Can Class 1 AC power and Class 1 DC control wiring share the same conduit?

Yes, and the section that says so is short and clear. In the 2023 NEC, Class 1 circuits live in Article 724, having been split out of the old Article 725.

The permission

Section 724.48(A) states that Class 1 circuits are permitted to occupy the same cable, cable tray, enclosure, or raceway without regard to whether the individual circuits are alternating current or direct current, provided all conductors are insulated for the maximum voltage of any conductor in the enclosure, cable tray, or raceway.

That last clause is the real condition. If the machine has 480 volt AC power conductors in the pipe, the 24 volt DC control conductors sharing that pipe must be insulated for 480 volts. In practice that means everything in the raceway is 600 volt rated THHN or MTW, not thermostat wire, not 300 volt rated tray cable, and not shielded instrumentation cable rated 300 volts.

Mixing Class 1 with the power supply circuit

Section 724.48(B)(1) permits Class 1 circuits and power supply circuits to occupy the same raceway, cable, or enclosure only where the equipment powered is functionally associated. A control circuit for the same machine that the power conductors feed is functionally associated. A control circuit for a different piece of equipment across the plant is not, and it belongs in its own raceway.

Section 724.48(B)(2) through (B)(4) add the cases for cables with and without a metal covering and for factory assembled control centers.

Conduit fill and derating

Section 724.51(A) applies the Chapter 9 fill rules where all conductors are 14 AWG or larger. Section 724.51(B) is the one that surprises people. Where Class 1 circuit conductors carry continuous loads exceeding 10 percent of the ampacity of each conductor, and there are more than three current carrying conductors, the ampacity adjustment factors of 310.15(C)(1) apply.

The corollary is useful on machine work. Control conductors that only energize a coil or a pilot light draw almost nothing and do not carry a continuous load above 10 percent of their ampacity, so they are not counted as current carrying conductors for adjustment. That is why you can stuff a machine conduit with control wires without collapsing the ampacity of the motor conductors sharing it.

Conductor sizes

Section 724.49(A) permits 18 AWG and 16 AWG conductors for Class 1 circuits where they are installed in a raceway, an approved enclosure, or a listed cable and the load does not exceed the ampacities in 724.49(A). Section 724.49(B) covers 14 AWG and larger, which follow the ordinary Article 310 rules. Section 724.43 requires overcurrent protection at the ampacity of the conductor, with exceptions in 724.43 for 18 and 16 AWG.

What you cannot do

Class 2 and Class 3 circuits are a different animal, governed by Article 725. Section 725.136(A) prohibits Class 2 and Class 3 conductors from occupying any cable, raceway, or enclosure with electric light, power, Class 1, or non power limited fire alarm circuits, with narrow exceptions in 725.136(B) through (I). If your DC control wiring comes from a listed Class 2 power supply, reclassifying it as Class 1 under 725.130 Exception No. 1 is possible, but then it must be wired entirely as Class 1 and it loses its Class 2 status throughout.

Field takeaway

Class 1 AC and Class 1 DC in the same conduit is explicitly allowed by 724.48(A). Insulate everything for the highest voltage present, confirm the loads are functionally associated under 724.48(B)(1), and only count conductors as current carrying for derating when they exceed 10 percent continuous per 724.51(B).