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I've searched online and found wire gauge calculators but I still can't seem to find a reliable answer.

What gauge wire do I need to carry 36 volts (DC) at 5 amps (peak) for 4 meters?

This cable will be used for a video arcade machine light gun, and the power drives a solenoid which is only active for a few milliseconds. The current draw should be zero until the light gun is fired which quickly activates a solenoid and then stops but the gun can be fired in rapid succession... (which can cause a fairly steady (?) draw of 5A to continuously activate the solenoid)

Would an 18 AWG wire be sufficient, or do I need to go bigger? ...or can I go smaller?

Voltage drop isn't too important because the solenoid circuit can use 24 - 36V and will be powered with a regulated 36V power supply. I just don't want the wire to heat up or catch on fire, heh.

edit: I'm looking to use a stranded wire for flexibility (in case that matters)...

edit: This is related to a previous question, where a cable length of "10 to 15 feet at the most" is mentioned.

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  • \$\begingroup\$ Keep in mind, that the current needs to go to AND back. Both the power and ground cables need to be able to handle the 5 Amps. And any calculation on voltage droop and wire resistance needs to be calculated for the entire wire run. \$\endgroup\$
    – Passerby
    Mar 13, 2014 at 23:47
  • \$\begingroup\$ Also, remember, a solenoid must follow Ohm's Law. A drop in Voltage with a fix resistance, means a drop in current. So why use a 36v supply instead of a lower voltage? Less current to deal with. \$\endgroup\$
    – Passerby
    Mar 13, 2014 at 23:49
  • \$\begingroup\$ Finally, keep in mind that AWG ratings are fairly conservative. And the Power Transmission ratings are for high voltage long distances. You want the chassis wiring rating, then halve it since its going in a bundle inside a metal shield. NEC is slightly less conservative. Either way, 18AWG is fine. And have you tried looking up what commercial arcade machine light guns use for wires? \$\endgroup\$
    – Passerby
    Mar 13, 2014 at 23:57
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    \$\begingroup\$ Consider putting a large capacitor and mosfet or other switching circuit in the gun itself. Then you don't have to deal with the full driving current in the wire going to the gun, nor the inductance and capacitance of the wire going to the gun. You just have to deal with the average current, which, even if fired in rapid succession, will be much less than the full continuous driving current of the solenoid. Should make for a more powerful "punch" as well. \$\endgroup\$
    – Adam Davis
    Apr 17, 2014 at 20:27

4 Answers 4

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DO NOT GO NEAR THESE LIMITS DO NOT GO NEAR THESE LIMITS DO NOT GO NEAR THESE LIMITS: -

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So what should you use? I'd use this calculator - it seems to do the job: -

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  • \$\begingroup\$ I meant to add that the top table is courtesy of Alpha Wire so I'd expect them to have more than half a clue about this sort of stuff. I'll also add that making a decent estimation of the RMS current may surprisingly result in a lower RMS current than 5A. If you can trace out the shape of the current and produce a picture then estimate just how many times per second this can be reasonably expected to happen you might find the RMS is more like 1A. \$\endgroup\$
    – Andy aka
    Mar 13, 2014 at 20:52
  • \$\begingroup\$ That calculator shows me 20 AWG wire is the limit for 5A but it doesn't seem to factor in voltage. Does that not affect the rating? \$\endgroup\$
    – SofaKng
    Mar 14, 2014 at 14:03
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    \$\begingroup\$ Not really, it's current through the wire that dictates power lost as heat. \$\endgroup\$
    – Andy aka
    Mar 14, 2014 at 14:05
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AWG 18 wire is fine for that current handling capacity. NEC ampacity is 14A for wire with 90°C insulation (reference Wikipedia for all figures)

The voltage drop of a loop 30' long is 5A * 6.385\$\Omega\$ per 1000' * 30/1000 or about 1 volt. If your supply is 36V, that's not very significant.

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  • \$\begingroup\$ The ratings are all based on temperature rise and hence average power absorbed and average RMS current. So that 14 amps is either continuous 14 A DC, 14 A RMS AC, 28 A DC with a 50 percent duty cycle, 140 A DC with a 10 percent duty cycle, etc. \$\endgroup\$
    – alex.forencich
    Mar 13, 2014 at 20:13
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    \$\begingroup\$ @alex.forencich Hi, I think you're forgetting the meaning of RMS. Heating is \$I^2\cdot R\$ so sufficiently fast pulses of current to even out the heating at 10% duty cycle would be limited to 44A, not 140A. It would be worse if the wire had time to change temperature appreciably during the pulses. \$\endgroup\$
    – Spehro Pefhany
    Mar 13, 2014 at 20:20
  • \$\begingroup\$ Ah, you're right, it does go with the square of the current. A 50% duty cycle produces half of the heat of a 100% duty cycle, but if you want to get the same heat then you can only increase the amplitude by 40%, not double. In this case, $P = I^2 * R * DC$ where $I^2 * DC$ is an approximation of the overall RMS current with I being the RMS current at 100% duty cycle and DC being the actual duty cycle. \$\endgroup\$
    – alex.forencich
    Mar 14, 2014 at 0:01
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What @Andy said.

Does the pulsed nature of the current reduce the requirement for the wire gauge? It could.

  • The current limit is driven by the heat generated by the wire resistance. The limit is actually an RMS current. You know what the max fire rate of your gun will be, so you can can calculate the duty cycle. Your RMS current will be lower than 5A.
  • When the 5A is flowing, the voltage drop will be the same whether 5A is a pulse or continuous. (Obviously.)
  • There may be a failure mode where the supply line is shorted in the gun. For instance, it can happen if somebody physically abuses it. To mitigate this, add a fuse at the supply end. The fuse should have a slightly lower rating than the wire. A slow fuse or a PTC fuse may be preferable.
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#18 wire should be sufficient. Light duty AC extension cords are #18, and are rated for 5 amps.

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  • \$\begingroup\$ If 5A runs only for a few milliseconds, then even 24 AWG may be sufficient. Based on data from here. \$\endgroup\$
    – Nick Alexeev
    Mar 13, 2014 at 19:31
  • \$\begingroup\$ Thanks... It will only run for a few milliseconds but what if the gun is fired repeatedly causing a fairly continuous draw of 5A? (but again, only running for a few milliseconds at a time) \$\endgroup\$
    – SofaKng
    Mar 13, 2014 at 19:35
  • \$\begingroup\$ Depending on the length of the cable, you may have to worry about voltage drop due to wire resistance, as well as wire heating due to the current. Also, you may want to use larger wire for durability, if the cable is subject to rough handling. \$\endgroup\$
    – Peter Bennett
    Mar 13, 2014 at 19:41

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