No, you generally cannot use conduit (especially if it's serving no electrical purpose) that spans the length of a joist as direct support for a fixture. Here's why: Building Codes and Electrical Codes

Building codes and electrical codes (like the National Electrical Code, or NEC) are very specific about how electrical boxes and fixtures are supported. They require that fixtures be supported independently of other systems. Using conduit as the sole support, especially if it's not carrying any wiring, is almost certainly a violation.

Intended Use of Conduit

Conduit is designed to protect wiring. It's not engineered to bear the weight of a fixture over a long span. Even if the conduit seems strong enough, it may deflect (bend) over time, leading to the fixture becoming unstable or even falling.

Safety

A failure of this type of improvised support could result in a fire hazard, electrical shock hazard, or physical injury.

Inspection

An electrical inspector would likely fail this installation.

Acceptable Ways to Support Fixtures



Directly to the Joist

The preferred method is to attach a junction box (the electrical box that holds the fixture's wiring) directly to the joist using screws, nails, or appropriate fasteners.

Using a Bar Hanger

For recessed lights or heavier fixtures, a bar hanger can be installed between joists to provide a secure mounting point. The junction box is then attached to the bar hanger.

Fixture-Rated Suspension Cables/Chains

Some fixtures are designed to be suspended from the joist using cables or chains specifically rated for the fixture's weight.

Dedicated Support Structure

In some cases, you may need to build a dedicated support structure between joists to hold the fixture.

In Summary: Don't use conduit as the sole support for a fixture, especially if that conduit serves no electrical purpose. It's unsafe, likely violates codes, and is not a proper installation method. Instead, use code-approved methods to mount your fixture securely. When in doubt, consult with a licensed electrician. Flag for review