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automotive wiring question....

toad67
Am wiring up some LED work and back up lights for my truck. The overhead work lights on my headache rack draw 2.5 amps each, for a total of 5 amps, and the lower backup lights draw 1.67 each for a total of 3.4 amps total. Each set are on their own separate toggled circuit from a hot post in my fuse box thru an in line fuse holder. What size of fuse should each circuit have to protect them? I can get a 5 amp for the bigger ones, but the next lowest is a 3 amp which doesn't quite cover the smaller ones. Thanks Fellas, and happy Turkey day to you all.

Todd

edit for question number 2:

The toggled switches are LED also. Does the switch ground wire for the light need to be the same size as all of the rest of the wire, or can I cheat and use some smaller stuff?
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Comments

11 comments

Date Votes
  • Cubslover
    That seems a bit high for LEDs.

    In any event, I use 125% of FLA (Full Load Amos) when sizing protection for circuits. Typically in the industrial segment.

    I?d run spade style fuses have less selection than glass. If you are running spade style fuses you?re limited to 7.5 and 5A respectively. If you have an inline glass fuse holder, you can better dial in your fusing.

    Going smaller on ground wires typically isn?t a problem, but it depends on just how much smaller. If you?re running 12-14ga wire for power, a 16ga ground should suffice although it?s not likely a massive cost savings.
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  • toad67
    quote:Originally posted by Cubslover
    That seems a bit high for LEDs.

    In any event, I use 125% of FLA (Full Load Amos) when sizing protection for circuits. Typically in the industrial segment.

    I?d run spade style fuses have less selection than glass. If you are running spade style fuses you?re limited to 7.5 and 5A respectively. If you have an inline glass fuse holder, you can better dial in your fusing.


    Thanks for the reply Brandon, that's what I planned on doing, but wasn't sure... They are spade fuses too..
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  • toad67
    Any thoughts on the ground wire size?
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  • dav1965
    I would use the same size wire. Like he said before smaller wire is fine. 1 size smaller than the main. Also how far is it going before it gets grounded.

    I would use the same size wire because i am for overkill. I have seen enough fires in my lifetime. Like he said above not much more expensive to use the same size as above.
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  • Horse Plains Drifter
    I have never heard that it's ok to use smaller ground wire, and I certainly do not subscribe to that. It may be accepted in an AC system where the ground is just a "safety". In a DC system however, the ground is one half of the system. Every bit of current that runs through the load has to be returned to the negative battery post via the ground wire. In a DC system, I would never consider a smaller ground wire. I would size my wires so they are able to easily handle the rated current of the fuse I installed. Keep in mind the fuse is supposed to be the weak link in the system.
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  • Cubslover
    quote:Originally posted by Horse Plains Drifter
    I have never heard that it's ok to use smaller ground wire, and I certainly do not subscribe to that. It may be accepted in an AC system where the ground is just a "safety". In a DC system however, the ground is one half of the system. Every bit of current that runs through the load has to be returned to the negative battery post via the ground wire. In a DC system, I would never consider a smaller ground wire. I would size my wires so they are able to easily handle the rated current of the fuse I installed. Keep in mind the fuse is supposed to be the weak link in the system.



    Your ground must be rated to the acceptable current in a DC system. He never stated which wire sizes he was planning. 16ga will certainly carry 5amps.
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  • Horse Plains Drifter
    quote:Originally posted by Cubslover

    Your ground must be rated to the acceptable current in a DC system. He never stated which wire sizes he was planning. 16ga will certainly carry 5amps.
    Correct, he never said what size wire he was using. However, both you and Dave stated it was fine to use smaller ground wire in a DC system. Bad advice IMO. I agree 16ga wire will carry 5 amps. So both wires need to be 16ga. There is nothing to gain by feeding a 5 amp circuit with a 14+ ga wire.
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  • jimdeere
    quote:Originally posted by toad67
    Any thoughts on the ground wire size?

    It should be big enough.
    Not don.
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  • toad67
    The power and ground for the lights is 14 gauge. The smaller wire size that I'm referring to is only to light the tiny red led in the switch to let you know that it is on.
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  • Horse Plains Drifter
    quote:Originally posted by toad67
    The power and ground for the lights is 14 gauge. The smaller wire size that I'm referring to is only to light the tiny red led in the switch to let you know that it is on.
    Oh heck, 22ga would be over kill for that, but that's probably as small as you can handily find.
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  • droptop
    Wire sizes/amps, allowable voltage drops, lengths are considerably different for 12V systems.

    https://www.engineeringtoolbox.com/amps-wire-gauge-d_730.html
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