My main drain line has rust spots that looks like they are ready start leaking. i want to replace this line with PVC. The drain line is galvanized pipe that measures 2 3/4 inches in diameter. my home improvement store only carries 2" and 3" PVC. They also sell the rubber boots to connect PVC to galvanized. Should i use 3" PVC for this job? Will the 3" rubber boot fit the galvanized pipe? Thanks
One thing to consider is that metal pipes are often grounded and may even have electrical connections using them as a ground (think appliances). Running a section of PVC will interfere with this so be certain that nothing is connected or run a seperate grounding line back to the other pipe or to the nearest feasible point.
Using the larger pipe will be better especially if it has solids passing through it (kitchen sink for example), don't be afraid to use online stores to find exactly what you're looking for. One option is to buy from the hardware store and return the fittings if they don't fit (although if you've used solvent weld etc then that may cause issues but nothing stopping you from sliding them over and checking before installing).
Bring your tape measure to the store!
If this is your main drain line (meaning all fixtures in your house drain into it) you'll want it to be at least 3 inches, since that is the minimum for any standard toilet. (Macerating toilets can have smaller lines but those are not very common in standard residential settings.)
As for the rubber boot, yes, it should be able to connect 2.75-inch galvanized pipe to 3-inch PVC without issue. Those connectors typically have a fair amount of play; for a 3-inch boot, you could probably connect either side to a pipe ranging in diameter from about 2.5 to 3.5 inches.