In section 66.9, the UL508A standard lists the following
color coding for internal control wiring.

Black - all ungrounded control circuit conductors operating at the supply voltage
Red - ungrounded AC control circuits operating at a voltage less than the supply voltage
Blue - ungrounded DC control circuits
Yellow - ungrounded control circuits or other wiring, such as for cabinet lighting, that remain energized when the main disconnect is in the "off" position
White or natural grey - grounded AC current-carrying control circuit conductor, regardless of voltage
White with blue stripe - grounded DC current-carrying control circuit conductor
White with yellow stripe - grounded AC control circuit current-carrying conductor that remains energized when the main disconnect is in the "off" position

The UL508A standard does not apply to wiring outside the industrial control panel.
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Please note that section 66.9 applies to industrial machinery such as:

Metalworking Machine Tools
Plastic Machinery, (injection molding, extrusion, blow molding, etc.)
Woodworking, Laminating and Sawmill machines
Assembly machines
Material handling machines and robots
Inspection and testing machines

They do not apply to general use industrial control panels.

On wire size --- there are two limiting factors ---

Maximum voltage drop you want to allow. You can calculate this using data from wire tables. This will be length dependent.

Maximum cable temperature. This is determined by the current thru the wire, its insulation rating, and the thermal dissipation characteristics of the cable.

vulcom1:

Shielding and twisting wire pairs do not have much to do with each other. Twisting reduces magnetic coupling over an area of several lengths of the twist independent of any non-magnetic shielding. Conductive shielding if non-magnetic ( copper or aluminum ) provides shielding of electric fields. At very high frequencies and microwaves things are somewhat different, but we are not talking about that part of the spectrum. A magnetic material wrapped around the cable could provide both magnetic and electric shielding.

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WilliamD:

Find a copy of "Reference Data for Radio Engineers", by ITT Corporation. My edition is 4, and on pager 51 is Wire Tables, Annealed copper (AWG). #10 has 0.9989 ohms/1000 ft. #16 has 4.016 ohms/1000 ft. And #20 has 10.15 ohms/1000 ft.

Fusing currents of wires is on page 55. #20 is 58 amps, #16 is 117 amps, and #10 is 333 amps. You can not run close to the fusing current, but it sort of defines an absolute maximum. This is obviously ambient temperature dependent. Melting point of the insulation is of greater importance.

If you had #20 with 58 amps thru it, then you would have a dissipation of 33 watts per foot, or 2.8 watts per inch. Take a 2 W wire wound resistor, it is about 1" long, dissipate 2.8 W in this and you can not hold your finger on the resistor. Its surface area is vastly greater than that of the #20 wire. Thus, the #20 wire would be much hotter.

Now reduce the current from 58 amps to 7 amps and the dissipation of #20 wire is about 0.04 W per inch.

Suppose you have a 10 ft wire length from controller to motor, that is 20 ft of loop resistance. For #20 wire this is 10.15/50 = 0.203 ohms. The total voltage drop at 7 amps is 7 * 0.203 = 1.4 V. Not of great importance relative to your source voltage of 60 V. You are dealing with a complex AC waveform so these are only approximations based on DC values, but more than adequate to guide you.

NC Cams:

Multiple strands of copper wire to make one conductor, whether these are twisted or not, has nothing to do with the twisting of two conductors carrying the same current in opposite directions.

When you have a current flow there is a magnetic field created from this current. This current can be in a wire or an electron beam as in a cathode ray tube.

If I make a loop of wire from point A to point B and pass a current thru the wire I will have a magnetic field created all along the the loop and the field will be in the same direction. If I twist this loop every 1 ft, then at each foot I reverse the direction of the magnetic field. If I go a number of feet away from the loop, then I get the superposition of the magnetic fields from a number of the 1 foot segments. These alternating direction magnetic fields then cancel one another.

On an ordinary twisted cable the pitch will be in the fractional inch range. In a CAT-5 cable it is about 0.4, 0.5, 0.6, 0.7 for the 4 different pairs. The pitches are staggered to reduce mutual coupling between the closely spaced twisted pairs.