55 watt lamps, and the single LED.

    Any suggestions? This is my first wiring addition to my new truck and I would rather not make a dumb mistake and melt something down or light it on fire.
     
  2. Trap

    Trap Well-Known Member

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    15 amps requires 14 gauge minimum. So continue with the 12 gauge. No use changing now if you already have that size. Or if you must then 14 gauge is the minimum size you can use if it's tight and you need more room. The wire is sized by the fuse and not the load that is hooked up to them, it needs to be big enough to blow the fuse in a short circuit.

    But really you should run a relay for the lights and just have the switches powering the relay. It will last forever that way.
     
  3. brow

    brow [OP] Well-Known Member

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    the problem I am running into with the relay idea is that i have SPDT switches, with on/on function. I am making a 3-way switch, with one of the switches in the bed one in the cab. But with a relay I hook up terminal 85 to my switch, run the two wires to my other switch, have then I have a single wire coming out of the second switch, and I have no idea where that is supposed to be wired to.


    In my crappy diagram, the top square is my battery, then the fuse is the small rectangle under that. the large square would be the relay, the fuse connects to terminal 30, the line to the right of that would be the ground, terminal 86. Bottom left corner (terminal 87) is going to my lights and indicator light. and the bottom right corner (terminal 85) is going to my switches. The far end of this is where I am confused as to what I would connect to

    relay circuit.jpg
     
  4. brow

    brow [OP] Well-Known Member

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    bump?
     
  5. cinch

    cinch Member Extraordinaire

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    It seems to me the wire from the switches needs to go to a source of power to trigger the relay. I would tap into the power feeding the cigarette lighter or auxillary power plug.
     
  6. skytower

    skytower Well-Known Member

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    55w 12v lights draw 5.6 amps each. LED's are rated in watts too. Take the wattage and divide by 12. That will give you the amp draw. Amps of each device are added together: 11.2 amps plus led=total. You only want to run %80 of the wire's max load. Use the fuse of the wire's max amp rating. Keep the same size wire throughout the positive side of the circuit. Ground can be the same or one size smaller and run to the body ground.
    I don't have my wire size/capacity chart handy. Search online for it.
    Happy wiring:)
     
  7. skytower

    skytower Well-Known Member

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    Oh yea...
    +1 on the relay. Lower amp switches and wires can be used on the control side of the relay, but use the size wires the table recommends.
     
  8. skytower

    skytower Well-Known Member

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    The coil of the relay needs power and ground to open one set of contacts and close the other. The coil side also runs through the switch.

    Hope that helps.
     
  9. Manlaan

    Manlaan Well-Known Member

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    First thing, just stick with 12ga for going to the bed to run the lights. It's not that much more expensive and it'll be good.

    Attached is how I'd probably end up running it using spst switches (or just not hook anything to the second throw on your spdt). (draw your source power for the switches wherever you want. I just did it from the battery so its clear its power). I also didn't show fuses, but you already know you need those. You could probably get away with even less than 18ga for the switches, but thats pretty much the minimum I use. Personally, I have a tendency to use 16ga instead of 18ga, but it really is overkill.

    If you do use external led's to indicate power, as in your original diagram, I'd run that off the return from the switch, not the main line for the lights as the 18ga wire can easily handle the needed to power for the led and you wont have to run extra wire.

    This way you can put an led switch in the cab and it should light up any time the bed lights are on. Downfall of this setup is if either switch is on, the lights are on. It doesn't end up working like the lights in your house, where either switch can toggle them on/off.

    As far as wiring it like a double switch in your house, I'm not sure and would have to follow the logic a bit to make it happen. Maybe something with relays at each switch might work but I cant quite get the flow right in my head.

    wiring.jpg
     
  10. brow

    brow [OP] Well-Known Member

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    Thanks Manlaan, and everyone else. I have my switches figured out, I have two SPDT switches that only have on/on functions, so when I wire these together I should be able to switch the lights on or off with either switch, regardless of which one I use first (these were a pain in the butt to find, ended up having to find a specialty marine supply switch online) and I will be mounting an LED indicator under the switch so I know when the lights are on, hopefully I avoid killing my battery to many times with this set up.

    After a bunch of internet searching I figured out how to wire my relay, I'm still slightly confused on the whole point of it, From what i can gather, the point of the relay is to not send the power for the accesory through the switch, but im hooking the back end of my switches up to a 12v source anyways. I don't know what I'm missing in the logic but i'll just run with it and see what happens

    The whole purpose of this Mod is due to camping season, followed by hunting season. I will be isntalling everything this weekend (barring any more damn rain) so I'll let you know how it all goes.
     
  11. skytower

    skytower Well-Known Member

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    Low current control circuit which can control a separate high current and differant or same voltage circuit. Low current switches are cheap and you can tap into a switched circuit in the vehicle to make your accessory shut off with the ignition without overloading it.
    Check out Oznium.com for dash switches w/led and low amp LED's for lighting. It'll last longer than the 55 watters you're using.
    The scion switches fit in my 08 regular cab with a little shaving.
    The only nice weekends around here are the ones I get called into work. Go figure.
     
