Chandler amendment to IPC 1003.3.7: how big must a grease interceptor be?

The 2024 International Plumbing Code (IPC) has its own sizing language for gravity grease interceptors in Section 1003.3.7. Chandler City Code 29-4.4 deletes the first sentence of that section and replaces it with a rule that the capacity "shall be sized per the City of Chandler DFU Conversion Chart." The chart is printed in the ordinance.

A footnote adds that establishments "resulting in total DFU's > 342 shall be designed by a professional engineer, registered in the State of Arizona."

• Gravity grease interceptor: a large in-ground or below-floor tank that slows kitchen wastewater so grease floats and solids settle. It is different from a small hydromechanical trap under a sink.
• Drainage fixture unit (DFU): a number the plumbing code assigns to each fixture for how much it can discharge. The city's sizing worksheet (Form UDM-062) converts by drain size, using IPC Table 709.2: a drain of 1 1/4 inches or less is 1 DFU, 1 1/2 inches is 2, 2 inches is 3, 2 1/2 inches is 4, 3 inches is 5 and 4 inches is 6.
• Total DFU: only the fixtures that drain to the interceptor count, not the whole building. The worksheet lists what to include: three-compartment and two-compartment sinks, food prep sinks, mop sinks, floor sinks, trench drains, floor drains, ice cream dipper wells and trash compactor washouts, among others.
• Dishwashers: the worksheet counts a dishwasher as 2 DFU for every gallon per minute of the manufacturer's flow rate, and asks for the make, model and a cut sheet.

It applies to gravity interceptors at any Chandler food service business that needs one. The chart does not size small hydromechanical grease traps, and the ordinance text does not say how the city sizes those. It does not apply outside Chandler city limits, and neighboring cities use their own sizing rules (compared below).

A tenant improvement in a retail center. The landlord's plans show a 500-gallon interceptor left by a previous tenant. That is below the chart's smallest size of 750 gallons, and if the new kitchen has more than 21 DFU draining to it the chart calls for at least 1,000. Whether an existing tank can stay in a tenant improvement is decided at plan review.

One fixture flips the size. A kitchen at 21 DFU needs 750 gallons. Adding a single 3-DFU sink moves it to 24 DFU and 1,000 gallons. That is why the fixture count matters before you finalize the layout.

A build-out copied from another city. A restaurant group reuses a Phoenix set of plans with a 500-gallon interceptor. Phoenix's own manual sizes by flow (an example there multiplies 12 DFU by 3 gallons per minute by a 12-minute retention time to get 432 gallons and rounds up to 500). In Chandler, the same 12 DFU sits in the first row of the chart and needs 750 gallons.

1. 1Have your plumber or designer fill out the city's Gravity Grease Interceptor Requirements worksheet (Form UDM-062): list each fixture that drains to the interceptor, its drain size and quantity, and multiply by the DFU value.
2. 2Read the size off the chart above. Over 342 DFU, hire an Arizona registered professional engineer.
3. 3Submit the fixture count and plumbing plan to Chandler Development Services when you apply for the plumbing permit. The city's Food Service Establishments page asks for exactly that.
4. 4Add the dishwasher make, model and flow rate, and attach the tank manufacturer's cut sheet, both of which the worksheet asks for.
5. 5Plan the maintenance access and the pumping records now. The city keeps the owner responsible for cleaning and repairs.

• "The IPC formula decides the size." In Chandler the ordinance chart replaces the first sentence of IPC 1003.3.7.
• "Bigger is wasted money." The city says correct sizing costs less over time because a smaller device "will require more frequent cleanings."
• "500 gallons is the standard." Mesa's minimum is 500 gallons. Chandler's chart starts at 750.
• "The chart covers every device." It covers gravity interceptors.