Stepper motor wire gauge

Can I use 22/4 AWG for stepper motors as I am having trouble getting 18/4 AWG in the UK

Short answer is, no.

When installing wire you never want to use a smaller gauge than what is required, So if you can not find 18/4 wire you should then look for 16/4 wire, in wire sizes the smaller the gauge number (from 18 to 16) the larger the wire and more voltage and or amperage load it can carry. If you step down in size (from 18 to 22) the smaller the wire and then you are taking the chance of the wire not carrying the current and catching things on fire. A lot a people get confused on wire sizes as they think the larger the number the larger the wire, but it is the opposite, the smaller the number the larger the wire. Hopes this helps.

Yes that is a great help Kenneth, I thought that it was the other way round and that the larger number was larger wire, I did ask, and the seller said it was 300v rated, so I will have another search for 16/4

Possibly. It will likely be stranded wire, but you need to know how many strands because it determines the actual cross section of copper.

Take a look at this chart. You’re only really concerned with the current rating (your voltage is low). You need something rated for the maximum amperage of your motors plus some margin, maybe 20% minimum. The NEMA 17 motors are 1.68 Amps/phase, so say 2.5 Amps for your wires.

You’re getting close with the 22 AWG wire, but it could feasibly work.

Thank you for that information Bill, it is good to know, I have NEMA 23 motors so I have decided to order the proper cable from Inventables, postage and duty is always a pain ordering from USA, but I dont want to cut corners just to save some money

In the future, note that you can simply run multiple smaller wires in parallel. It’ll take up more space in the drag chain, but you’ll meet the safety limits for the wire and might save some money too.

That (the skin effect) is only true for alternating current of sufficient frequency. For the frequency and thickness of the wires we’re dealing with here, it doesn’t come into play.

Actually, the skin effect does come into play for the stepper motor wires. Remember that the voltage is chopped to limit the current to the motor. That’s a square wave. A square wave is made up of an infinite number of frequencies of a sine wave.

“In particular, it has been found that square waves are mathematically equivalent to the sum of a sine wave at that same frequency, plus an infinite series of odd-multiple frequency sine waves at diminishing amplitude:”

@LarryM Mathematically, yes, a perfect square wave is infinite in the frequency domain. That’s actually how you know that these square waves aren’t perfect square waves, because it would require infinite frequencies, and since they’re real signals they don’t contain infinite frequencies.

That’s ignoring that the motors are micro-stepped, which approximates a sine wave, not a square wave.

Solid 18 AWG wire has a radius of just over one half of a millimeter.

So, unless you’re sending a full step to your motors more than 11,000 times a second, the skin effect does not come into play.

It’s also worth noting that stranded 18 AWG wire is not sufficiently different unless the stands are individually insulated. That is, unless it’s litz wire.

It’s not the stepping, it’s the current limit chopping. I haven’t put a 'scope on it so I don’t know the frequency.

Chopping isn’t alternating the current, it’s modulating it. The skin effect is produced from alternating current, so it doesn’t really matter what frequency the chopping is for that effect. It only alternates during stepping.

There is alternating current as the coil of the motor is an inductor. When current is flowing in one direction the magnetic field is building in the coil. When the voltage is removed then the inductance returns current in the form of back emf. Since you have current flowing in both directions in the supply wire then you have AC current.

All of this is academic in that it most likely has no effect on the original question as the magnitude of the effect in this situation is extremely small, but present.

Quite the opposite, actually. Inductors resist changes in current, so cutting the voltage certainly does not reverse the current. The current continues to flow in the same direction and falls off exponentially. I assume you’ve studied electronics at some point, so you may recall the dualism with the capacitor and voltage. Flipping the current would be akin to the capacitor flipping to negative voltage when current stops.

Yep. Sorry. My error. That’s what I get for trying remember things too far back.