jar546
Forum Coordinator
According to the 2018 International Plumbing Code, Section 709.3 "Size of Fixture Drains", the minimum size of a trap arm for a bathtub is 1-1/2 inches in diameter. Is this correct?
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What is the minimum size of a trap arm for a bathtub?
- Thread starter jar546
- Start date
steveray
SAWHORSE
How about this too?
Bathtubs shall be equipped with a waste outlet that is not less than 11/2 inches (38 mm) in diameter. The waste outlet shall be equipped with a watertight stopper. Where an overflow is installed, the overflow shall be not less than 11/2 inches (38 mm) in diameter.
407.2 Bathtub Waste Outlets and Overflows
Bathtubs shall be equipped with a waste outlet that is not less than 11/2 inches (38 mm) in diameter. The waste outlet shall be equipped with a watertight stopper. Where an overflow is installed, the overflow shall be not less than 11/2 inches (38 mm) in diameter.
jar546
Forum Coordinator
Wasn't the question about a trap arm?
Are you trying to put a trick question out here or what?
Per my book...
2018 IPC 709.3 Conversion of GPM Flow to DFU Values
Where discharges to a waste receptor or to a drainage system are only known in gallons per minute (liters per second) values, the drainage fixture unit values for those flows shall be computed on the basis that 1 gpm (0.06 L/s) of flow is equivalent to two drainage fixture units.For the correct reference...
2018 IPC Table 709.1 - Drainage Fixture Units for Fixtures and Groups
^b A showerhead over a bathtub or whirlpool bathtub attachment does not increase the drainage fixture unit value.
2018 IPC Table 709.1 - Drainage Fixture Units for Fixtures and Groups
FIXTURE TYPE | DRAINAGE FIXTURE UNIT VALUE AS LOAD FACTORS | MINIMUM SIZE OF TRAP (inches) |
| Bathtub ^b (with or without overhead shower or whirlpool attachments) | 2 | 1-1/2 |
jar546
Forum Coordinator
Nope. Trying to create accurate content. Having more than one person who chooses the same answer gives it more clout. Maybe I am wrong and looked too fast.....
And for those who follow the UPC...
2018 UPC Table 702.1 Drainage Fixture Unit Values
2018 UPC Table 702.1 Drainage Fixture Unit Values
PLUMBING APPLIANCES, APPURTENANCES, OR FIXTURES | MINIMUM SIZE TRAP AND TRAP ARM ^7 (inches) | PRIVATE | PUBLIC | ASSEMBLY ^8 |
| Bathtub or Combination Bath/Shower | 1-1/2 | 2.0 | 2.0 | -- |
wwhitney
Registered User
Under the UPC, the tub trap arm may be 1-1/2", but once the vent comes off, any horizontal drain carrying the tub has to be at least 2". A 1-1/2" drain is good for 2 DFU vertically (with some limitations) but only 1 DFU horizontally, and a tub is 2 DFU.
Cheers, Wayne
steveray
SAWHORSE
I was just indicating that ALL signs point to 1.5"......
Mech
Registered User
Per Table 709.1, bathtubs have a dfu load of 2 and require a minimum 1-1/2" trap. Table 710.1(2) permits 3 dfu on a 1-1/2" horizontal branch, so I will say a 1-1/2" minimum trap arm is the answer.
Paul Sweet
Sawhorse
I remember that 2" was the minimum size for waste piping under a slab, but I can't find it npw. Maybe it's a requirement that was dropped but I continue to use as good practice.