Many of the current rating charts you'll find are for NEC (National Electrical Code ) standards for industrial and residential wiring with ambient temperature of 30C. If you use between 5 to 15% the maximum fusing current you generally fall within an acceptable working current of the wire your using without excessive temperature increase bearing in mind the temperature rating of the wire insulation. If your interested here are some of the temperature ratings of various wire insulation's Also to play around here's a calculator that will tell you the approximate maximum current rating of copper wire Wire fusing in free air You'll see that copper can handle a fair bit of current before fusing. Anyways, I just wanted to go out of my way to muck things up for you further, by providing the source that I make use of professionally and personally. Tefzel insulated wire is the defacto standard here in the US Aerospace due to it's mechanical and thermal properties. One thing to be cognitive of is that those tables assume Tefzel insulation (MS22759/blah PTFE/ETFE or some such shite), which isn't inexpensive when contrasted with other wiring that has different insulation. PDF is the start of the electrical wire rating section. Given these are the basic tables that all avionics/mechanic technicians can employ on all aircraft from small single engine piston poppers to multi-engine jet airliners, they are conservative. The tables are approved for selection of conductors used in aircraft based on current flow (intermittent/continuous) and whether they are bundled or in free air. Being in the Aerospace and Defense sector here in the US, I always fall back to the tables in FAA AC 43.13-1B Chapter 11. I hear ya on lack of consistency across various sources. \ You can also use multiple wires in parallel, like computer power supplies do with pci-e 6 pin cables, where there's 2-3 pairs of AWG16 or AWG18 wires giving 12v to the video card. Voltage = Current x Resistance (ohm's law) so depending on thickness of the wire, you will get less voltage drop in the cables. From there, it's like me and others have explained. The inductor, diodes and capacitors that are part of the buck regulator which converts the high voltage to low voltage, making it possible to get higher current on the output. From the barrel jack connector, size the wires so that you'd have as low of voltage drop as you need. at 5A you may have already 19.9v at the barrel connector, because there's 0.1v drop on the cable between laptop adapter and barrel jack, unless the laptop adapter has voltage sense or intentionally outputs more than 20v to counteract that. For example, let's say you have a laptop adapter that outputs 20v at max 5A but comes with a 2 meters length of cable and a barrel jack connector at the end. You determine how much voltage drop you're willing to tolerate between the power supply and your voltage regulators or whatever you have in the device. You can also calculate the amount of power dissipated in these 2 ft of wire : power = Current 2 x resistance = 100 x 0.01277 = 1.277 watts so you have to think how warm the wires will be above ambient (temperature inside a product), or how will this heat affect connectors, will heat be heatsinked into circuit boards through connectors? If you use a pair of awg 18 wires, the current will be spread across the pair of wires, so each wire will do 5A and will warm up less and you'll have a lower voltage drop. in total so a total resistance of 12.77 mOhm or 0.01277 ohm If you apply Ohm's law : voltage = current x resistance, then it's quite easy to figure out you have a voltage drop of v = 10A x 0.01277 = 0.1277 volts on the wires and therefore you may get approx. or 20.95 mOhm per meter according to Wikipedia : So if you output 6v at 10A and you have 1ft between source and consumer, then you have 2 ft. For example, a 18 AWG wire is supposed to have around 6.385 mOhm per ft. I guess it also depends on how big of a voltage drop you're willing to tolerate.
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