Can you run feeder conductors and branch circuit conductors in the same junction box?

Yes. Nothing in the NEC separates feeders from branch circuits by function. The controlling rule is 300.3(C)(1), which permits conductors of different systems, meaning 600 volts nominal or less alternating current and direct current circuits, to occupy the same equipment wiring enclosure, cable, or raceway, provided all conductors have an insulation rating at least equal to the maximum circuit voltage applied to any conductor in the enclosure.

What that means on a normal job

A 208Y/120 volt feeder to a subpanel and a 120 volt lighting branch circuit both use conductors with a 600 volt insulation rating, so they can share a junction box, a wireway, or a raceway with no code problem. A 480 volt feeder and a 120 volt branch circuit can also share a box, again because standard building wire is rated 600 volts and 480 volts is below that. Where it fails is when you mix a 300 volt rated cable, such as some control cable and some Class 1 remote-control cable, into a box that also carries 480 volt conductors. Then the 300 volt insulation is below the maximum applied voltage and 300.3(C)(1) is violated.

Where the code does force separation

Class 2 and Class 3 circuits under Article 725 must be separated from power conductors. Section 725.136(A) prohibits Class 2 and Class 3 conductors from sharing a cable, cable tray, enclosure, raceway, or similar fitting with electric light, power, Class 1, non-power-limited fire alarm, or medium power network-powered broadband conductors, unless one of the exceptions in 725.136(B) through (I) applies.

Emergency circuits under Article 700 are stricter. Section 700.10(B) requires wiring from an emergency source to emergency loads to be kept entirely independent of all other wiring, with a short list of permitted exceptions such as transfer equipment enclosures and listed luminaires supplied from two sources.

Fire pump circuits under 695.6(A) and 695.14 also require independence from other wiring.

Practical cautions

Even where it is legal, count your conductors for box fill under 314.16 and for ampacity adjustment under 310.15(C)(1) if more than three current-carrying conductors run together for more than 24 inches. If the junction box also functions as a pull point for large raceways, 314.28 dimensions apply. Section 408.3(A)(2) prohibits using a panelboard or switchboard gutter as a raceway for conductors feeding through unless the conditions of 312.8 are met, which caps the cross sectional area at 40 percent of the wiring space and total conductors plus splices at 75 percent.

Field takeaway

Same insulation rating, same box, no violation. Keep Class 2, emergency, and fire pump conductors out. Label the feeder inside the box so the next person does not treat it like a 20 ampere circuit.