  12. brow

    brow [OP] Well-Known Member

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    oznium doesn't have the style of switch that I needed, hardly anywhere does
     
  13. Manlaan

    Manlaan Well-Known Member

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    Actually, now that I think about it, pretty easy to do the lights like in a house. Indicator lights will have to be run from the return line between the switch and the relay though, but not really an issue if you make the bed switch the first switch and the cab switch the second in the series.

    The point of the relay is to not have to run high amps through the switch/other places that dont need it. What this accomplishes is not having to run more expensive and more space consuming wire. Also, all switches aren't created equal, so a smaller current through them will help them last longer.

    wiring.jpg
     
  14. Manlaan

    Manlaan Well-Known Member

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    For switches, the best place I've found for good variety and to do what ever is needed is http://www.delcity.net They do have a $25 minimum order though, but have all sorts of configurations (spst, dpst, spdt, dpdt) and many sizes and styles. Of course not all configs are available for all styles, but...
     
  15. brow

    brow [OP] Well-Known Member

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    yup, product number 73470 are the ones I got from delcity. Is there a specific reason why the indicator light would need to be run off the switch side of the relay and not the accesory side?
     
  16. Manlaan

    Manlaan Well-Known Member

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    There's no reason you cant run the led off the high amp side if you want. It seems like a lot of extra work since usually the indicator light will be next to the switch and you already have a lead going to the relay that will only have power when both switches are in the same direction. Thats basically the reason I did my second diagram with the bed being the first switch in the loop to make it easiest to display the led.

    Not exactly sure how you're running the light power to the bed, but unless you're running the power down the center console, it'd take a 5-6 feet extra wire just for the indicator. Unless you're thinking of adding an indicator light for the switch in the bed, which really isn't necessary since you have the actual lights themselves for that.


    I put the surf n turf switches into my panel as well and they do look quite nice, but with that I'm limited to only 6 switches in a single din area (I have a 1 din head unit and use the bottom din for my switch panel instead of a shelf). I'm thinking of moving everything to the mini rockers that are 1/3 the size of the large ones and making cutouts above the switch to put icons on some type of transparency. It then will give me a way to see the light in the dark (I'll run the backlight off the same wire that does the backlight of the dash), and lets me add in a few more things I've been wanting to play with doing, like bed lights, backup lights, roll bar lights, and maybe even some rock lights. Then there's wiring in a compressor as well and I wouldn't mind a slot for a usb charger. So much to do and so little space...
     
  17. BradleyScottETC

    BradleyScottETC Class IV Category 8 Elite VIP Member (Only)

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    Did you make that up?
     
  18. Manlaan

    Manlaan Well-Known Member

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    Well, from one point of view, he is correct. If you put a 100amp fuse on some 20ga wire, the wire will blow before the fuse does, making for bad things. This is what I took it as his meaning, although phrased kinda wrong.

    I think with what you're getting at, he is wrong. The wiring should be done for the application with some leeway (I like to go 1-2 levels above required), and fused to what the max amps the application will use so the failure point is always at the fuse. I know the charts have built in leeway also, but I figure better safe than doing it all over again.
     
  19. Trap

    Trap Well-Known Member

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    No that is the correct way to size the wire otherwise the wire will become the fuse and burn your truck up. If you don't want to use that size wire run a smaller fuse but keep in mind lighting a bulb is a dead short at first so there is a huge inrush. those bulbs drawing 11.2 amps after the fuse so don't run anything smaller than # 14 after the fuse. That will have the 80% load safety factor built in. If you run a relay the switch wire can be # 16 or even less but i would not go to small because bigger wires are generally stronger and more trouble free.

    I don't care what that damn chart says I'm a industrial Electrician.

    Obviously with decades of car fires around somebody is dropping the ball. Wires can short out so you want to make sure they can carry 15 amps in this case without catching on fire to blow the fuse and turn the power off the circuit. The fuse should blow at 13 amps so it's already really close. The bulbs draw 11.2 lit so if you sized the fuse down to 12 amps it would blow when you turned the lights on. Changing the fuse size will not save you with this setup, it's too close to the 80% of the fuse rating. The chart is about voltage drop because it's DC. I thought you where already running #12 so continue unless you have a spool of #14 lying around. Bigger wires have added mechanical protection. Don't want a bird or rock taking the wires out and burning down your truck now do you ?
     
  20. BradleyScottETC

    BradleyScottETC Class IV Category 8 Elite VIP Member (Only)

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    I'm really NOT trying to start a crap-storm here, but this stuff your writing is got me REALLY scratching my head.

    Well, it kind of gets me thinking about your credibility when you try to say that #14 wire is bigger than #12, and that load isn't a factor in wire size. Maybe it's just your wording, but those two statements are completely false, and from one Commercial Technician to another, just simply stating your job title doesn't prove that your knowledge is superior to his literature.
     
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