The plumbing code most Valley cities use sets no depth. IPC 1003.3.7 sends gravity interceptors to IAPMO/ANSI Z1001 and the maker's instructions. Mesa is the exception: its code requires an outlet tee with a down spout of at least 18 inches. Elsewhere, follow the listed unit's drawings and your approved plans.
Why the outlet depth matters
The outlet tee is the pipe that lets treated water leave the tank. The National Precast Concrete Association describes grease rising to the top and heavier food particles settling into a solids zone at the bottom. Where the outlet draws from decides which of those layers can follow the water out.
NPCA's guidelines say "An outlet baffle or tee is necessary," and they list proper interior tee length or baffle depth among the design points. They also say "A space is needed at the bottom for sludge accumulation." A tee that's missing, broken or cut short changes where water leaves the tank. That's why it shows up on inspections. See what happens when a grease interceptor baffle or tee breaks.
What the IPC says (2021 and 2024)
Section 1003.3.7 reads the same in both editions on this point. The IPC says gravity grease interceptors "shall be designed and tested in accordance with IAPMO/ANSI Z1001" and "shall be installed in accordance with manufacturer's instructions." Where there are no manufacturer's instructions, they're installed in compliance with ASME A112.14.6 and IAPMO/ANSI Z1001. The section sizes the tank by peak flow and a 30-minute retention time. It gives no outlet depth.
What each Valley city requires
| City | Code on this point | Outlet tee depth in the code? |
|---|---|---|
| Phoenix | 2024 IPC 1003.3.7 as amended (Ord. G-7397, effective Aug. 1, 2025) | No. Design and testing to Z1001; install per maker's instructions and the AHJ |
| Mesa | City Code 8-4-22 | Yes. Outlet tee with threaded cover and a down spout of at least 18 inches |
| Scottsdale | Revised Code 31-85 (amends IPC 1003.3.7) | No. The amended section sets the sizing formula only |
| Chandler | City Code 29-4.4 (amends IPC 1003.3.7) | No. Swaps the first sentence for a DFU sizing chart; the rest of the 2024 IPC section stands |
| Tempe | City Code 8-600 (amends IPC 1003.1 and 1003.3) | No. The Public Works Director can add requirements |
Phoenix rewrote 1003.3.7 but kept the design rule: interceptors are "designed and tested in accordance with IAPMO/ANSI Z1001" and installed per "manufacturer's instructions and the Authority Having Jurisdiction." Its additions are about the box, not the tee. A 500 gallon interceptor needs at least two compartments and two manways, 750 gallons and up need two compartments and three manways, and every manway is at least 20 inches inside diameter. We also found no outlet depth in Phoenix's June 2024 FOG manual.
Mesa is specific. Mesa's City Code 8-4-22 says an interceptor must be "Constructed with outlet piping with a tee connection and a threaded cover with a minimum of an eighteen (18) inch down spout." The inlet gets "a ninety degree (90°) elbow and minimum of an eighteen (18) inch down spout." Just so you know, that's a minimum length for the down spout. The code doesn't state a distance from the tank floor.
Scottsdale replaced 1003.3.7 with its sizing formula, total DFUs times 3 gpm times the retention time. Its grease interceptor guidelines add that plans "may be required to be prepared and stamped by an Arizona Registrant." Neither sets a tee depth.
Tempe left 1003.3.7 as written, and its amendment to 1003.3 adds that the Public Works Director "may provide additional requirements." Chandler changed only the sizing sentence, so the Z1001 and manufacturer's-instructions language still applies there.
For how the tank itself gets sized, see what size grease interceptor a restaurant needs.
How other states set it
Some places outside Arizona do write a number, and they don't agree. Washington's rule for grease interceptor tanks says the outlet tee or baffle must extend "Into the liquid to a point between six inches and twelve inches from the bottom of the tank," and above the liquid far enough "to allow scum storage and venting." Connecticut's fact sheet for outdoor interceptors calls for inlet and outlet tee fittings "that extend within 12 inches of the bottom and at least 5 inches above the liquid level of the tank." Neither rule applies in the Valley. They show why copying a number from another state's rule is a bad idea.
The manufacturer's design usually decides
On a listed tank, the outlet comes with the unit. Infiltrator Water Technologies, for example, says its interceptors are installed by a licensed plumber under "state and/or local regulations and approvals, which supersede the manufacturer's installation instructions." It also says "Infiltrator requires the use of the supplied inlet and outlet tees." So if a tee breaks, the replacement should match the maker's part and drawing, not a guess.
What to do
- 1Pull the plans and the unit's submittal. The outlet detail lives there.
- 2In Mesa, check the down spout length. At least 18 inches, on a tee with a threaded cover.
- 3Inspect the tee after every full pump-out, when you can actually see it.
- 4Match replacements to the original design, and ask your city whether the repair needs a permit.
While you're looking, check the sampling port and venting too. HQ Plumbing & Air coordinates a subcontracted pumper to empty the tank, then jets and inspects the interceptor. We don't pump tanks ourselves.