Is there a way to check the quality of a USB cable without an autopsy?
Well... if you search (I just did) for USB3 AWG 22 or 24 cable, that has some hope of giving you that actual gauge when you purchase one advertised as such. Not all of the wires need to be that heavy, depending on the USB specification (see the Wikipedia USB page for an idea of that can of worms) but the power lines do. You still need to be careful, since I have bought cables online that said they were one gauge and turned out to be another (those were standard 12 volt power cables, not USB). Watch the buyer feedback on those.
As for what to search for (you may know all this.. if so, sorry!):
So, voltage drop = current * resistance of wire. Stranded wire resistance per foot varies some, but probably not enough to worry about in this case. One chart I found listed resistance of 24 gauge wire at 21-26 (approx) ohms per thousand feet. Assuming the connectors are properly made, I discount that resistance. For 1 amp of power (12 watts at 12 volts, as it happens, or for usb,5 watts at 5 volts), and 10 foot cable:
voltage drop = 1*(10*26/1000) or about .26 volts.
(forgot to add: If they just tell you the power required, not the current, you can figure that out as Current = Power/Voltage. 12 watts/12 volts = 1 amp, 1 watt/5 volts would be .2 amps, etc.)
Continuing with the 1 amp example:
For 34 gauge, the resistance jumps to 237 ohms/1000 feet or a 2.3v drop in 10 feet. That is a drop from whatever the starting voltage is (for USB, that would be 5v of course). This would be bad. AWG 28 stranded is about 65 ohms/1000 feet (.65v drop).
I think they only use one power wire in USB2.x and 3.x (USB C is another kettle of worms as well... Interesting and with some new features, but not really important here).
I also have a USB extender cable that has a power jack on it, I believe, near the "far" end. I expect that is to either bump up the capacity or something else. Any longer run I always terminate into a powered hub, then make the last hop shorter but with hopefully good (AWG 24 or similar) cables.
Hope I got this right... Got my EE in 1981, so am a bit rusty..
Edited by PartlyCloudy, 27 February 2021 - 11:36 AM.