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How much current can a short run of 26AWG cable support?
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Zero999:

--- Quote from: Marmotta on July 06, 2019, 03:39:08 pm ---Thanks for the advice. The power supply I'm using is rated for 5V 4A, but it does have short circuit protection and over-current protection.

https://www.mouser.co.uk/datasheet/2/670/sdi24-ud-1500492.pdf

Looking at everything, it seems that in theory it should be safe enough to use 26AWG. I think I'll just run it for a while and see if there are any repercussions.

--- End quote ---
No, not that safe. The problem is, if there's a short circuit in the device at the end of the cable, the cable will go up in smoke. You need to put a fuse, preferably at the start of the cable, but at least before the device, so it blows, before the cable gets too hot.
L_Euler:
The JST connectors can be made up with 24AWG wire--I do it all the time--and you can also use parallel wires.
janoc:

--- Quote from: L_Euler on July 07, 2019, 09:56:42 pm ---The JST connectors can be made up with 24AWG wire--I do it all the time--and you can also use parallel wires.

--- End quote ---

That the wire can fit there doesn't mean that this is safe. If the connector is rated for 1A then trying to push 3A through it will not end well, regardless of the wire you use. The common JST PH/XH series aren't designed for high currents.

Better choice is something like Molex KK 396 / KK .156 (see: http://tech.mattmillman.com/info/crimpconnectors/#kk156 )
6PTsocket:

--- Quote from: Zero999 on July 06, 2019, 03:04:47 pm ---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/

--- End quote ---
If we are measuring small drops, fuses are not resistance free either. Check a few out.

Sent from my SM-G900V using Tapatalk

L_Euler:

--- Quote from: janoc on July 08, 2019, 09:18:43 am ---
--- Quote from: L_Euler on July 07, 2019, 09:56:42 pm ---The JST connectors can be made up with 24AWG wire--I do it all the time--and you can also use parallel wires.

--- End quote ---

That the wire can fit there doesn't mean that this is safe. If the connector is rated for 1A then trying to push 3A through it will not end well, regardless of the wire you use. The common JST PH/XH series aren't designed for high currents.

Better choice is something like Molex KK 396 / KK .156 (see: http://tech.mattmillman.com/info/crimpconnectors/#kk156 )

--- End quote ---

That's why I also said use parallel wires (ie. multiple pins for the same power source.)  And just to see how it would end, I'm going to set up a JST with 26AWG pushing 3A followed by dual 24 AWG.  Results to follow.

So I ran the test this evening, but only on 26 AWG.  After testing with 26, the expected results for 24 were obvious.

Completely unscientific, non peer reviewed test:

* Wire - 20 CM, 26 AWG solid, properly crimped to male and female JST pins and inserted in single carriers
* Current - 3 Amps AC sourced by variac
* Ballast Resistor - 0.3 Ohm, 10 W
* Temperature of female connector at start - 25.6 C
* Test Time - 15 min
* Temperature of female connector at Stop - 35.5 C (However, I attribute the temp rise to heat conduction through the resistor lead.  The resistor got pretty hot.  There was no detectable temp rise at the male end.)
* Wire Voltage Drop - 138 mVAC
* Calculated Wire Resistance - 45.9 mOhmConclusion:  3A on a JST pin with 26 AWG wire is completely doable.  Most standards are incredibly conservative and can generally be safely bent if you understand what you're doing.
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