Running threads shall not be used on RMC for connection at what?
At couplings. NEC 2023 Section 344.42(B) states plainly that running threads shall not be used on conduit for connection at couplings. The same language appears in 342.42(B) for intermediate metal conduit. If you are studying for a journeyman or master exam, that is the answer the question is looking for.
What a running thread is
A running thread is a long stretch of thread cut on the end of a conduit so a coupling can be spun all the way back onto it, letting you join two fixed lengths of conduit that cannot be rotated. It is the field workaround for a run where neither end can turn, such as tying into an existing stub or closing a loop between two anchored points.
The problem is mechanical and electrical. Cutting a long thread removes metal from the wall of the conduit, thinning it and reducing its strength exactly where the joint carries load. The coupling also does not bottom out on a shoulder the way it does on a standard thread, so the joint never develops the metal to metal contact pressure that a threaded coupling depends on for its equipment grounding path. Section 250.118 recognizes rigid metal conduit as an equipment grounding conductor, and that recognition assumes properly made up joints. A running thread joint can loosen with vibration and corrode, turning a code recognized ground path into a high impedance connection.
What to use instead
The correct fitting is a three piece union, commonly called an Erickson coupling. It has a body and two threaded halves that pull the two conduit ends together without rotating either length, and it is listed for the purpose so both the mechanical strength and the grounding continuity are maintained.
Section 344.42(A) also permits threadless couplings and connectors, which must be made up tight and, where buried in masonry or concrete or installed in wet locations, must be the concretetight or raintight type respectively. A listed split coupling is another option in tight spots. Section 344.28 requires reaming to remove burrs, and 344.24 limits bends to four quarter bends or 360 degrees total between pull points.
Related threading rules
Where conduit is threaded in the field, 344.28 requires a standard cutting die with a 1 in 16 taper, which is 3/4 inch per foot. Field cut threads must be coated with an approved corrosion resistant compound under 344.14 when the conduit is in contact with earth or in a corrosive environment, because cutting removes the galvanizing.
Field takeaway
Running threads are prohibited for connection at couplings on RMC and IMC. Use a three piece union or a listed threadless coupling when neither conduit end can be rotated.