There's a little more to this than meets the die ...
In nearly every 'ordinary' instance, electricians use ordinary 'plumbers' NPT dies.
However, when you are dealing with an established name like Rigid, or doing some specialty work, it's easy to get the wrong replacement inserts. Plus, the inserts are brand specific.
The best bet is to take the old inserts to a plumbing supply house and have them select the proper replacements.
Without getting too bogged down in the specifics, there are other thread patterns that also comply with "npt" (they simply have tighter tolerances); inserts with extra throat clearanc (for coated pipe), and inserts with different cutting edge profiles (for cutting different materials). There are also some oddball applications where a different threadform altogether is required - but that situation probably doesn't apply here.
