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Can I fuse a thicker wire with a thinner wire?

Discussion in 'Technical Chat' started by codemancer, Apr 17, 2022.

  1. Apr 17, 2022 at 4:55 PM
    #1
    codemancer

    codemancer [OP] Well-Known Member

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    I am trying to run power from the battery to the bed of the truck for an air compressor. Max draw is 30A and the compressor uses a 40A fuse. I am planning to run about 25ft of #4 wire to the battery, but the fuses I have on hand are only #8 size wires with 40A fuses. Is it OK to downsize at the battery for a very short run (~1ft)? It will be similar on the compressor side where I splice into the existing wiring.

    thanks!
     
  2. Apr 17, 2022 at 5:35 PM
    #2
    Dm93

    Dm93 Test Don't Guess

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    As long as fuse amperage is correct for the smallest wire in the circuit you can put together whatever wire sizes you want, you may see a slight bit of voltage drop with smaller wire but the main thing is that the fuse is sized correctly for the wire gauge.
     
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  3. Apr 17, 2022 at 7:08 PM
    #3
    codemancer

    codemancer [OP] Well-Known Member

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    It is correct for a short run (meaning the 8 AWG fuse wire can easily handle 40A max for 1ft), but I wasn’t sure if it was additive like that. So if I interpret you correctly it sounds like it should work fine.

    I actually went ahead and cut this out of the equation and just ordered a 40A inline circuit breaker fuse that can attach directly to a 4 AWG wire so I don’t have to worry about it. Though I’ll still end up reducing on the compressor end.

    thanks for the response!
     
  4. Apr 17, 2022 at 7:14 PM
    #4
    Key-Rei

    Key-Rei Well-Known Member

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    Get an appropriately sized fuse block for the the wire or get a fused distribution panel like a Blue Sea Fuse box.

    Find out the peak amp draw of your compressor most fuses are oversized or at least sized for peak draw (just starting) rather than load (already running) so the current in-rush doesn't blow the fuse.
     
  5. Apr 17, 2022 at 7:18 PM
    #5
    codemancer

    codemancer [OP] Well-Known Member

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    i have an aux system for everything but this so I didn’t want to go overboard with a distribution system for just 1 accessory.

    i ended up getting 25ft of 4 AWG wire and some lugs and one of these: https://www.amazon.com/dp/B07FF8TDSS

    The compressor is a Viair 400P which is rated for a “max” of 30A but is fused to 40A. So that’s why I chose 40A for the battery side as well. Think this will work OK?
     
  6. Apr 17, 2022 at 7:26 PM
    #6
    Key-Rei

    Key-Rei Well-Known Member

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    Should I have a bigger Vi-air compressor with a 100% duty cycle I run on #10 wire and 30a fuse no issues.

    But... YMMV.

    :notsure:
     
  7. Apr 17, 2022 at 7:29 PM
    #7
    codemancer

    codemancer [OP] Well-Known Member

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    Oh wow ok. Everywhere I looked it seemed like I needed #6 at a minimum over the approximate 25ft to prevent the wire from melting haha. Would have been nice to work with something smaller. But oh well. I guess larger can’t hurt.
     
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  8. Apr 18, 2022 at 8:07 AM
    #8
    MR5X5

    MR5X5 Well-Known Member

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  9. Apr 18, 2022 at 10:47 AM
    #9
    codemancer

    codemancer [OP] Well-Known Member

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    That's the exact calculator I used to determine I should use #4 wire. To be fair, my measurement of the distance is pretty loose (somewhere ~20 feet most likely, but might need a few extra feet for vertical portions of the run, thus my number of 25ft). The #4 will be harder to work with but in the end I think it's the safest option.
     
  10. Apr 18, 2022 at 3:06 PM
    #10
    MR5X5

    MR5X5 Well-Known Member

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    Plenty safe. The 30A rating is for inrush current. Operating current will be significantly less. All good in the end though.
     
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  11. Apr 18, 2022 at 3:19 PM
    #11
    codemancer

    codemancer [OP] Well-Known Member

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    Great, thank you!
     
  12. Apr 18, 2022 at 3:19 PM
    #12
    Dm93

    Dm93 Test Don't Guess

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  13. Apr 18, 2022 at 3:21 PM
    #13
    codemancer

    codemancer [OP] Well-Known Member

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    Yeah, it's sounding like from anecdotal evidence I probably could have stuck with the original #8 wire I had ordered. Oh well, I have 30 feet of #4 wire now and a bunch of components for that haha.
     
  14. Apr 18, 2022 at 4:49 PM
    #14
    Dm93

    Dm93 Test Don't Guess

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    Look at it this way, you have some headroom for future electrical projects back there now:D
    Mine lives in my toolbox and I just checked I'm running a 25A fuse.
    20160827_115150.jpg
     
  15. Apr 18, 2022 at 9:39 PM
    #15
    6 gearT444E

    6 gearT444E Certified Electron Pusher

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    the fuse is there to protect the wire, not the compressor which is why it’s important to size the fuse to the appropriate gauge wire. You want the fuse to blow before the wire melts. Inrush currents can be protected by an additional inverse time breaker added in series to the fuse but it’s really overkill for the application of a simple vehicle mounted air compressor. At that point the protective elements will cost you more than the component you’re trying to protect.
     
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