# Stepper Motor Wire Questions

Is the normal wire size for the Shapeoko 3 XL stepper motors 22AWG? I saw some discussion of 18 gauge elsewhere on this site.

Also, some of my cables seem short. I’m either going to have to put the X&Z motor/cable connections inside the drag chain or invert the PCB cover so the holes are in the top to reach comfortably.

The large Holes in the most recent control box, IS on the top. The USB connections are on the left, or facing towards the rear of the machine.

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What wire size/gauge are they using?

The controller enclosure has gotten updated:

Not sure what gauge wiring is used in current kits — on the Shapeoko wiki, 18–20 gauge is recommended, but that may be rather conservative for the lengths used.

Mine are 22. Any reason they need to be lower?

I did some calculations on the back of my napkin and I think 22AWG is fine for this. I may play with the numbers more after I get some coffee in me and wake up.

Resistance per 1000 ft
22 AWG = 16.14Ω
18 AWG = 6.385Ω

If you assume the stepper cable is 8ft length and we double it because we have both a supply and return from the motor winding we get 16ft wire length.

Resistance per 16 ft
22 AWG = 0.25824Ω
18 AWG = 0.10216Ω

Stepper Winding Resistance per manufacturer is 1.3Ω

Total Circuit Resistance
22 AWG = 1.55824Ω
18 AWG = 1.40216Ω

The power supply can easily overcome the difference in resistance but it does have to generate additional power to do so. Assume the winding current is 2A (amps) so the formula for power is I^2 * R

Rough Power Estimate Total Circuit
22 AWG = 6.23296W
18 AWG = 5.60864W

Power Excluding the Motor - Cable only
22 AWG = 1.03296W
18AWG = 0.40864W

So about 0.6W more is wasted in each 22 AWG wire compared to the 18 AWG wire. That’s per circuit so 4 Motors x 2 Windings = 8 circuits so about 4.8W of wasted power and more work the 30W power supply has to do. So is 18 AWG better technically — Yes; but 22 AWG does work and works across many machines so it’s an efficiency vs cost tradeoff design decision and I can’t fault the designer for it given the target cost of the machine. I’d say the designer was successful given there are not machines failing all over the place.

Again, I really need to finish my coffee as I’d bet there is an error somewhere in this musing. This brings back memories; I did the motor driver designs used in millions of IBM typewriters and Laser Printers during my career.

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Probably not a continuous current in the motors, but I see your point. You know you won’t see a change in your electric bill at that level, and the larger wire will take up more space, etc.

steppers DO have a continuous current whenever the machine is on. They hold in place until told to move. But 22 awg is plenty for these.

Maybe the wiki needs to be updated where it talks about 18 or 20 awg as the minimum. Especially since the machines shipped come with 22 awg.

I’d rather be conservative — this was also a consideration of availability — does anyone have a source for 4-conductor shielded 22-gauge wire which is as convenient as a carrel at a big box store?

Maybe say something like 22AWG is the minimum recommended ideally 18 or 20AWG offer better performance. You should not say 18 to 20AWG minimum if the system comes from the factory with 22AWG.

What is the point of recommending something which isn’t easily sourced?

Do currently shipped Shapeokos come with shielded cables?

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No, they’re not. It’s my understanding we recommend not enabling hard limits unless you replace the wiring with shielded.

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OK so the point I was trying to make was that the recommendation seems to read “if you need to replace the wires for your motors and limit switches, you should get shielded wire when none comes with the machine of a larger gauge than what the machine comes with”. That appears to say that the wiring that comes with the machine is inadequate, if you replace the electrical wires, get thicker wires that are shielded. While it may offer better performance, I don’t know that you really want to say that without better qualification.

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