Re: Pipe Sizing
the two are different thread patterns. the NPT is like for hoses and such and could be looked at as a rough thread. the IPT or Iron Pipe Thread is the common industry standard for copper threading, black iron pipe threading, brass fitting threading, etc.
Now at lowe's or home depot, i think it is watts brand, that sells like a 100 different little baggies of fitting and such. this is the same place where you buy the different lengths of hoses and stuff. in the plumbing section.
but they are different and won't match up without the adaptor.
Process of elimination. Good luck.