From the 2006 IRC. (bold is mine)
- P2801.5 Required pan.
Where water heaters or hot water storage tanks are installed in locations where leakage of the tanks or connections will cause damage, the tank or water heater shall be installed in a galvanized steel pan having a minimum thickness of 24 gage (0.016 inch) (0.4 mm) or other pans for such use. Listed pans shall comply with CSA LC3.
- - P2801.5.1 Pan size and drain.
The pan shall be not less than 11/2 inches (38 mm) deep and shall be of sufficient size and shape to receive all dripping or condensate from the tank or water heater. The pan shall be drained by an indirect waste pipe having a minimum diameter of 3/4 inch (19 mm). Piping for safety pan drains shall be of those materials listed in Table P2904.5.
- P2904.5 Water-distribution pipe.
Water-distribution piping within dwelling units shall conform to NSF 61 and shall conform to one of the standards listed in Table P2904.5. All hot-water-distribution pipe and tubing shall have a minimum pressure rating of 100 psi at 180°F (689 kPa at 82°C).
One type of pipe *NOT* included in Table 2904.5 is *PVC*.
The drain pipe from water heater drain pans is no longer allowed to be PVC (except, of course, in Florida, where water cools off immediately after exiting the relief valve and is suitable for bathing in right then and there 
).