What is the maximum unsupported span for strut channel?

There is no fixed maximum. Span is what you read out of the manufacturer's load table for that part once the load, the support condition, and the deflection limit are settled. Change any one of those and the allowable span changes with it.

Does strength or deflection control strut span?

On longer spans, deflection usually controls. The channel reaches the sag the specification allows before it approaches a strength limit, which is why a run that holds the load can still be wrong. Check the deflection column in the table, not just the load column.

Is span measured center to center of the supports or between them?

Check the table. Manufacturers state which basis their published spans use, and center to center and clear distance are not the same number on a real installation. Reading a table on the wrong basis quietly overstates the span you are allowed.

Can strut channel cantilever past the last support?

Yes, but the allowable cantilever is much shorter than the allowable span for the same channel and load, and a table published for a simple or continuous span does not describe a cantilever. Use the manufacturer's cantilever data, or have it engineered.

What is the fastest way to fix a span that does not work?

Add a support. Shortening the span reduces deflection quickly and it is usually cheaper than upgrading material along the whole run. Going to a deeper profile is the next lever, then a heavier gauge, then changing the support condition.

What is the maximum span for strut channel between supports?

Maximum span is not a property of the channel. It is what falls out of the manufacturer's load table once you fix three other things: the load the run carries, the support condition, and the deflection your specification will accept.

Change any one of those and the span changes with it. Which is why a note that says "support at maximum spacing" without stating the load and the deflection limit is not a spec. It is a question with the inputs left out, and somebody in the field ends up answering it.

Why is span an output and not an input?

Load tables are printed the opposite way from how people read them: rows of spans, and the load each span allows for that part. You are not looking up a span. You are finding the longest span where the allowable load still covers what you are hanging, at a deflection your spec can live with. That answer belongs to that part number and that job, and it does not travel to the next one.

It is also why the same channel gets specified at very different spacings in the same building. A run of light conduit and a run carrying filled pipe are not the same problem.

What decides the span?

The load comes first: how much, and whether it is spread along the run or concentrated at a point. Those do not affect the channel the same way, and every table states which one it assumes. More on that in how much weight strut channel can support.

Then the support condition. A single simple span, a run continuous over several supports, and a cantilever past the last support give three different answers for the same channel and the same load. Reading a simple-span table and then building a cantilever is one of the common ways a run ends up overspanned.

Then the channel itself, profile depth first and gauge second. Depth buys more than thickness does, which is why back-to-back and combination sections exist. The gauge side of that tradeoff is in 12 gauge vs 14 gauge strut channel.

Then the deflection limit, usually written as a fraction of the span. A tighter limit pulls the allowable span in. There is no universal value: the one that applies comes from the project specification, the equipment manufacturer's requirements, or the engineer of record.

Why does deflection usually decide before strength does?

On longer spans the channel reaches the sag limit well before it approaches a strength limit. In practice that means the failure people actually get is not a collapse. It is a run that sags: a drain that stops draining, a door that binds, a tray that settles into something below it, a line of conduit that looks wrong to the inspector and gets rejected.

It also explains why shortening a span is such an efficient move. Deflection falls off much faster than the span does, so one more hanger usually buys more than one more gauge of steel.

How do you get a longer span?

Four levers, roughly in order of cost. Reduce or redistribute the load. Add a support, which is the cheapest fix and the one people skip because the layout is already drawn. Go deeper in the profile, or to a back-to-back or combination section. Or change the support condition so the run is continuous over its supports instead of a series of simple spans, which only counts if the run is actually built that way.

If none of those reach, the answer is not a bigger table. It is a different support system, or an engineer sizing a member for the job. That threshold arrives sooner than most people expect on runs over people or critical equipment, on anything seismically braced, on anything a jurisdiction wants stamped, and on anything carrying a dynamic load such as a pump, moving equipment, or a run that gets bumped by traffic.

The Strut Channel Support Configurator walks profile, gauge, finish, and a span-and-load sanity check in that order, which is a fast way to sanity-check a configuration before you go to the catalog. Its load figures are illustrative assumptions, not verified engineering data, so confirm every value against the manufacturer's published load tables and a qualified engineer. It is one of the buyer-decision tools in the Rethink catalog.