3 Compartment Sink Plumbing Diagram

What You Actually Need to Know About Three-Compartment Sink Plumbing

The 3 Compartment Sink Plumbing Diagram is more straightforward than people make it out to be, but there are enough code quirks that if you wing it you will end up with a sink that either drains too slowly or fails health inspection on a technicality. I have seen both happen. The basic layout is simple: three basins in a row, each with its own drain connection to a common waste line, with separate water supply feeds for hot and cold to each compartment, and a garbage disposal or strainer setup depending on local code. The real work is in the details. When you pull a plumbing diagram for a three-compartment sink, you are looking at a combination of a floor plan view and a riser or schematic view. The floor plan shows where the sink sits relative to walls, the water supply entry point, and the drain stub-out location. The schematic view shows pipe sizes, slope, venting, and how the P-traps tie into the waste stack. Both views matter. Ignoring one will get you a pipe that does not fit or a vent that is in the wrong place. Here is the typical configuration you will see on a proper diagram:

• Hot and cold water supply lines running along the back wall, with individual shut-off valves for each basin
• A gooseneck or commercial prep faucet on the wash compartment, and regular faucets on the rinse and sanitize compartments
• Three P-traps, one per basin, all tied into a wet vent or a common waste arm before hitting the building drain
• A floor drain or floor sink downstream if your jurisdiction requires it for commercial kitchens
• A grease interceptor connection if this is a food service installation and local code demands it

Let me give you a real example from a job I worked last year. We were retrofitting an older restaurant kitchen and the original diagram showed the waste line exiting straight back through the wall with a single half-inch vent coming off the second compartment. That setup looked fine on paper. It was wrong in practice. When we ran three full basins at once during a pass, the first compartment would siphon dry within seconds because the vent was undersized and positioned too far downstream. The fix was moving the vent connection to just after the first P-trap and upsizing it from half inch to three-quarter inch ABS. We also added a second vent takeoff near the third basin so the trap seals stayed intact under heavy load. That changed the whole dynamic of the system. Most code jurisdictions require the following minimums for a three-compartment commercial sink. These are not suggestions. A health inspector will cite you for anything below them. Supply lines: Three-quarter inch copper or PEX for the main feeds. Individual branch lines to each faucet can be half inch, but if you are running multiple faucets simultaneously you will notice pressure drop across the board and it affects rinse and sanitize cycle effectiveness. I switched to five-eighth inch PEX on the branch runs in a recent install and the flow consistency improved noticeably.

Waste lines: Two-inch PVC or ABS from the P-traps to the building drain. Do not go smaller. Three compartment sinks discharge a lot of water fast, especially when someone is draining a large pot into the wash bay. A one-and-a-half inch waste arm will gulp and aerate instead of moving water cleanly. It sounds minor until you are dealing with standing water between compartments during a rush. Venting: Each P-trap needs a proper vent connection within the allowed distance from the trap weir. For a two-inch waste line, that distance is typically five feet under most model codes. The vent must connect upstream of the trap seal, not downstream, or you are just venting sewage odor instead of protecting the water seal. This is the mistake I see most often on renovation jobs. The plumber is tired, the diagram is unclear, and they tie the vent into the wrong side of the fitting.

How the Physical Installation Actually Works

When you are hooking this up for real, here is the order that prevents headaches. Start with the drain assembly. Each basin gets a strainer bowl fitted into the drain hole with a gasket and lock nut. The tailpiece drops down to the P-trap. Do not use thread tape on the strainer threads. These are designed to seal with a compression gasket, and tape interferes with that. Tighten by hand plus a quarter turn with channel locks. Over-tightening cracks the plastic housing or distorts the gasket and you get a slow leak that you will not find until it has damaged the cabinet below. From each P-trap outlet, run a two-inch waste arm sloped at a quarter inch per foot toward the main stack. Use a wye fitting to combine the first and second compartment drains, then a second wye for the third. Keep the fittings long-radius where possible. Standard wyes create more turbulence and slower drainage than sweep configurations. The difference is small on a light run but it adds up when all three basins are draining simultaneously. The vent connection comes off the waste arm between the wyes, positioned within five feet of the first P-trap weir. From there, the vent goes up through the ceiling or wall, turns ninety degrees with two forty-fives instead of a single true elbow to maintain airflow, and ties into the vent stack. That ninety-degree turn detail matters more than people realize. A true elbow can create a partial blockage for air movement during a drain event. Two sweep elbows keep the path open.

