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I am looking to directly power a Raspberry Pi for my 3D printer from its existing power supply. The terminals on the power supply appear to be ~15 amps max output at 24V. I already have a buck converter wired up to the USB on the Pi to bring the voltage down to ~5V that the Pi wants. What I don't know how to handle is the proper size wires to use in this situation.

I have a lot of 20 gauge wire that is generally used on 3D printers, but I am worried about wiring the RPi directly from the power supply with this wire. I also have 14 gauge wire, but it is too large to fit into the terminals on my buck converter, and even if it did, the buck converters are 5 amp max so that would just move the "problem" to the buck converter.

Any help in how to properly wire this up would be greatly appreciated. I would prefer crimp connections where possible (my solder work is sub par at best), and would classify myself as very much "hobbyist" and basically assume I know nothing about electrical, so as specific an answer as possible would really help.

To be clear, from my understanding, because the 14 gauge wire can handle the max output of the power supply, it should be used. But I also know that the max actual draw of the RPi is far lower than that, and is within the acceptable range of say, a 20 gauge wire. My concern is running 20 gauge for this circuit back to the power supply could have the potential of a fire in a short, but I have no idea how I'm supposed to remedy that. As running 14 gauge wire (even if I could) would just offload the short risk to the buck converter or maybe even the Pi.

JYelton
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Justin Packwood
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  • Also you may find it helpful to review [this question about selecting a power supply](https://electronics.stackexchange.com/q/34745/2028) to understand that the Pi will pull only the current it needs, which then informs your choice of wire gauge. – JYelton Jun 13 '22 at 15:58
  • Please do not post an edit when an edit has already been made (doing so reverts that edit). You had spelling errors and capitalization issues which I corrected, but your edit reverted. – JYelton Jun 13 '22 at 16:01
  • Sorry about that, I was told by the site to add some clarity with an edit so I must have done that at the same time. Hopefully my edit adds more clarity tho? – Justin Packwood Jun 13 '22 at 16:01

4 Answers4

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I linked to other questions that should cover the basics of your question (in comments, but I'll list them below as well).

Per your additional information, regarding the concern of a fire resulting from a short, you are correct in addressing this concern. If you use a wire gauge that is too small for the power supply, but adequate for the load, there is potential that during a short condition that wire would become quite hot and potentially start a fire.

The simple answer is to use a fuse or circuit breaker. This is exactly what is done in home wiring, for example. The circuit breakers are limiting current for each circuit based on its intended usage and how it has been installed. For example, in the US, 15 or 20 ampere breakers are common for circuits that use 14 and 12 gauge wire, respectively. The upstream "power supply" (the mains grid) can easily supply more current which would happily start a fire during a short were it not for the breakers or fuses.

Just remember that wherever you install a fuse, the wire should be sized according to the max current that the power supply can deliver on the upstream or input side.

Additional resources:

JYelton
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  • Thanks for all your help! And all the other answers as well! I will look into sourcing a fuse that I can fit in line between the buck and the power supply (limited space but im sure I can find something) – Justin Packwood Jun 13 '22 at 16:13
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I have a lot of 20 gauge wire that is generally used on 3D printers, but I am worried about wiring the RPi directly from the power supply with this wire.

You can use 20 gauge wire but, fit a fuse so that if anything went wrong on that circuit spur, the 20 gauge wire wouldn't exceed its ratings by virtue of it being protected by the fuse.

Andy aka
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The Official Answer

Often times wire size is related to your fault scenarios. If your RPI was to experience a fault and short 5V and ground together, there would be a large amperage going through the wiring. The question becomes how much is that amperage, and how long does it last, before circuit protection kicks in. Even then, it's also a question of what the circuit protection will do (not all types are a hard cut-off, like PTCs or a regulator going into thermal limit).

You want your wires to survive such scenarios. And so you'd want to understand what circuit protection you are using and design to that level.

Here is a nice table for relating AWG to ampacity.

The Hobbyist Answer

20 AWG would be fine so long as it isn't too long. Try to keep it as short as reasonably possible, and beware of making sure your ground wire goes back to your power supply before joining with motor wires (the voltage induced by the motors over wiring would "lift" your RPI).

Standard disclaimers for not burning down your house apply.

Smith
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  • Beat me by seconds! – Adam Lawrence Jun 13 '22 at 16:03
  • Thanks for your answers (both of you!). I am able to use strong crimp connections for all connections, and I actually want to wire directly back to the power supply to be sure that the rpi is not on the same wires as my motors, so I'm glad that was a good call! Is there some simple / hobby level protection I could add in line with these wires or something where I could use 20 guage and still be "safe" about it? – Justin Packwood Jun 13 '22 at 16:08
  • I am currently leaning towards just using 20 guage and being very meticulous with my crimps, and very short runs. But if there is something I can add to that circuit to make it a bit safer in the event of a short I would love to do that – Justin Packwood Jun 13 '22 at 16:09
  • Found my answer to the followup question in another answer! Thanks again! – Justin Packwood Jun 13 '22 at 16:12
  • For circuit protection, I'd first look at the power supply. If it is rather strong and would supply a LOT of current in a fault scenario, or if it doesn't like faults and would blow up instead of failing gracefully, then my usual approach would be to put in a PTC. – Smith Jun 13 '22 at 22:02
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From the power supply, for safety you must use wire which will not cause a hazard when operated at the maximum current the power supply can continuously output. If your PSU can actually output 15A, you must use wire which can handle 15A between the PSU and the buck in case there's a fault and the PSU just sits there at maximum current. 14AWG would be appropriate for this.

Between the buck and the Pi, you will need to use wire which is sized based on the maximum buck output current OR the maximum power supply output current, whichever is higher. The rationale for this: if the buck MOSFET fails short, it's like the PSU is directly connected to the Pi and all the wiring may see the fault current. Otherwise the first set of wires may be fine but the second set may start smoking.

This advice is meant for long-term, unattended safe operation of your circuit. Of course, if you size the wire based on just how much normal load current is expected, under normal operation you will not have problems - only abnormal operation will be potentially smoky. If you are just doing hobby work and understand the risks, you can use the smaller gauge wire. Just don't leave the circuit on unattended

Adam Lawrence
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  • In case of a short circuit in the Raspberry Pi, the Raspberry Pi would catch fire anyway. What's the advantage of making sure the wires don't catch fire? – user253751 Jun 13 '22 at 16:42
  • The Pi will have gone through its own safety inspections prior to market, and a single abnormal should not result in a hazardous situation. The fixturing is something *you* build and as such *you* are responsible if it causes damage. – Adam Lawrence Jun 13 '22 at 18:21