3 compartment sink grease trap sizing determines whether your kitchen passes health inspections or gets shut down. One wrong calculation leaves you with an undersized trap that fails when inspectors arrive.
Key Takeaways:
- Standard 3-compartment sinks require 15-25 GPM flow rates, driving 30-50 pound grease trap sizing
- Maximum 10-foot distance between sink and grease trap to prevent grease cooling and solidifying
- Uniform Plumbing Code mandates specific trap sizing based on fixture capacity method calculations
What Size Grease Trap Does a 3-Compartment Sink Need?
A 3-compartment sink requires a grease interceptor sized between 30-50 pounds based on its GPM flow rate calculation. You can’t guess this number. The fixture capacity method determines exact sizing requirements regardless of how much water you think you’ll use.
Standard 24x24x14 inch compartments hold 22 gallons each. When all three compartments drain simultaneously, they generate 15-25 GPM flow rates depending on drain configuration. This flow rate drives your grease trap sizing under commercial grease trap regulations.
| Sink Configuration | Compartment Size | Total Volume | GPM Flow Rate | Required Trap Size |
|---|---|---|---|---|
| Standard 3-bay | 24″x24″x14″ | 66 gallons | 15-18 GPM | 30-35 lbs |
| Large 3-bay | 30″x30″x14″ | 102 gallons | 20-25 GPM | 40-50 lbs |
| Deep 3-bay | 24″x24″x16″ | 75 gallons | 18-22 GPM | 35-45 lbs |
Your actual usage doesn’t matter. A grease interceptor must handle maximum possible flow, not average daily volume. Health departments size traps for worst-case scenarios when all fixtures run at capacity.
Most restaurants get this wrong by focusing on typical usage instead of peak capacity. The fixture capacity method prevents this mistake by calculating based on drain size and compartment volume, not estimated water usage.
How Do You Calculate GPM Flow Rate for 3-Compartment Sinks?
The fixture capacity method calculates GPM flow rate through a specific process that determines your grease trap requirements. You measure compartments, identify drain sizes, then apply drainage fixture units to get final GPM numbers.
Measure each compartment’s dimensions in inches. Length times width times depth gives you cubic inches per compartment. Multiply by three for total sink volume.
Identify drain pipe diameter for each compartment. Standard 3-compartment sinks use 1.5-inch or 2-inch drains. Each 1.5-inch drain adds 3 GPM to total flow rate calculation under fixture capacity method rules.
Apply drainage fixture units based on compartment volume. Compartments over 20 gallons each count as 3 drainage fixture units. Under 20 gallons counts as 2 units per compartment.
Convert drainage fixture units to GPM using standard conversion tables. Nine fixture units equals 15 GPM. Twelve fixture units equals 18 GPM. This gives you the minimum GPM rating your grease trap must handle.
Add 20% safety margin to final GPM calculation. Code requires this buffer for peak flow conditions. A 15 GPM calculation becomes 18 GPM minimum trap capacity.
Actually, you need to verify local code variations before finalizing calculations. Some jurisdictions use different conversion ratios or require larger safety margins. Check with your local health department before purchasing equipment.
Where Should You Install a Grease Trap Under a 3-Compartment Sink?
Maximum distance is the distance limit between any grease-producing fixture and its grease trap. This means you can’t install the trap wherever convenient. Fats oils and grease (FOG) solidify when they cool, creating blockages in connecting pipes.
Ten-foot maximum distance prevents grease cooling below 70°F and solidifying in pipes before reaching the trap. Hot dishwater keeps FOG liquid during this short travel distance. Go beyond 10 feet and you’ll get solid grease plugging your drain lines.
Under-sink installation works best for most 3-compartment setups. You mount the trap directly below the sink cabinet, minimizing pipe runs and maintaining proper temperatures. External installation only makes sense when space constraints prevent under-sink mounting.
You need 18 inches of clearance around the trap for maintenance access. Health inspectors check this during routine visits. Cramped installations that prevent proper cleaning access will fail inspection regardless of proper sizing.
One thing to consider: floor-mounted traps require different clearances than wall-mounted units. Floor units need access from above for lid removal. Wall units need side access for outlet cleaning. Plan your space accordingly before installation begins.
What Are the Plumbing Connection Requirements for 3-Bay Sink Grease Traps?
Connection plumbing requirements specify exact pipe sizing, routing, and venting for code compliance. The Uniform Plumbing Code mandates these standards to prevent backups and ensure proper trap function.
• Use 3-inch minimum inlet pipe from sink to trap. Smaller pipes create velocity problems that prevent proper grease separation. Each compartment connects to this main inlet line through individual 2-inch branches.
• Install proper venting within 6 feet of the trap inlet. Vents prevent vacuum formation that pulls water from trap seals. Poor venting causes sewer gas odors and trap seal failures during heavy drainage.
• Maintain 2% minimum slope on inlet pipes. This equals 1/4 inch drop per foot of horizontal run. Insufficient slope allows grease to accumulate in pipes before reaching the trap chamber.
• Place cleanouts every 50 feet and at direction changes. Cleanouts provide access for clearing blockages without tearing out walls. Install them at trap inlet and outlet connections for maximum access.
• Use Schedule 40 PVC or cast iron materials only. Thin-wall materials can’t handle commercial kitchen temperatures and chemical exposure. ABS pipe deteriorates from hot grease and cleaning chemicals.
Actually, some local codes require different materials or pipe sizing. Check your jurisdiction’s amendments to UPC standards before starting plumbing work. What works in California might not meet requirements in New York.
What Code Requirements Apply to 3-Compartment Sink Grease Management?
Uniform Plumbing Code Section 1003 requires grease interceptors for all commercial dishwashing operations. This includes every 3-compartment sink in restaurants, regardless of size or volume. No exceptions exist for small operations or limited-service establishments.
Local health departments enforce these requirements through permit inspections and routine compliance checks. They verify proper sizing, installation distances, and maintenance records during visits. Violations result in fines, closure orders, or permit suspension until corrections are made.
Code requirements include specific installation standards beyond just trap sizing. Proper venting, cleanout placement, and pipe materials all carry equal weight during inspections. Miss any requirement and you fail the entire grease management system review.
You must maintain detailed maintenance logs showing cleaning frequency, waste disposal, and pump-out records. Inspectors review these logs to verify compliance with local ordinances. Missing documentation can trigger violation notices even when equipment functions properly.
Actually, some jurisdictions add requirements beyond UPC minimums. California requires enhanced treatment systems in certain areas. Texas mandates specific maintenance contracts with licensed haulers. Research your local amendments before finalizing system design.
Frequently Asked Questions
Can you use a smaller grease trap if you have a low-volume 3-compartment sink?
No. Grease trap sizing depends on fixture capacity, not actual usage volume. Even low-volume 3-compartment sinks require properly sized grease interceptors based on their maximum potential flow rate according to grease trap sizing calculations.
Do all three compartments of a sink drain into the same grease trap?
Yes. All three compartments typically connect to a single drain line that feeds one grease trap. The trap must be sized for the combined flow rate of all compartments draining simultaneously during peak kitchen operations.
What happens if the grease trap is too far from the 3-compartment sink?
Grease cools and solidifies in the connecting pipes, causing blockages and compliance failures. Most codes require grease traps within 10 feet of the fixture to prevent this problem and maintain proper FOG temperature during transport.