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O.T. Need ampacities for low current applications

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  • O.T. Need ampacities for low current applications

    All the references I find seem to be for house wiring like my Ugly’s manual, and cut off at 14ga. I am looking for the smallest wire to safely conduct 1 amp at 120 volts. Length would be 6’.

  • #2
    Here's a site:

    https://www.powerstream.com/Wire_Size.htm

    It says 29 Ga, but for 6' you could go lower.

    Mike

    Comment


    • #3
      Safely probably means the wire won't burn up at that current level, even if it's contained within a poor heat conductive material or space. There are tables that show this- open air wiring can be allowed a higher current because the heat generated can be dissipated fast enough that the temperature rise in the wire stays within limits.

      The other thing you would want to consider is how much loss of voltage you can tolerate in your application. Passing 1 amp where the voltage is say 100 volts would give you most of the power at the load, whereas is your supply voltage was 2 volts for instance, you might dissipate most of your power in the wire, with little left for the load.

      A good example is a thermocouple in a furnace or water heater. The operating voltage is on the order of perhaps .1 volt, so the resistance in the wire and the connections is of high importance. To run an led array from a 12 volt source for instance, is much less critical.

      Carrying this further, if your voltage was going to be high, say 300 volts, then your choice of wire would be largely dictated by insulation. I have seen wire which is meant to operate from 300 to 400 volts sources, and the insulation is quite a bit thicker- a higher proportion of the overall diameter of the wire. This would mean there is less copper for the diameter, and this is more important as the wire size goes down- ie the gauge number goes up.
      Last edited by darryl; 02-19-2021, 04:38 PM.
      I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

      Comment


      • #4
        The voltage rating of the insulation is a factor. Might start with finding single conductor wire with the appropriate insulation and work from there. What's the application?

        Comment


        • #5
          I made a spreadsheet for wire sizes. It shows ampacity based on temperature rise as well as cross sectional area (for bundled wires and windings) and surface area (for bare wire in free air). Current that can be safely carried by a certain wire size depends on many factors, such as ambient temperature, insulation type, and wire length to a heat sink such as a connector or PCB.

          http://enginuitysystems.com/pix/elec...s/WireSize.xls

          According to that chart, #24 AWG (317 sq mils) will carry 1.18 amps based on cross-sectional area and NEC, but #29 AWG (100 sq mils) will carry 1.12 amps based on watts/sq in surface area.

          Another way to estimate current capacity is to know that three wire sizes correspond to twice (or half) of the ampacity. So if #12 is 20 amps, #15 is 10 amps, #18 is 5 amps, #21 is 2.5 amps, and #24 is 1.25 amps.
          Last edited by PStechPaul; 02-19-2021, 05:02 PM.
          http://pauleschoen.com/pix/PM08_P76_P54.png
          Paul , P S Technology, Inc. and MrTibbs
          USA Maryland 21030

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          • #6
            Nice spreadsheet - thanks Paul!
            Location: North Central Texas

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            • #7
              Two ways to figure capacity of a wire. In one, the temperature rating of the insulation is the limiting factor, so you need to specify the type of insulation.

              In the other, voltage drop is the limiting factor. Often 2 or 5 percent, So you need to know the wire length and the supply voltage.

              Most tables you find use one or the other, and make some assumptions. That's why the tables often dont agree.

              Ed
              Last edited by ed_h; 02-21-2021, 12:01 AM.
              For just a little more, you can do it yourself!

              Comment


              • #8
                Then you get into what I do... specs, specs specs, but the plan is stamped "min 16awg". lol

                Comment


                • #9
                  For 6 ft of length I'm wondering if the wire is exposed. I'd be more worried about the durability of the insulation. Smaller size hookup wire isn't intended for being out where things can rub or thump against it or people walk on it.
                  Chilliwack BC, Canada

                  Comment


                  • #10
                    Horst The above answers are correct for current capacity. HOWEVER, everything can and will change depending on the application. So, what are you trying to use this small wire for? Will it be protected from flexing, pulling and other abuse? What environment will it be exposed to (heat, cold etc...). Any other info will help in making a safe choice. Also, remember all wiring is rated for its maximum safe current, more or less... So if you use a 29 AWG wire it MUST be fused for around 1 amp so that if it is plugged into a 15 amp outlet and it accidentally gets shorted it blows the fuse and not the wire in a fireball! For example, in my younger days I did some amateur pyrotechnics for stage bands on the cheap. The flashpots were loaded with a smallish amount of black powder and an electric igniter. The igniter was a 1 inch piece of 30 AWG bare wirewrap wire connected to a 16 AWG extension cord with the female end cur off. The small wire was at the bottom of the charge and the cord went back to a home made control board. The whole mess hooked up to a car battery and when the proper switches were pressed the battery was connected to the igniter cord. The 30 AWG wire flashed and poof boom went the flashpot. The point is that small wire is the failure point in most applications and can be a safety hazard. Please be careful!!
                    Robin

