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Commercial Grease Interceptor Sizing per IPC 1003

Reference Compliance Updated Jul 24, 2026 ยท Plumbing

Why this matters

A grease interceptor undersized for a commercial kitchen sends FOG (fats, oils, grease) downstream into the building sewer, public collection main, and ultimately the treatment plant. The owner gets fines, the line backs up into the dining room, the public-works inspector locks the food-service permit. IPC 2021 Section 1003 prescribes when an interceptor is required and how to size it; the local jurisdiction may amend up but rarely down. Sizing is not a guess: the math is in the code, and the inspector will check it on plan review.

When an interceptor is required

IPC 1003.3.1 requires a grease interceptor on every commercial discharge from food-preparation sinks, soiled-dish sinks, prerinse sinks, woks, dishwashers (where local AHJ permits dishwashers on grease service), and floor drains receiving wash-down from food-prep areas. Hand-wash lavatories, restrooms, and customer toilet rooms must NOT discharge through the grease interceptor; their flows dilute the FOG matrix and break the interceptor's gravity separation. Pre-rinse spray-valve sinks discharge through. Three-compartment dish sinks discharge through.

Two device categories

Hydromechanical Grease Interceptor (HGI), formerly grease trap: indoor, point-of-use device sized by flow in gallons per minute (gpm) and rated for grease retention in pounds per gpm. PDI G101 and ASME A112.14.3 list flow ratings 10/20/25/35/50/75/100 gpm with corresponding grease capacities. Installed under or near sinks; pumped monthly or quarterly.

Gravity Grease Interceptor (GGI), exterior: typically precast concrete or HDPE, 750 to 5,000+ gallons, installed exterior to the building between fixtures and the building drain connection. Sized by retention time per IPC 1003.4.2 or by local AHJ tables (Uniform Plumbing Code tables, jurisdictional tables in major cities). Pumped quarterly to annually by licensed grease haulers.

HGI sizing by IPC 1003.3.4

Sum the drainage fixture units (DFU) or the actual flow in gpm of all connected fixtures. For a typical three-compartment sink:

  • Compartment volume = L x W x D (inches), divide by 231 cubic inches/gallon.
  • Fixture flow rate = compartment volume / 60 seconds, then x 60 = gpm equivalent if drained in 1 minute; IPC uses a 1-minute drain assumption.
  • Apply 75 percent fill factor (sinks are not drained from full to empty under pure gravity at design flow).

For a three-compartment sink at 24 x 24 x 15 inches per compartment (8,640 cu in per compartment, 37.4 gallons), drainage at 1-minute fill = 37.4 gpm per compartment. Sized at 75 percent: 28 gpm. Three compartments draining simultaneously is unusual; the code allows sizing by the worst-case simultaneous discharge. The selected interceptor is the next-size-up listed unit; for 28 gpm peak, select a 35 gpm HGI (Zurn Z1170, Watts FT Series, or equivalent).

GGI sizing by IPC 1003.4

IPC 1003.4.2 allows sizing by:

  • Drainage Fixture Unit method: tabulated minimum interceptor liquid capacity for the connected DFUs.
  • Volume method: peak hourly flow x retention time (typically 30 minutes), per Uniform Plumbing Code Appendix H or local AHJ.
  • Manufacturer's published sizing guides (Schier GB-250 through GB-3000 series have flow-based selection tables).

A 100-seat full-service restaurant with prerinse, three-compartment sink, two prep sinks, two floor sinks, and one disposer typically lands on a 1,000 to 1,500 gallon exterior GGI. A 250-seat operation with a steam table line, multiple woks, and a dishwasher on grease service lands on 2,000 to 3,000 gallons.

Installation notes that catch in plan review

Vent the interceptor per IPC 1002.4; a sealed interceptor without venting flows incorrectly and inspector flag is automatic. Use a flow control fitting upstream on HGI per ASME A112.14.3 to prevent over-velocity (which carries grease through the unit without separation). Maintain a downstream sample port on GGI for inspector access. Slope tributary lines at 1/4 inch per foot minimum for 2 inch and smaller, 1/8 inch per foot for 3 inch and larger; flatter slopes allow FOG to coat the pipe walls. Do not connect ground-up food-waste disposers (garbage disposals) upstream of a HGI in most jurisdictions; the slurry blinds the baffles. AHJ-specific: many cities ban disposers in commercial food service entirely.

Maintenance and FOG management plan

Pump-out frequency depends on the 25 percent rule: when accumulated FOG plus settled solids reach 25 percent of the wetted depth, the interceptor must be cleaned. HGI typically every 2 to 8 weeks; GGI every 60 to 120 days. The building must keep a manifest for each pump-out; many municipalities require the manifest as a condition of the food-service permit renewal. Drain-line jetting on the kitchen branch every 12 months extends interceptor life by knocking down wall-coated FOG before it reaches the interceptor.

References

  • IPC 2021, Section 1003 Interceptors and Separators
  • ASME A112.14.3-2018 Grease Interceptors
  • PDI G101 Standard for Testing and Rating Grease Interceptors
  • Uniform Plumbing Code 2021, Appendix H Sizing of Gravity Grease Interceptors
  • EPA 832-F-12-001 Best Management Practices for Restaurant Food Service Operators

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