Schedule 80 PVC has a thicker wall than Schedule 40 at the same nominal size, so its pressure rating is higher. The outside diameter is the same, which is why both fit the same socket size. The inside diameter of Schedule 80 is smaller. For drain, waste and vent (DWV) lines, use Schedule 40 DWV pipe and fittings, not Schedule 80. For cold-water pressure lines, either schedule can work if the whole system’s rating covers the pressure. A heavier wall does not make PVC suitable for hot water, compressed air or drainage fittings it was not made for.

Pressure ratings at room temperature (Charlotte Pipe)

These are Charlotte Pipe’s maximum working pressures for solvent-cemented pipe at 73°F:

Nominal size Schedule 40 pipe Schedule 80 pipe
1/2 inch 600 psi 850 psi
3/4 inch 480 psi 690 psi
1 inch 450 psi 630 psi
1-1/2 inch 330 psi 470 psi
2 inch 280 psi 400 psi
4 inch 220 psi 320 psi

The pipe is rarely the weak point, though. Charlotte’s manual gives three further limits:

  • Fittings: Charlotte recommends rating socket fittings at 60% of the pipe’s working pressure for the same size and schedule.
  • Threads: molded or cut threads are rated at 50% of a solvent-cement system, and Charlotte recommends threading only Schedule 80 (PVC or CPVC), not Schedule 40.
  • Lowest-rated part: the whole system must be derated to its lowest-rated component, whether that is a valve, union, flange or threaded adapter.

Heat cuts the rating sharply

Charlotte multiplies the 73°F rating by a temperature correction factor. For PVC, the factor is 0.62 at 100°F, 0.40 at 120°F and 0.22 at 140°F. Above 140°F, PVC is not rated at all. In Charlotte’s own example, 6-inch Schedule 80 PVC drops from 280 psi to about 62 psi at 140°F. The same derating applies to heat from outside the pipe, including sunlight on a dark surface.

That is why Schedule 80 PVC is not a hot-water supply pipe. CPVC is a different material with its own products and ratings. Do not substitute one for the other.

Never use PVC for compressed air

Charlotte’s warning is direct: “NEVER test with or transport/store compressed air or gas in ABS / CPVC / PVC pipe or fittings.” Failures can be explosive. This holds for Schedule 80 too. Use a piping system rated for compressed air.

Why Schedule 80 is wrong for drains

Charlotte “does not recommend using Schedule 80 pipe in combination with Schedule 40 DWV fittings.” Its reasons:

  • The smaller inside diameter of Schedule 80 leaves a ledge at every DWV fitting hub. The ledge can catch waste and lead to clogs.
  • Schedule 80 pressure pipe (ASTM D1785) is “not marked or listed in the model plumbing codes for DWV applications.”
  • Pressure-pattern fittings lack the sanitary turn that lets waste pass unobstructed. There is no ASTM standard for Schedule 80 DWV fittings.

For drains, use Schedule 40 DWV pipe and fittings marked ASTM D2665. Charlotte notes that cellular-core (foam-core) Schedule 40 under ASTM F891 is lighter and less resistant to damage than solid-wall pipe. Where a buried drain needs more strength against soil or traffic loads, Charlotte points to cast iron soil pipe, not Schedule 80 PVC.

Sunlight

Charlotte says UV exposure discolors PVC and reduces its impact strength, but only at the surface. Pressure rating and stiffness are not adversely affected. It recommends painting exposed pipe with a water-based latex paint, never a petroleum-based one.

Before buying a repair section

  1. Read the printline on the existing pipe: size, schedule, ASTM standard (D1785 for pressure, D2665 for DWV, F891 for cellular core) and pressure rating.
  2. Confirm what the line carries and its operating temperature.
  3. Match fittings to the same pattern: pressure fittings (D2466 for Schedule 40, D2464/D2467 for Schedule 80) or DWV fittings (D2665).
  4. Use the primer and cement the maker specifies, and wait out the full cure time before pressurizing.

These figures come from one manufacturer’s manual. Use the ratings published for the exact pipe and fittings you buy, and follow local code.

Sources