                    Happily working on my second million Gave up on the first

                    Comment


                    • #11
                      Yes. absolutely ^^^^^^^^^^^^^^^^^^^^

                      Although I will comment that thin copper wire is no sort of fuse. I once needed a fuse and thought of that. Errr...NO!

                      The thinnest wire I had, possibly 30 ga, maybe thinner, was easily able to withstand 30A. I do not recall if, or at what point it did open, but it was not a current of interest, way higher than needed.

                      Yes, the wire became very hot, and yes it was an ignition source..
                      2801 3147 6749 8779 4900 4900 4900

                      Keep eye on ball.
                      Hashim Khan


                      It's just a box of rain, I don't know who put it there.

                      Comment


                      • #12
                        Have a look at a voice coil from a speaker. That's pretty fine wire and routinely handles severals of amperes. They do get some cooling though, and they can't be too rugged or you'd never get to see a smoke show

                        Agreed, the application for your fine wire is a big part of determining what the gauge should be.
                        I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

                        Comment


                        • #13
                          That's the whole point of proper fusing. The fuse IS the small wire.



                          Originally posted by rdfeil View Post
                          Horst The above answers are correct for current capacity. HOWEVER, everything can and will change depending on the application. So, what are you trying to use this small wire for? Will it be protected from flexing, pulling and other abuse? What environment will it be exposed to (heat, cold etc...). Any other info will help in making a safe choice. Also, remember all wiring is rated for its maximum safe current, more or less... So if you use a 29 AWG wire it MUST be fused for around 1 amp so that if it is plugged into a 15 amp outlet and it accidentally gets shorted it blows the fuse and not the wire in a fireball! For example, in my younger days I did some amateur pyrotechnics for stage bands on the cheap. The flashpots were loaded with a smallish amount of black powder and an electric igniter. The igniter was a 1 inch piece of 30 AWG bare wirewrap wire connected to a 16 AWG extension cord with the female end cur off. The small wire was at the bottom of the charge and the cord went back to a home made control board. The whole mess hooked up to a car battery and when the proper switches were pressed the battery was connected to the igniter cord. The 30 AWG wire flashed and poof boom went the flashpot. The point is that small wire is the failure point in most applications and can be a safety hazard. Please be careful!!
                          Paul A.
                          SE Texas

                          And if you look REAL close at an analog signal,
                          You will find that it has discrete steps.

                          Comment


                          • #14
                            Most lamps and household extension cords use #18 AWG or smaller wire, yet are plugged into receptacles fused for 15 or 20 amps. Most fuses and breakers will hold overcurrent of 1.5 or 2 times rating for a minute or so. I have seen plugs with fuses rated for the wire size of the cord, but they were rather old. I think many European plugs have fuses, but not in the US.

                            They are available but are about $35 each:

                            https://www.mcmaster.com/fused-plugs/
                            http://pauleschoen.com/pix/PM08_P76_P54.png
                            Paul , P S Technology, Inc. and MrTibbs
                            USA Maryland 21030

                            Comment


                            • #15
                              Originally posted by darryl View Post
                              Have a look at a voice coil from a speaker. That's pretty fine wire and routinely handles severals of amperes. They do get some cooling though, and they can't be too rugged or you'd never get to see a smoke show

                              Agreed, the application for your fine wire is a big part of determining what the gauge should be.
                              Voice coils dissipate essentially ALL of the incoming power. Only a couple percent typically goes into anything audio, and not all that comes out as sound, there are various acoustic losses.

                              The wires use the magnet structure as a heat sink. I have seen speakers with the ends of the voice coil black, and the middle fine.... Too much excursion at high power, so those areas were poking out past the magnet structure.
                              2801 3147 6749 8779 4900 4900 4900

                              Keep eye on ball.
                              Hashim Khan


                              It's just a box of rain, I don't know who put it there.

                              Comment

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