For the supply side, run your hot and cold mains along the perimeter wall. Branch off with tee fittings at each faucet location. Install individual shut-offs inside the cabinet so you can isolate one compartment without killing the whole system. If you are working in a tight cabinet space, use compression fittings instead of solder or solvent. They take longer to perfect but they save you from dealing with a torch in a confined area and they make future repairs much easier.

Common Failures and What to Check Before You Close the Walls

I want to be direct about the problems that actually come up, not the theoretical ones from the code book. Trap siphoning under load: This is the number one issue. When the third compartment drains rapidly while the first two are still running, negative pressure can pull the seal out of the first two traps if the venting is marginal. Check that every trap has an unobstructed vent path before you seal anything up. Pour three gallons of water into each basin simultaneously during your test. If any trap gurgles or loses its seal, your venting is inadequate. Grease buildup in the waste arm: The wash compartment handles soiled dishes and cooking water. Even with a good strainer, fine grease particles get through and deposit on the pipe walls. Over time this reduces effective diameter. I recommend cleaning the waste arm with a drain snake every six months in a commercial setting. It takes fifteen minutes and prevents a two-hour emergency call later.

Inconsistent water temperature across compartments: If the hot water branch to the wash basin is significantly longer than the branches to the other two, you will lose heat before the water reaches the faucet. This matters for sanitization. The rinse and sanitize cycles depend on specific temperatures, and if your hot water arrives lukewarm you are not meeting health code requirements. Keep branch runs as equal in length as practical, or install a recirculation loop if you are doing a high-volume installation. Incorrect P-trap depth: Commercial sinks sit higher than residential ones. Make sure your P-trap depth matches the drain height of the basin. A standard residential P-trap will not reach the drain outlet on a commercial three-compartment sink without extension pieces, and extensions introduce more joints where leaks can develop. Measure first. Buy the right trap assembly.

Practical Diagram Download and Reference Notes

There is no single universal 3 Compartment Sink Plumbing Diagram because pipe sizes, vent configurations, and local code amendments vary by jurisdiction. What you should do is start with the model code diagram from your local plumbing code authority, then adjust for your specific layout. The International Plumbing Code and the Uniform Plumbing Code both have standard three-compartment sink configurations in their fixtures chapters. Cross-reference both if you are unsure which applies in your area. For a downloadable reference, I keep a simplified schematic on hand that shows the standard wye-to-wye waste configuration with vent placement marked at the five-foot maximum distance. It is not official code, but it is accurate for most residential light-commercial conversions and it catches the common errors I described above. Search for IPC or UPC three-compartment sink details in the official code manuals, and keep a printed copy on site during installation. Diagrams on a phone screen are fine for reference, but they are useless when you are three feet up in a cabinet trying to figure out which fitting to use.

When This Setup Will Not Work

Three-compartment sinks assume you have adequate water supply pressure and a properly sized vent stack. If you are working in a building with already marginal water pressure, adding a third compartment sink will compound the problem. The rinse and sanitize compartments will suffer first because they rely on consistent flow rates. In those cases, a two-compartment sink with a separate handwash station may be the more practical choice. It is not about code compliance. It is about whether the system can actually deliver the water volume and temperature that the sanitization cycle requires. Similarly, if your building's vent stack is already at capacity with other fixtures, adding another vent takeoff for the sink may require a complete vent stack upgrade. This is not something you can solve with a branch vent and a fitting. You have to check the existing vent diameter and the fixture unit count already on the stack. A quick way to estimate is to add up the fixture units: a three-compartment sink counts as fifteen fixture units for drainage and ten for venting. If the stack is already near its limit, you will need to upgrade the vent stack diameter or run a new vent directly to the roof. Skipping this step means the traps will fail intermittently, usually at the worst possible time. The bottom line is that the plumbing diagram is only as good as the conditions it connects to. Get the diagram right and then verify the conditions match. If they do not, adjust the design before you buy any pipe.