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| How much current can a short run of 26AWG cable support? |
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| Marmotta:
I'm trying to figure out what sort of gauge wire I need to use for power delivery in my project. I'd ideally like to use 26AWG so I can use a relative small JST connector, but I don't know if it can safely handle the current. The board I'm powering recommends a 5V 3A power supply, but the most I've managed to record it drawing is using a cheap digital multimeter is 0.85A. JST seem to have a flat recommendation of 1A for 26AWG and other online calculators have just confused me. My run of cable would be about 20cm long. Any help would be great. https://www.arcade-museum.com/manuals-videogames/N/NeoGeo%201%20Slot%20MV1C%20Users%20English.pdf https://www.powerstream.com/Wire_Size.htm http://www.jst.fr/doc/jst/pdf/current_rating.pdf |
| ArthurDent:
The concern you should have is what is going to be the voltage drop with your cable and connectors. If you need 5 volts at the device, 5 volts into the cable may be real iffy. For 20cm of cable you have 2 wires so you actually have 40cm of #26 AWG wire. Check the resistance of the wire per meter and calculate the resistance total then times the current will be the voltage drop. If this lower voltage at the end of the cable is acceptable to your circuit, go for it. The current handling capacity of wire is rather subjective and depends on open air, in a bundle of other wires, etc., so these values will vary wildly. I'd conservatively use the lowest current value listed as the limit then calculate the maximum drop for the total length of the wire I want to use in the application. I've had precision voltage sources where millivolt drops are unacceptable but just connecting an LED at the end of a long cable wouldn't be noticed. |
| rstofer:
The 3A thing reminds me of a Raspberry Pi requirement for the 3B+. The thing about the Pi is that they are pretty serious about the ampacity and attempts to use smaller wall warts have been met with failure. Some people get lucky and their system works with a lesser supply for a while. The only way I would undersize the supply is if I knew for a fact that under any operating condition, the current was less than specified. I'm not sure how I would determine that. A DMM averages things so I would have to come up with a way to capture the peak current. |
| Electro Fan:
https://www.powerstream.com/Wire_Size.htm |
| Zero999:
According to Wikipedia, the resistance of #26 is 133.9mΩ/m. You have s total run of 40cm or 0.4m, so that's a resistance o 0.4*0.1334 = 0.05336. If the current is 0.85A, the voltage drop = V IR = 0.05336*0.85 = 0.045356V, so with 5V at one end, the voltage at the other end will be 5 - 0.045356 = 4.954644V, which will be no problem. https://en.wikipedia.org/wiki/American_wire_gauge#Tables_of_AWG_wire_sizes The only other thing you need to consider is over-current protection. The cable should be protected by a fuse or the power supply's current limit, which should be below the cable's rating. If the power supply can provide a much higher current, than the cable's rating and there's a short circuit, the cable insulation can melt, short circuit and present a fire risk. As mentioned above, the current rating depends on the temperature rating of the insulation, whether the cable is bundled and the ambient temperature. Wikipedia says 1.3A, for #26 assuming the insulation is rated to 60°C, the ambient temperature is 20°C and it's not bundled with other conductors. You'll probably want it to run at higher ambient temperatures than 20°C, but the cable insulation will also be rated to over 60°C. According to the article below, PVC cable insulation (the most common type) is commonly ratted to 105°C. The next column in the Wikipedia table says 2.2A, with 70°C rated insulation, so it's probably safe to protect the circuit with a 2A or even 3A fuse, as cable insulation can probably take that current without damage. https://www.performancewire.com/insulated-wire-protection/ |
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