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  1. #1
    Elite Member sandman2234's Avatar
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    Default How to determine fuse size needed for lights

    I pulled the lights off the Duetz tractor before I sold it, and wanted to mount them on the John Deere 2555. I have a bunch of 7.5 amp fuse holders that I would love to wire into the circuit but don't have a clue what amps the lights draw. Really don't want to play hook up enough lights till a fuse blows, then back out of it some, so does anyone have an easy way to determine what size fuse I need? I have a reasonable idea of what I am doing, just no way to figure out the amp draw. Is there an easy way?
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  2. #2
    Veteran Member dstig1's Avatar
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    Default Re: How to determine fuse size needed for lights

    Do you have an ammeter? The clamp on style are the easiest. Hook up to 12v unfused and test.

    The other thing you can try is measuring resistance, but I doubt that will be accurate. V=IR so 12v/Resistance = Amps, theoretically. Only works if lights are a simple resistive load and that the resistance doesn't change when they are lit.

    What watts are the bulbs? It should say on them somewhere. I= VA so 55 watt lamps would draw 4.58A on 12V You might also be able to look them up online if you can find the model numbers.

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  3. #3
    Super Member 94BULLITT's Avatar
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    Default Re: How to determine fuse size needed for lights

    Be careful when you hook the light to a meter. Be sure you have the correct setting, like 10amps or you will blow the fuse in your meter

  4. #4
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    Default Re: How to determine fuse size needed for lights

    Watts are equal to Volts times amps, and 'most" lamps give you an indication of their output printed on the bulb base or envelope "somewhere". If not, buy new bulbs!

    So a 35 watt utility lamp on 12V draws about 3 amps

    A pair of 55 watt automotive head lamps is pushing 10 amps.

    The wiring and switches for a pair of 35 watt lamps would be well protected with the 7.5 amp fuses

  5. #5
    Veteran Member JerryK's Avatar
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    Default Re: How to determine fuse size needed for lights

    The fuse is to protect the wiring..... So if you have wire that is capable of 30 amps, you can fuse it with UP TO 30 amps.... Automotive wire rating is done by a different standard than that which covers home wiring.. with the home wiring standards weighted on safety. The SAE standards for auto wiring is more ' this is good enough '. Here is a link for basic auto wiring... Automotive Wire FAQs from Allied Wire and Cable | Distributor of electrical wire, battery cable, automotive primary wire and more. Basically, you tear into a piece of wire.... clean and mic the small strands... then count the strands. I did it for my Mahindra when I added four LED work floods to an existing work lamp circuit... It was one of those ' Gee, that little wire is good for 30 amps..!!! ' moments.
    You also do not want to fuse the circuit too close to the draw of the load. Fuses that draw near their rating 'age' and will usually not last long. A fuse is like a little resistor... if you pass a certain current thru it, it will heat up/melt... and open. Or if you try to pass a large current thru it, it will fail like a flash bulb. Good luck, Jerry.

  6. #6
    Super Star Member LD1's Avatar
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    Default Re: How to determine fuse size needed for lights

    You dont size the wire to the load as Jerryk pointed out. Rather the fuse is sized to protect the wiring. So you have to select a wire size that will handle the load and then protect that size wire with the appropriate fuse.

    With a 12v system, voltage drop is a factor. So longer runs need a larger sized wire.

    This is a good chart Wire Gauge Amps Ratings for 12 volt Automotive Systems

    So if you want to run 2 lights, and they are 55w lights, you are just a tad under 10A. I'd select a wire that is capable of 15A. That would be 18ga wire for up to 10' of length and the 16ga for up to 20' of lenght.

    The 7.5a fuse holders wont work unless you wire each light individually, or are much lower wattage lights.
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  7. #7
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    Default Re: How to determine fuse size needed for lights

    When you decide on the size of the fuse it is best to locate the fuse as near to the power supply as you can. Doing this protects the device you are powering and the entire length of wire from the fuse to the device.

    Also when using clamp on ammeters... They are typically designed to measure A/C current rather than D/C (read the manual for your meter).
    Last edited by 1fastbob; 10-02-2012 at 08:27 AM.
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  8. #8
    Super Member crazyal's Avatar
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    Default Re: How to determine fuse size needed for lights

    I think you can put away all the math books and just use some common sense. You can all but assume that you'll need a 10 amp circuit for two lights as long as they are 55w or less. The lights could be 35w and only require a 7.5a fuse but you may end up replacing the bulb when it dies with a 50w or 55w bulb in which a 7.5a fuse will be too small. I also think you'll find 35w isn't a lot of light and you'll want to upgrade (assuming you have a large enough alternator in your Deere). Can you replace the fuse with a 10 amp and just use the holder? I think you'll find the wire diameter is the same between 7.5a and 10a.
    Kubota L4240,Case 580K backhoe, Case 450 Dozer

  9. #9
    Veteran Member Jay4200's Avatar
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    Default Re: How to determine fuse size needed for lights

    FYI - I just installed dual pairs of 55W fog lights on my kubota this past weekend. Each pair of lights were factory-fused at 15A, and came with all connections made with 18ga wire.

    I tossed the factory wiring set-up, and ran low-voltage outdoor lighting wire (heavy-insulation 12ga stranded), with each set of lights taking one conductor of the 12 ga wire (only hot is run - ROPS is ground), and both sets running off of a common 30A fuse at the main block.

    JayC

  10. #10
    Epic Contributor Soundguy's Avatar
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    Default Re: How to determine fuse size needed for lights

    always fuse to protect wireing. make sure switch is up for load as are sockets.

    to determine minimum fuse rating needed for lamps.. use E=IR EI=P

    with power you can determine amps.... then you are good..

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