Why are GRC couplers threaded NPS when GRC is NPT?

pv_n00b

Senior Member
Location
CA, USA
If GRC is threaded NPT and all equipment that receives GRC is NPT why are the couplers used to connect two sections of GRC together threaded NPS? It makes no sense to me and no one I have talked to seems to know why this is. There are many theories though. Does anyone know why they are NPS? It seems to go against the need for a good seal and solid electrical connection.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
If GRC is threaded NPT and all equipment that receives GRC is NPT why are the couplers used to connect two sections of GRC together threaded NPS? It makes no sense to me and no one I have talked to seems to know why this is. There are many theories though. Does anyone know why they are NPS? It seems to go against the need for a good seal and solid electrical connection.
Methinks that the taper-straight connection will wedge tighter once tightened, and the conduits will meet in the middle and prevent a gap that can catch fishtape.

A taper-taper connection, like plumbing pipe, prevents the pipes from touching each other, and leaves a gap. It works for plumbing because the medium is fluid,
 

Russs57

Senior Member
Also gives you leeway on where the lock nuts end up.

I seem to recall some NEC stuff about minimum of 5 threads engagement but I could be wrong.

Is it a generally accepted truth with plumbing fittings that if you cut your threads a little under sized, so you have more threads in the fitting, you have a mechanically stronger joint. Useful for when you are building railings and such. For sealing against water/gas/steam pressure you cut standard or a little over sized.

I suspect LarryFine is right.
 
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