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Will this wire burn me or melt my clothes ?

  1. Jan 16, 2009 #1
    Will this wire burn me or melt my clothes ??

    I am making a heated motorcycle jacket - so 13 volts DC (actual measurement while riding).

    For heating wire I am using 30 gauge (7 strand x 40 AWG) tinned copper wire with Teflon insulation (good to 250C) which has a resistance of 0.102 ohms per foot (actual measurement as versus 0.103 from tables).

    I will sew 22 feet of this wire into my jacket so that gives me:
    2.24 ohms, 5.8 amps, and 75 watts.

    75 watts over 22 feet is 3.43 watts per foot.

    The practical question is will this wire burn me or melt my polyester jacket ??

    I do Not need the exact temperature of this wire. There are no second or third degree issues as the wire is sewn into polyester insulation between my jacket shell and liner. So there is NO air circulation, fan, or any complications.

    There must be a formula for calculating the APPROXIMATE temperature of this wire.

    Any help will be greatly appreciated as I am definitely not an electrical engineer.

    thanks, Bob
  2. jcsd
  3. Jan 16, 2009 #2


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  4. Jan 17, 2009 #3
    Re: Will this wire burn me or melt my clothes ??

    I don't have a source of 13 volt power unless I am riding my bike down the road.

    The wire will be spread evenly throughout my jacket - so coiling it up isn't a good test - although that would give me a max worse case.

    The 102 watt blanket is a good example, but I don't know how many feet of wire they are using - so the temperature per foot could be quite different. But it is a good example because there is no air circulation in the blanket. I remember using one of these somewhere and that the High setting was too hot. Of course, I will be riding at 70mph in 40F weather, so that should cool my jacket more than an electric blanket on a bed.
  5. Jan 17, 2009 #4


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    Re: Will this wire burn me or melt my clothes ??

    If you have a car battery, that'd be able to supply close to 13V at sufficient current for long enough to determine whether or not it gets uncomfortably hot. Also, I'm assuming you're sewing in something so that it'll help diffuse the heat a little? Not so much that you lose more heat to the outside than gets put into you, but something to diffuse it a tad (the difference between a 40 W soldering iron applied to you, and a 40 W electric blanket applied to you).

    EDIT: That's not only a neato idea, but also customizable: if you find the jacket is too hot, you can add some more wiring, and you can cut it down to do the opposite!
  6. Jan 17, 2009 #5
    Re: Will this wire burn me or melt my clothes ??

    OK, with my bike at idle, the battery will be sufficient to test how hot this wire gets. I have a 16 foot test piece of 32 gauge wire I can just plug in and wait a few minutes check it with a thermometer.

    Still there ought to be a formula I can use in my spreadsheet to help me do the design.

    The heat is spread around because the wire is sewn evenly throughout the jacket.

    My current Jacket design has 4 parallel loops of wire:
    Loop 1 is 14 feet of 30 gauge
    Loop 2 is 22 feet of 30 gauge
    Loop 3 is 30 feet of 32 gauge
    Loop 4 is 48 feet of 32 gauge
    The Gloves will have 7 feet each of 32 gauge and be in Series together.

    The plan is to be able to plug the Gloves into any Loop 1 to 4 and get 4 to 28 watts of heat.
    Then plug in any additional Loops to add more heat to the Jacket - with the combinations I will be able to get just about any heat level I need for the weather.
  7. Jan 17, 2009 #6


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  8. Jan 18, 2009 #7
    Re: Will this wire burn me or melt my clothes ??

    I followed the suggested thread - it basically says its way too complicated - at least the example in that thread.

    My application is simplified - ie - the wire is sewn into the polyester insulation in my jacket. So there is NO air circulation or heat sink to carry off the heat.

    These variables are known:
    13 volts DC
    22 feet of 30 gauge stranded & tinned copper wire
    2.34 ohms of resistance
    5.8 amps of current
    75 watts

    So I have started trying the experimental approach - because all you guys keep saying this problem is way to complicated for humans.

