Pipe diameter needed for a commercial grease waste line serving four commercial sinks
Short answer
A commercial grease waste line serving four commercial sinks requires a minimum 3-inch pipe diameter sloped at one-quarter inch per foot, increasing to 4 inches for three-compartment pot sinks or flatter slopes. Each individual sink connects via a 2-inch fixture drain. This sizing handles the 12 to 24 drainage fixture units generated while maintaining adequate flow velocity to prevent grease buildup, line choking, and kitchen sewer backups.
A commercial grease waste line serving four commercial sinks requires a minimum pipe diameter of 3 inches at a one-quarter inch per foot slope, expanding to 4 inches for three-compartment pot sinks or shallower slopes.
Commercial kitchen waste lines handle hot water laden with fats, oils, and grease (FOG) that rapidly cool and solidify against pipe walls. Under standard model plumbing codes like the International Plumbing Code (IPC) and Uniform Plumbing Code (UPC), undersizing grease waste piping leads to rapid line choking, sewer backups, and costly health code violations.
If you only do one thing: Size the common grease branch line to 3 inches minimum for standard sinks, but upgrade to 4 inches if connecting three-compartment pot-washing sinks.
- Drainage fixture unit calculations: Commercial food service sinks carry between 3 and 6 Drainage Fixture Units (DFU) each under IPC Table 709.1, bringing a four-sink setup to an aggregate load between 12 and 24 DFUs.
- Horizontal branch pipe sizing: A 3-inch horizontal branch pipe pitched at 1/4 inch per foot (2% grade) supports up to 20 to 35 DFUs, providing sufficient capacity for four hand, prep, or standard two-compartment sinks.
- Heavy-duty sink sizing: When serving four large three-compartment scullery or pot sinks, or when structural constraints limit horizontal slope to 1/8 inch per foot, install a 4-inch diameter pipe to handle the simultaneous surge discharge.
- Individual fixture connections: Rough in a dedicated 2-inch trap arm and fixture drain for each individual sink before manifolding into the common 3-inch or 4-inch grease waste trunk line.
- Grease interceptor flow rate: Route the grease waste pipe directly to an indoor hydromechanical interceptor rated between 35 and 75 gallons per minute (GPM) or an exterior gravity interceptor before tying into the municipal sanitary sewer lateral.
- Cleanout placement: Install full-size cleanouts every 50 to 100 feet and at every horizontal change of direction greater than 45 degrees to allow hydro jetting, such as ARS/Rescue Rooter Scour Jet® service, to clear fat deposits.
- Watch out for: Pitching 3-inch grease lines at less than 1/4 inch per foot, which drops flow velocity below the 2 feet per second needed to keep suspended fats in motion.
- Watch out for: Installing short-turn 90-degree elbows, which trap grease build-up and prevent sewer inspection cameras or hydro-jetter nozzles from passing through fittings.
- Watch out for: Routing commercial dishwasher effluent exceeding 140°F through the common grease line, which liquefies fats and forces grease past the trap into municipal sewer mains.
Calculate total fixture compartment volumes and aggregate DFU values against local municipal plumbing amendments before pulling rough-in permits. Contact ARS/Rescue Rooter at 866-399-2885 for commercial plumbing design, pipe installation, and grease line maintenance.