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​Grease Trap Sizing Calculations

See 1994 Uniform Plumbing code (UPC) Section 1011 and Table 10-2.

The total storage capacity in gallons of the fixtures discharging into the grease trap shall not exceed 2.5 times the certified GPM flow rate of the grease trap. This limitation is in addition to the restrictions found in Table 10-2 below.

​​​Total Number of Capacity Fixtures Connected

​Required Rate of Flow Gallons Per Minute (GPM)

​Grease Pounds (LB)

​1

​20

​40

​2

​25

​50

​3

​35

​70

​4

​50

​100

Grease Interceptor Sizing Calculations​

See 2006 Uniform Plumbing Code, (UPC) Appendix H

​​​Minimum Interceptor Size​

​500 gallons (2 compartments)

750 gallons or more  (3 compartments)

​Minimum Retention Time

​12 Minutes -Systems without garbage disposal(s)

​12 Minutes -Systems without garbage disposal(s)

Flow Rate

For the purpose of interceptor sizing, a value of 3 gallons per minute (GPM) is assigned for each connected fixture unit determined from application of Table 7-3 or Section 702.0. (Fixture and pipe size shall be as required elsewhere in the 2006 Uniform Plumbing Code)

Alternative: The actual maximum waste flow rate of individual fixtures may be substituted for the standard 3 GPM per fixture unit flow rate when substantiated by engineering calculations and manufacturers data for such individual, specified fixtures.

Floor sinks and trench drains (trough drains) are assigned fixture unit values based upon trap and trap arm sizes or as shown in UPC Table 7-3.

Example - Grease Interceptor Sizing

Description of Installation

A grease interceptor installed to receive wastes from multiple fixtures including: 1 dish washing machine floor sink, a grease extracting hood, 1 tilt kettle floor trough, 1 mop sink, a 3-compartment pot sink, 1 pre-rinse sink with disposal, and 4 floor drains. No solids interceptor is used; therefore a 17-minute retention time is required.

Step 1 - Determine the Hydraulic Loading of each Fixture in Fixture Units (FU)​

  1. Dish machine receptor (floor sink) with 4" trap and trap arm = 8 FU {Section 702)
  2. Grease exacting hood receptor (2" floor sink) = 3 FU (Table 7-3)
  3. Tilt Kettle trough (minimu​m 2 " trap and trap arm) = 4 FU (Section 702) ...Mop (service) sink = 3 FU (Table 7-3)
  4. 3-compartment (commercial) sink = 3 FU {Table 7-3)
  5. Pre-rinse sink with garbage disposal (2" trap and trap arm) = 4 FU
  6. 4 Floor drains x 2 FU each = 8 FU
Step 2 - Establish the Total Hydraulic Loading on the Interceptor in Fixture Units

​​​8 FU + 3 FU + 4 FU + 3 FU + 4 FU + 8 FU = 33 Fixture Units

Step 3 - Determine the Minimum Size Interceptor Required in Gallons

  • The 17-minute retention time applies to all fixtures even though all fixtures may not discharge simultaneously.
  • (33 FU) x (3 GPM) x (17 minute) retention time = 1683 gallons minimum capacity required
​Solution
Choose the next larger available size 3-compartment interceptor (i.e.: 1700, 1750, 1800, or 2000 gallon capacity)