    I have a 12 volt, 3 amp battery charger.
    I hooked up a 16 foot, 32 gauge stranded & tinned copper wire
    resistance of this wire is 145 ohms per thousand feet
    this wire should draw 5.17 amps

    It got hot to the touch but you could hold it.
    I wrapped it around a thermometer and I think it maxed my human thermometer at 108F

    The battery charger has some sort of safety circuit in it and it keeps popping - so maybe this wire is drawing more than 3 amps (as it should if it was available)

    So now I have a secondary question - will a 12 v, 3 amp battery charger put out more amps it the draw is there - and pop the battery charger's safety circuit ??

    AND ?? also pop the fuse on my 10 amp ammeter ??
  9. Jan 18, 2009 #8


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    Re: Will this wire burn me or melt my clothes ??

    Yes, a power supply puts out as much power as the load draws or fails trying.
  10. Jan 18, 2009 #9
    Re: Will this wire burn me or melt my clothes ??

    why don't you just wire in a potentiometer to vary the voltage... an easy way to dial down the heat if it's a bit much.
  11. Jan 18, 2009 #10
    Re: Will this wire burn me or melt my clothes ??

    Apparently - my ammeter read 14.78 before I could pull the plug - and I was ready with my hand on the wire.
    The battery charger says 3 amps in big letters right on it.
    Apparently this means 3 amps until it thinks about poping its safety/overload circuit.

    So when I have my 16 feet of 32 gauge test wire attached to the battery charger - how in the world can I measure what amps are really being drawn ?? If I put my ammeter on the circuit (it will then be in parallel with my test wire) ?? it will just draw lots of amps instead of reading what amps are being drawn by the wire ?? if that is the case what good is an ammeter ??
  12. Jan 18, 2009 #11
    Re: Will this wire burn me or melt my clothes ??

    Motorcycles only generate a limited amount of juice (mine is 300 watts or 25 amps) and it takes 100 to 150 watts for all the lights, turn signals, etc. So in my heated clothing design I have limited myself to 150 watts (jacket, gloves, pants, socks).

    If I only need 1/2 power for heating on a particular day - a potentiometer will still draw 150 watts instead of 75 watts. I would rather turn on my running lights (70 watts total) than waste watts in a potentiometer.
  13. Jan 19, 2009 #12
    Re: Will this wire burn me or melt my clothes ??

    OK, so I guess I am making progress using my battery charger

    I cut a 32 foot piece of 32 gauge wire
    with 0.145 ohms per foot
    so 4.64 ohms
    11.94 volts from battery charger
    2.57 amps
    30.7 watts
    0.96 watts per linear foot

    Measured temperature (I wraped wire 6 times around bulb on thermometer and then held coil in place with my finger and thumb) was:
    107.4 F

    I think this worked because the wire only draws 2.57 amps which is less than 3 amps the battery charger is happy putting out.

    To my way of thinking the measured 107.4 is higher than it would be in actual operation because:
    when wire is sewen into jacket it is straight lines not Coil - ?? right ??

    But this is only one data point and I will need lots of data points to deduce a formula for how hot this wire gets in various lengths, with 13 volts not 12, 30 and 32 gauge, etc.

    Doesn't anybody have a formula for the Simplest Condition (no air circulation, etc)
  14. Jan 19, 2009 #13


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    Re: Will this wire burn me or melt my clothes ??

    Put the ammeter in SERIES with the test wire to measure the current through the wire.
  15. Jan 19, 2009 #14
    Re: Will this wire burn me or melt my clothes ??

    You're intent is to get heat to your skin. At a temperature of 107F this is going to take a very, very long time. But the smallest air movement will carry this away. Without leathers, buffeting from the air will pump most of it out through the smallest seams. You need a lot more power.
  16. Jan 19, 2009 #15


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    Re: Will this wire burn me or melt my clothes ??

