Final answer:
Electrical nonmetallic tubing (ENT) should never be threaded because the plastic material can easily be damaged or crushed.
Step-by-step explanation:
The type of raceway that should never be threaded is electrical nonmetallic tubing (ENT). ENT is a type of raceway that is made of plastic and is used to protect electrical wires and cables. It is not suitable for threading because the plastic material can easily be damaged or crushed.