you find the definitions and wire sizes in American Wire Gauge, it has a chart for comparison of AWG to cross-section area.
As a quick reminder,
6 mm² corresponds to 9 AWG
5.3 mm² corresponds to 10 AWG
2.5 mm² corresponds to 13 AWG
1.5 mm² corresponds to 15 AWG
0.75 mm² corresponds to 18 AWG
0.5 mm² corresponds to 20 AWG
In engineering, wires are not measured in wire diameter but in cross-section area, because the cross section is directly proportional to its strength and weight, and inversely proportional to its resistance. The cross-sectional area is also related to the maximum current that a wire can carry safely. Wire diameter is not.
There is a nice comparison chart for American Wire Gauge to IEC 60228 in this file:
60 Hz is EMI dirt and it is a high current, since it is a .75 kW motor, with 120 V there flows 6.25 A AC. Control signals for the stepper motors are also very dirty and the currents are high, up to 6 A, stepper motors are driven by power electronics. There is nothing that should not be shielded.
You mean EMI will disturb stepper signals more than it will disturb router operation?
But shielding does not only prevent EMI to act on a cable but also to come from a cable. So if you don’t want the stepper signals to be disturbed, I believe both should be shielded. Both carry dirty power signals. And you don’t know what additional wires you will drag in your drag chain in the future.
If you replace the cable of the router anyway, why would you use an unshielded cable at all? For cost? But you do this only once and if you have EMI problems it is sometimes very, very hard to find the cause. I’ve learned to avoid them where I can.