Plumbing code sizes a grease interceptor from the fixtures that drain into it, so it is large enough to trap fats, oils, and grease before they reach the sewer. Sizing uses the drainage load and flow of the connected sinks and equipment, with a minimum size for gravity interceptors serving a commercial kitchen.
The Uniform Plumbing Code is published and copyrighted by IAPMO. This page explains the section in our own words with a short excerpt only. Read the full official text at the source.
A grease interceptor only works if it is big enough. Too small, and it fills with fats, oils, and grease faster than it can trap them, so grease escapes into the sewer and clogs the line. UPC grease-interceptor rules set how to size the unit from the fixtures that drain into it, so a commercial kitchen keeps grease out of the public sewer.
What the code requires
The code sizes an interceptor from the drainage load of the fixtures connected to it, not from a guess. The core idea is stated directly:
Grease interceptors shall be sized in accordance with the anticipated flow and the fixtures discharging into them.
Two sizing approaches appear in the codes. A hydromechanical grease trap, the smaller unit found under or near a sink, is sized by the flow rate it must handle, in gallons per minute, based on the fixtures draining to it and a retention factor. A gravity grease interceptor, the large in-ground tank outside, is sized by volume, using the number and type of fixtures, their drainage load, and the time waste sits in the tank so grease can separate and float. Local rules commonly set a minimum size for a gravity interceptor at a food-service establishment, and the size has to be reviewed and approved during permitting. An undersized unit fails because it cannot give the grease time to separate before the water moves on.
When this comes into play
This governs every new or remodeled commercial kitchen. Picture a new restaurant adding a three-compartment sink, a prep sink, and a dishwasher: the interceptor has to be sized for the combined load of those fixtures, and the city reviews that sizing before the kitchen opens. Add fixtures later and the interceptor may need to be resized. Get it wrong and the kitchen faces grease backups and code violations.
What this means for you
If you run or are opening a food-service business, the interceptor size is set by your fixtures and flow, so plan it into the design rather than buying the smallest unit that fits. An interceptor that is correctly sized still has to be cleaned on schedule to keep working. In Phoenix, a food-service business also falls under the city's FOG grease program, and the kitchen equipment draining to the interceptor has to follow UPC 803 indirect waste piping. For the applied sizing question, see what size grease interceptor a restaurant needs, and for maintenance, see how often to pump a grease trap.
Full text and source
The UPC grease-interceptor rules are part of the Uniform Plumbing Code, published and copyrighted by IAPMO. The excerpt above reflects the sizing approach as adopted; Phoenix enforces the 2024 UPC with local amendments and its own minimum sizes, so verify the current method and section numbers. Read the interceptor rules on UpCodes, or confirm local amendments through the City of Phoenix: phoenix.gov/pdd.
Keep Reading
- UPC 1003: Which Fixture Traps Are Allowed (and Banned)
- UPC 1007: Trap Seal Protection for Floor Drains
- UPC Section 1101: Storm Drainage and Roof Runoff
- UPC 1208: Approved Fuel Gas Piping Materials
- What size grease interceptor does my restaurant need?
- How often should a restaurant grease trap be pumped?
- What is FOG (fats, oils, and grease) in plumbing?