How are grease interceptors sized?

The two kinds of interceptor

Hydromechanical interceptors are the compact units, usually inside near the kitchen, rated in gallons per minute of flow with a grease capacity in pounds — the rule of thumb rates grease capacity at roughly twice the flow rating. Gravity interceptors are the large tanks, usually buried outside, sized in liquid gallons — hundreds to thousands — and they work on retention time: the tank must hold the peak flow long enough, commonly a 30-minute retention basis, for separation to happen. Which type your project uses is decided by the fixture load, the available space, and the local authority — many jurisdictions publish their own sizing tables and approved-unit lists that override generic methods.

What goes into the calculation

Start with the inventory of fixtures that discharge through the interceptor: three-compartment sinks, pre-rinse stations, dish machines where the local code routes them in, floor sinks receiving kitchen waste, wok and kettle drains — each contributing flow and grease load. Fixtures that must NOT connect matter too: restroom waste never routes through the interceptor, and some jurisdictions keep dish machine discharge out or require it in. Peak flow is estimated from the connected fixtures draining simultaneously, then the unit is selected whose rated capacity holds that flow at the required retention. The grease production of the menu itself — a fry-heavy operation loads an interceptor faster than a café — drives the pumping interval the owner will live with.

Why sizing goes wrong

The classic failures: the interceptor sized from the plumbing plan before the final equipment list, so the added fryer and bigger three-comp sink never entered the calc. The unit placed where it cannot be pumped — an interceptor that is hard to service will not be serviced, and an unserviced interceptor is a decoration. And the local ordinance discovered late: many cities impose their own minimum sizes, sampling structures, or manifest and pumping-report requirements regardless of the plumbing code's math. Sizing is a calculation plus a jurisdictional check, and the check is the part that gets skipped.

What Torch delivers

Torch sizes grease interceptors from the final kitchen equipment and fixture schedule — the same schedule that drives the gas, water, and exhaust design — and checks the result against the local ordinance's minimums, placement rules, and sampling requirements before the set is issued. The plumbing set shows the interceptor, its rating, its venting, and a location a pumper can actually reach. When the health or sewer authority wants the sizing calculation, it is in the submittal, not reconstructed after the correction letter arrives.

The bottom line

Interceptor sizing is a flow-and-retention calculation run on the final fixture list, then checked against the local grease ordinance. Size it late or place it badly and the unit fails in service, not on paper.

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