Are EMT and rigid conduit fittings interchangeable?

Not unless the fitting is listed for both. NEC 358.6 requires EMT fittings to be listed, and 110.3(B) makes the listing marking enforceable. Combination fittings marked EMT/IMC/RIGID are acceptable on all three.

Are compression connectors raintight?

Only when marked raintight. A standard compression connector is concrete tight. Outdoor and wet locations need a fitting listed for the location per NEC 314.15.

How do I connect rigid conduit to EMT?

Use a listed combination coupling with a threaded hub on the rigid side and a set screw or compression body on the EMT side.

Can you use a rigid compression connector on EMT?

Only if the fitting is listed for EMT. Section 358.6 states that fittings used with electrical metallic tubing shall be listed, and 110.3(B) requires listed equipment to be installed in accordance with the instructions included in the listing. The carton and the fitting body tell you what raceway it is approved for, and many rigid and IMC threadless fittings are not approved for EMT.

Why they are not interchangeable

The wall thickness is completely different. A 3/4 inch rigid metal conduit wall is about 2.4 mm, or 0.094 inch. A 3/4 inch EMT wall is about 1.4 mm, or 0.055 inch. Outside diameters are close, which is why a rigid threadless connector will slide over EMT and feel like it fits, but the gland ring and compression nut were designed to bite into a thicker wall. On EMT, that same ring can crush the tubing out of round or fail to develop the mechanical pull-out strength the fitting was tested for.

The electrical consequence matters more than the mechanical one. Section 250.118(4) recognizes EMT as an equipment grounding conductor. That listing depends on a fitting that maintains a low impedance metal-to-metal path under fault current. A connector that is not tested on EMT has no verified fault current rating on EMT, so you have quietly compromised the ground path for the whole run.

What to look for on the fitting

Many manufacturers now make combination fittings marked for EMT, IMC, and rigid, and those are perfectly legal on all three. The marking is usually stamped on the body or printed on the box as "EMT/IMC/RIGID" or "Listed for EMT". If the box only says rigid or IMC, put it back.

Direction matters too. Compression connectors have a shoulder and a gland ring. The ring goes on the raceway with the tapered face toward the nut, and the raceway must bottom out against the internal shoulder before you tighten. If the tube is not seated, the connection is mechanically weak and the ground path is marginal.

Wet and outdoor locations

Section 358.10(B) permits EMT in wet locations where protected from corrosion, and 314.15 requires boxes and fittings in damp or wet locations to be listed for the location. Standard set screw connectors are not raintight. If the run is outdoors, on a roof, in a car wash, or in a cooler, use compression fittings specifically marked "raintight" or "concrete tight and raintight". A plain compression connector is concrete tight only unless it carries the raintight marking.

Transitioning between raceway types

When you actually need to go from rigid to EMT, use a listed combination coupling. The rigid side is threaded, the EMT side is set screw or compression, and the fitting is tested for exactly that transition. Section 344.42(A) requires couplings and connectors used with rigid metal conduit to be made up tight, and where buried in masonry or concrete they must be the concrete tight type.

Field takeaway

Read the fitting. Combination EMT/IMC/RIGID compression connectors are legal on EMT. A fitting marked only for rigid or IMC is not, no matter how well it slips on. Use a combination coupling for the transition, and use raintight compression fittings anywhere the raceway is exposed to weather.