Just wondering, why is tub drain line smaller

Shower drain pipes are 2" but tub drains are 1-1/2". Why is this? Since most tubs also have showers in them why is the drain allowed to be smaller?

I would think draining a full bathtub would put more water down the pipe at a faster rate than just taking a shower.

Just curious.

This is a FAQ. The answer is really quite simple. In case of a clog, a tub will hold a great deal of water giving a margin of safety before the big flood. A shower base will hold very little water so there is very little margin of safety. The 2" drain is less likely to clog at least to the point of causing an overflow. No doubt there are many showers that have been installed either before this requirement came into being or installed after the fact but were never inspected. They work fine until that fateful day of the big clog.

Makes perfect sense. Thanks.

A second reason is a tub has an overflow, a shower doesn't, so if you do overfill a tub, it still won't overflow into the room...you want to dump a shower very quickly if you say drop a washcloth over the drain or stand on it before you realize water is rising.