Short definition
A cap is a closure fitting that slips over the outside of a pipe end and seals it. It’s the female counterpart to a plug, which threads or pushes into a fitting. Caps come in every common pipe material — copper (sweat or push-fit), PVC and ABS (solvent-weld), galvanized (threaded), and PEX (crimp or expansion) — sized to the pipe they fit.
What it is
You’ll use a cap any time a pipe needs to be terminated. Common scenarios: a removed fixture leaves a supply stub-out that needs to be sealed, you’ve roughed in a future fixture line that won’t be active until the next phase, or you’re pressure-testing a section of pipe and need to close one end.
A test cap is usually a thin-wall, low-cost version intended to come back off after the pressure test; a permanent cap is full-pressure-rated and meant to stay.
Common variants and what a cap is not
- Cap vs. plug. A cap is female (slips over the pipe end); a plug is male (threads or pushes into the open end of a fitting).
- Test cap vs. permanent cap. Test caps are cheap, designed for temporary use during pressure testing. Permanent caps are pressure-rated for service.