Good question. The only reason I can say is because the showerhead in a shower is constantly running water as you stand in it. Also, the showerpans are only a few inches deep to hold any water if draining slowly. A tub is always 14.5" deep or more and does have a safety drain on top in case of accidental forgetting when the tub is filling.

Most codes will have a 2" drain on the shower.

Hope this sheds a little light.

Toadangel,

I ask because I am replacing a tub with a shower receptor that is the same size and I already have the 1.5" drain in place. I would love to just convert it down from the 2" to the 1.5 line, but that would be a no-no.

While I am sure that the answer will cause a tirade of argument, the answer is basically simple.

Under both the IRC (international Residential Code) and the UPC (Uniform Plumbing Code) the standard size for a tub drain & P-trap is 1-1/2" and under the IRC the tub waste arm (drain line) may be 1-1/2" but under the UPC an 1-1/2" drain line is limited to 1dfu (drainage fixture unit) whereas under the UPC a tub is rated at 2dfu's, therefore under the UPC the tub fixture arm must also be 2".

in the case of a tub that has an internal lever waste stopper the drain opening is required to be equipped with a trim cover, which is the little flat metal cover with all the small holes in it. Code requires that the combined cross sectional area of all those little holes must be equal to or greater than the cross sectional area of the tailpiece. Understanding that the tailpiece is 1-1/2" it then stands that the cross sectional area of the tailpiece is 2.25" If we then make a cover with 1/4" diameter holes the we must have a minimum of 36 holes. By carefully arranging those holes in a circular patter they can make the overall diameter of the trim cover 1-3/8" or 1-7/16". The problem is that an adults foot is large enough that if the person taking a shower were to stand directly on the drain opening their foot could effectively cover all the holes and water could not drain out. Fortunately the walls of a tub are high enough that the tub would serve as a reservoir to hold the excess water until the person moves away from the drain, and in a worst case scenario, the tub has an overflow that will prevent water from spilling over the top of the tub.

On the other hand, a shower pan is required to have a minimum of a 1/4" per foot pitch towards the drain opening and the threshold is required to be a mere 2" above the finished height of the drain opening. If we had an 1-1/2'drain opening someone could inadvertently stand on the drain opening without realizing it, or perhaps drop a washrag on the drain opening and the water would quickly build up and overflow over the threshold. By requiring a 2" drain it then becomes necessary to have 64 1/4" holes in the trim cover, and allowing for enough metal to provide the necessary strength in the trim cover, the minimum diameter of the trim cover is now 3-1/2" to 4". Now I am sure that someone will come forward and say they have big feet, but on average due to the contours of the bottom of your foot you cannot cover all the holes in the drain opening with your foot.

This explains why a Master Plumber I paid installed a new 1.5" drain when I asked for a 2". He said he didn't have room for a 2" at a pinch point, and he reduced the line to 1.5" everywhere, starting right under the shower grate.

B.t.w. many years ago both codes called for 1.5" pipes. I don't know what discussion went on prior to the one code getting stricter.

A 1.5" line will carry shower water just as far as it will carry tub water. A 2" pipe will carry water farther because it has more self-venting capacity.

I'm going to have to agree with lazypup's comment about people with large feet not being able to block a 2" drain. I recently put in a shower with a 2" drain (kerdi) and my size 14's can't seem to block the water off. I've tried several times.

For those car wash type showers Barry refs in the OP, a 3" drain pipe is needed.