We know its confusing when working between an exhaust pipe's NPS (Nominal Pipe Size) and the pipe size a rain cap fits. The source of the confusion is the NPS size and the pipe's outside diameter (OD) - they don't refer to the same size. For example, a 4 inch NPS pipe has an actual outside pipe diameter of 4.5 inches. Confused? Don't feel bad...it still confuses us! So we created a table that eliminates the confusion and provided it for you to use below.
|Nominal Pipe Size (NPS)||Pipe Outside Diameter (inches)||Steel Rain Caps||Aluminum Rain Caps||Stainless Steel Rain Caps|