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Wiring functioning prior to fusing?

  1. Dec 28, 2015 #1
    I have a wire of a certain volume, the fuse current is rated at 82A(18AWG wire), can the wire still function normally at 70-75A continuously? I understand that at 82A the wire will melt, but can it still function normally at 75A? The resistance of the wire is about 0.042 ohms, just wanted to see if it can handle that amount of current, would the insulator melt?

    Maybe provided with reasonable cooling or heat transfer system, we could make it beyond 82A maybe to 100A+?
     
  2. jcsd
  3. Dec 28, 2015 #2

    berkeman

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    What's the application? Can you provide a lot more information about this?
     
  4. Dec 28, 2015 #3

    anorlunda

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    Whoa, my chart says that 18AWG wire is rated for 7 amps, not 75. To get 75 A, you need 4AWG or better.

    The conditions for ampacity of a given guage are:
     
  5. Dec 28, 2015 #4

    jim hardy

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    @ Mr Phi
    Have you used a vacuum sweeper at home? MIne says it's 10 amps and the cord is 18 AWG and the cord gets warm to the touch when i vacuum the floor.
    10 amps is the rating for #18 portable cord with 90 degC insulation, type SJOOW
    http://www.awcwire.com/productspec.aspx?id=portable-cord-sjoow-cable
    and the vacuum cleaner factory uses the smallest they can get away with .
    Depending on price of copper, #18 90degC might be cheaper than #16 60degC .


    What would be the temperature rise at 75 amps ?
    (75/10)2 X whatever is temperature rise at 10 amps?
    Precise measurements by UnScientificWildA**Guess method indicate a modest 5degF rise at 10 amps,
    so at 75 amps i'd expect a (75/10)2 X 5 = 187.5 degF rise.
    And that'd raise the resistance of the copper by 0.004 per degC ,
    and 187.5/1.8 X .004 = 75% more ohms,
    further increasing power dissipated in the cord hence raising its temperature even more.
    That's Thermal Runaway, where the positive feedback takes over, and it's how fuses work.

    Probably your 80 amp melt current for #18 is for bare wire in air , not insulated and not in conduit. Did they give a time?

    Tap into your everyday experiences when contemplating such musings. Learning is mostly discovering what you already know.

    old jim
     
    Last edited: Dec 28, 2015
  6. Dec 28, 2015 #5

    anorlunda

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    You mean melting point for solid wire bare in air. Cheap stranded house wire is not braided or twisted, short strands are just laid into a channel of the insulator. So the wire fails at the insulator's melt temperature, not the metal's melt temperature. Buy an extension cord in the dollar store, then cut open the wire to see what I mean. It is shocking (no pun) to see how little metal they put in those cords, and if they show an AWG rating on the label, I doubt if it is accurate.
     
  7. Dec 28, 2015 #6

    jim hardy

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    You mean they're not even continuous from end to end ? That's scary.
     
  8. Dec 28, 2015 #7

    berkeman

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    Agreed, that is scary. I haven't seen that myself, but I don't take apart inexpensive cords or wire. Yikes.
     
  9. Dec 28, 2015 #8

    dlgoff

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    Worse yet is the termination in the molded wall plug. Talk about getting warm.
     
  10. Dec 28, 2015 #9

    berkeman

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    Yeah, no wall socket will support that current. Thread closed.

    @PhiowPhi -- PM me the answers to my questions, and I'll re-open the thread...
     
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