    I don't agree with that, 75Watts is similar to the basal metabolic rate of a human and seems like a good starting point to me. Sure you still want good insulation between the heating coil and the outside of the jacket to stop the heat being drawn away to rapidly.

    DrAlloway, I don't know if this is too hard to do (physically) but one thing that would be a huge help in distributing the heat (to prevent hot spots that could potentially burn you or the jacket) would be if you could wrap the wire flat in Al foil (to make it like a foil ribbon instead of a plain wire). I guess that doing this might make it too hard to insert into the jacket, but it would definitely aid in distributing the heat and keeping the temperature more uniform.
  17. Jan 19, 2009 #16
    Re: Will this wire burn me or melt my clothes ??

    So I checked around and found that heated jackets run anywhere from 65 to 100 Watts. I think the answer is somewhere in between 75 Watts and 'alot more'. These jackets are designed to retain heat, and reduce excess air circulation. It would depend a great deal on what kind of jacket is being modified.
  18. Jan 19, 2009 #17
    Re: Will this wire burn me or melt my clothes ??

    Is this true? I thought they stepped down the voltage via multi tap transformer sort of idea, so lower voltage in the circuit, lesser wattage draw.

    If they function as a resistor than what you say is true.

    To be honest, Im not sure.
  19. Jan 19, 2009 #18
    Re: Will this wire burn me or melt my clothes ??

    And, after googling it for a second, I see you are totally right, as im sure you know already.
  20. Jan 19, 2009 #19
    Re: Will this wire burn me or melt my clothes ??

    Well, you really need to specify some kind of thermal model here. In the very simplest case, where the wire is assumed to be totally thermally insulated, the temperature will linearly climb without bound (i.e., you'll either catch on fire or part of the circuit will fail). Obviously, that's not very realistic. It really comes down to figuring out how much heat is going to be lost to the environment; you want the heater to output something close to that (much lower, and you will still get cold; much higher, and you will eventually overheat). This, in turn, depends on how the wire is coupled into the jacket, how insulative the jacket is, how cold the ambient air is, how humid the ambient air is, and how fast the air is moving over the outside of the jacket. I don't have an intiutive idea of what the total heat loss you're looking at is, but you will need to posit at least a preliminary idea in order to proceed with the design.

    Said another way, in this application, YOU are the heatsink :]

    Building in some facility to control the amount of heating is also a good idea: this will extend the range of climates you can use the jacket in, and keep you maximally comfortable in all of them. However, you will still need to build in some reasonable upper and lower limits on the design, in order to prevent fire/overheating on the one end, and to eliminate useless operating points on the other extreme. Once you have some reasonable, safe operating limits in place, you can use the variability to experiment in the field (record the temp/windspeed/humidity, and then drive around adjusting the temp until it's just right. Then go back to the lab and see how much current you were drawing at that setting).

    Also be sure to check that the melting points of any materials in the jacket close to the heating wires is not prohibitive.
  21. Jan 19, 2009 #20
    Re: Will this wire burn me or melt my clothes ??

    With Heated Clothing on a motorcycle one usually wears:
    base layer - for dead air space and moisture wicking
    lightweight long johns
    heating layer
    protective layer (usually leather with insulated liner and with armor).

    Commercial suits (costing about $700) include
    80 to 100 watts for jacket
    28 watts for gloves
    44 watts for pants
    28 watts for socks

    Most guys riding around have their heated layer on half power when the air temp is between 32 and 40F.

    So I am confident on the watts needed to keep me warm.
    However, commercial suits have a Heat Controller (high power triac or 555 timer) which they sell for $100 - and I am too cheap to buy --

    So I am using multiple parallel loops of heating wire to enable me to vary the temperature. I just need to make sure that no one loop is going to be too hot and burn me, especially in gloves and socks because it is very hard to sew lots of feet of wire there.
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