Sizing trap and drain of shower unit ?

[Wgoodrich]

The way I am picking up the IRC 2000 rules the plumbing units for drains allows a 1 1/2" branch drain. Then the rules for traps require 2" trap for that same shower unit. Then the IRC frowns on reducing a drain size in the drain system. Anybody got an explaination or am I missing something?

Curious

Wg

[DUNBAR PLUMBER]

This is going to be an educated guess coming back at you.

In my Kentucky State Codebook,


Shower stalls carry a minimum requirement of 1.5" drain, trap arm and counts as 1.5 fixture units.

A bathtub carries the same exact minimum requirements. In the majority of cases, the same valve assembly used for bathing on both a bathtub and a shower stall carries the same GPM flow on these units. In contrast to the sizing of drain to fixture, I am sure the calculation follows the maximum flow rate from the fixture in correlation to pipe size in order to properly carry wastewater without malfunctioning and backing up.


Now in my opinion, I personally like 2" for the reason that it takes longer for the drain to cake over with soap on top of the standing water in trap. 1.5" tends to cake over quicker.

These days the codes are either enforced or not from state to state........and I don't understand the theories behind that.......I feel it should be across the board.

But I have used 2" X 1.5" bell reducers on shower pans and tie into drain stack.

Why they don't make a shower drain assembly fitted for 2 different pipe sizes, I don't know. I do know they make one assembly where a simple bushing fits right into the underside of drain assembly, but it is a deep drain config, and might make it difficult if you need to keep piping up high in the joist spaces.


Good question, Hope I shed a little light from my end of the globe. Let me know what the final answer is from the inspectors in your area. :) :cool:

[Wgoodrich]

I was on the understanding that your plumbing Code in Kentucky was the International Residential Code 2000 for dwellings . If so you have the same rules as I do.

Check the following copied sections below that prompted my confusion logic and the rules.

COPIED SECTION IRC 2000;

TABLE P3004.1
DRAINAGE FIXTURE UNIT (d.f.u.) VALUES FOR VARIOUS PLUMBING FIXTURES

TYPE OF FIXTURE OR GROUP OF FIXTURES DRAINAGE FIXTURE UNIT
VALUE (d.f.u.)a
Shower stall 2

TABLE P3005.4.1
MAXIMUM FIXTURE UNITS ALLOWED TO BE CONNECTED
TO BRANCHES AND STACKS
NOMINAL PIPE SIZE ANY HORIZONTAL ANY ONE VERTICAL
(inches) FIXTURE BRANCH STACK OR DRAIN
1 1/2 3


TABLE P3201.7
SIZE OF TRAPS AND TRAP ARMS FOR PLUMBING FIXTURES
PLUMBING FIXTURE TRAP SIZE MINIMUM
(inches)
Shower 2



Notice the size of branch drain is allowed to be 1 1/2" minimum, yet the trap of that same drain at the receptor must be 2" minimum. To me this would mean a reducer installed after the trap that would drastically slow the drain flow. This is what bothers me. Why in the world would the minimum safety rules require a 2" trap then allow a reducer to 1 1/2" drain per code rules?

I feel I am missing something but I build the rules of the IRC as above and it tells me a 2" trap but the drain may be a size smaller. Duh?

Curious

Wg