Can different voltage circuits share a conduit?

Yes, under NEC 300.3(C)(1), provided every conductor in the raceway has an insulation rating at least equal to the maximum circuit voltage applied to any conductor in that raceway.

Can 300 volt cable go in a 480 volt conduit?

No. A 300 volt rated cable does not meet the insulation rating required when 480 volt conductors are present, which violates NEC 300.3(C)(1).

Can 300 volt rated cable share a conduit with 600 volt conductors?

Q: Can control wiring share a raceway with power conductors?

Class 2 and Class 3 circuits generally cannot, per NEC 725.136(A). Class 1 circuits may share a raceway with power conductors only where the equipment is functionally associated, per 724.48(A), and the conductors must be rated at least 600 volts.

No, not if the 600 volt conductors are energized at a voltage above 300 volts. NEC 2023 Section 300.3(C)(1) states that conductors of circuits rated 1000 volts nominal or less shall be permitted to occupy the same equipment wiring enclosure, cable or raceway only where all conductors have an insulation rating equal to at least the maximum circuit voltage applied to any conductor within the enclosure, cable or raceway. The rule keys on the highest voltage present, not on the rating printed on any individual conductor.

Applying it

Put a 300 volt rated cable in a raceway with 480 volt conductors and the 300 volt insulation is exposed to 480 volts if either conductor faults to the other. That is exactly the scenario the section exists to prevent. If the higher voltage conductors are actually operating at 240 or 208 volts, both conductors are inside the 300 volt rating and the arrangement is compliant, even though one is marked 600 volts. Higher rated insulation is always acceptable in a lower voltage circuit.

Where 300 volt cable shows up

Most control cable, thermostat cable, Class 2 and Class 3 cable, and many instrumentation cables are rated 300 volts. Article 725 has its own separation rules that usually settle the question before 300.3(C) does. Section 725.136(A) prohibits Class 2 and Class 3 conductors from occupying the same raceway, cable tray, cable, enclosure or fitting as electric light, power, Class 1, non power limited fire alarm or medium power broadband circuits, with limited exceptions in 725.136(B) through (I) for barriers, separate compartments and reclassified circuits. So a thermostat cable does not belong in the power conduit regardless of its voltage rating.

Generator and derived system cases

On the load side of a generator, the same rules apply. Engine control wiring, remote start conductors and annunciator wiring are commonly 300 volt Class 2 circuits and need their own raceway from the power conductors. Where control conductors are Class 1 under Article 724, 724.48(A) permits Class 1 circuits to occupy the same raceway with power conductors only where the equipment powered is functionally associated, and 724.49 requires Class 1 conductor insulation rated at least 600 volts in that case.

Practical checks before you pull

Read the cable print legend rather than assuming. Many jacketed cables show a voltage rating on the outer jacket that differs from the rating of the individual conductors inside. Verify that the raceway fill still works under Chapter 9 Table 1 after the extra cable, apply 310.15(C)(1) adjustment if more than three current carrying conductors are present, and remember 300.3(B), which requires all conductors of the same circuit including the grounded conductor and equipment grounding conductor to be contained in the same raceway.

Field takeaway

Every conductor sharing a raceway needs insulation rated for the highest voltage present in that raceway. Three hundred volt cable stays out of a 480 volt pipe, and Class 2 and Class 3 cable stays out of the power raceway entirely under 725.136.