Can PV DC conductors share a junction box with lighting circuits?

No. NEC 690.31(B) prohibits PV source and output circuits in the same box, raceway, or cable as non-PV conductors unless a partition separates them.

Can inverter AC output share a conduit with other circuits?

Yes. Inverter output circuits are not PV source or output circuits, so normal Chapter 3 rules apply, including the 300.3(C)(1) voltage rating requirement.

How often do PV DC circuits need labeling inside a building?

Every 10 feet and at every junction box, pull box, conduit body, and point of penetration, per NEC 690.31(D).

Can solar system wires share a conduit with non-solar circuits?

Not the DC side. Section 690.31(B) states that photovoltaic source circuits and photovoltaic output circuits shall not be contained in the same raceway, cable tray, cable, outlet box, junction box, or similar fitting as conductors of other non-PV systems, unless the conductors of the different systems are separated by a partition.

Why the DC side is walled off

A PV source circuit is energized by daylight, not by a breaker. Opening the AC disconnect does nothing to it. A service technician who kills the panel and opens a junction box has a reasonable expectation that everything inside is dead, and a live 400 volt DC string in that same box is exactly the hazard 690.31(B) exists to prevent. That is also why 690.31(D) requires PV circuits inside a building to be marked "PHOTOVOLTAIC POWER SOURCE" or "SOLAR PV DC CIRCUIT" at intervals not to exceed 3 m, or 10 feet, and at every junction, pull box, and conduit body.

The partition allowance is real but seldom practical. It means a listed barrier inside a wireway or gutter, not a piece of scrap sheet metal you cut on site. In an ordinary 2 inch conduit there is no way to install one, so the answer for a conduit is simply no.

What about the AC side

Inverter output circuits are AC power production circuits, and they are not PV source or PV output circuits. They fall under Article 705 and general Chapter 3 rules, so they may share a raceway with other AC circuits of the same system voltage. Section 300.3(C)(1) requires all conductors in the same raceway to have an insulation voltage rating not less than the maximum circuit voltage applied to any conductor in that raceway, which is easy to satisfy with 600 volt rated THWN-2 on a 240 volt system.

Two or more separate PV systems may share a raceway with each other under 690.31(B) as long as no non-PV circuits are present, provided the conductors of each system are identified. Multiple strings of the same array in one conduit is normal practice.

Fill and derating still apply

Grouping strings means grouping current carrying conductors, so Table 310.15(C)(1) adjustment factors kick in above three. Section 690.31(A) requires PV wiring methods to be suitable for the environment, and rooftop conduit gets the ambient adjustment of Table 310.15(B)(1) plus the rooftop provisions in 310.15(B)(2). Chapter 9 Table 1 caps fill at 40 percent for over two conductors.

Also note 690.31(C)(1) for single conductor cable. Type PV wire or distributed generation cable is permitted in exposed outdoor locations for PV source circuits within the array, and 690.31(C)(2) requires cable ties or supports to be listed and identified for securing and supporting in outdoor locations.

Field takeaway

Keep the DC home run in its own conduit and its own boxes. Label every 10 feet and at every box. Combine PV strings with each other freely, combine inverter AC output with other AC circuits normally, and never mix DC PV conductors with anything that is not part of a PV system.