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g/plumbing • Posted by u/pipefitter_pat •

Grease trap sizing for a commercial kitchen — any good rule of thumb?

Bidding a new restaurant build, 120-seat, full commercial kitchen. AHJ requires an indoor grease interceptor. PDI standard says to size based on drainage fixture units and flow rate, but the owner's architect just told me "size it for 50 gallons per meal."

That's not a real sizing method. Has anyone dealt with architects giving nonsensical grease trap specs? The PDI method gives me a different number than the "50 gallons per meal" guess.

What's the right approach and how do you document it for the AHJ?

💬 4 replies

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u/backflow_ben •

In my jurisdiction, the health department has their own form that supersedes the architect's "method." Get the local health department requirements first before doing any calc — they often have preapproved sizing tables for common kitchen configurations. Saves everyone time.

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u/water_heater_wally •

Following this thread — I've only done residential so far. Didn't realize commercial plumbing sizing was so different.

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u/pex_pioneer •

The architect told you to size for 50 gallons per meal because that's what a vendor told them. Vendors love to sell bigger equipment. Document the PDI calc, note that the architect's suggestion is not a recognized sizing standard, and submit both to the AHJ. Let them pick.

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u/drain_dave •

Use the PDI G-101 standard or your local health department's requirements — whichever is more stringent. The "gallons per meal" rule-of-thumb is outdated and rejected by most AHJs for commercial work. Document the PDI calculation: number of fixtures, flow rate (GPM), retention time, and you'll get an interceptor capacity in gallons. Most health departments have a worksheet.

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