How to Plumb a Double Kitchen Sink Drain with a Garbage Disposal

Begin by ensuring a safe and clean work environment. Place a bucket and several rags under the sink to catch any residual water. Turn off the electrical power to the garbage disposal and the dishwasher at your home's main circuit breaker panel. Verify the power is off by attempting to turn on the disposal. Clear out all items from the under-sink cabinet to provide ample space to work.

If the garbage disposal is not already installed, mount it to the left-hand sink basin according to the manufacturer's instructions. This typically involves attaching a mounting flange to the sink drain opening using plumber's putty and then lifting and locking the disposal unit onto the flange. Before installing, locate the dishwasher inlet nipple on the side of the disposal. Use a screwdriver and a hammer to firmly tap the plug inside this nipple, knocking it out into the disposal's grinding chamber. Reach in and remove the plastic knockout plug before proceeding.

Attach the flanged tailpiece (a short, straight pipe) to the basket strainer on the non-disposal (right-hand) sink. Secure it with a slip nut and washer. Next, take the continuous waste tee (a T-shaped fitting with one side arm) and attach it to the bottom of the flanged tailpiece. The side arm of the tee should be pointing towards the garbage disposal (left-hand sink). Hand-tighten the slip nut for now.

Attach the 90-degree discharge tube to the garbage disposal's side outlet, securing it with the provided gasket and flange/screws. Point the outlet of this tube directly towards the waste tee on the other sink. Now, you need to connect this tube to the tee. This usually requires a straight piece of pipe. Measure the distance between the end of the discharge tube and the inlet of the waste tee. Add about 1/2 inch to each end for insertion into the fittings. Mark this measurement on a straight piece of 1-1/2 inch PVC pipe and cut it to length using a hacksaw or tubing cutter. Smooth the cut edges with a utility knife or sandpaper. Connect this cut pipe between the disposal's discharge tube and the waste tee using slip nuts and washers.

The P-trap assembly consists of two parts: the J-bend and the trap arm (a longer pipe with a bend at one end). Assemble the P-trap by attaching the J-bend to the bottom outlet of the continuous waste tee. Secure it with a slip nut and washer, but keep it loose enough to swivel. Attach the trap arm to the other end of the J-bend. The straight end of the trap arm should be pointed towards the drain pipe outlet in the wall (the stub-out).

Align the trap arm with the drain stub-out in the wall. It is very likely the trap arm will be too long. Hold it in place and mark where it needs to be cut to fit into the wall fitting. Cut the trap arm to length with your hacksaw or tubing cutter. Deburr the cut end. Slide the slip nut and washer onto the trap arm, then insert the arm into the wall fitting. Tighten all the slip nuts in the entire assembly: the ones on the tailpieces, the waste tee, the P-trap, and the trap arm connection at the wall. Hand-tighten first, then use channel-type pliers for a final quarter-turn.

Locate the dishwasher drain hose. Slide a hose clamp over the end of the hose. Push the end of the hose firmly onto the dishwasher inlet nipple on the side of the garbage disposal that you cleared earlier. Position the hose clamp about 1/4 inch from the end of the hose and tighten it securely with a screwdriver. It is highly recommended to create a 'high loop' in the drain hose by routing it up as high as possible under the sink (touching the underside of the countertop) before it connects to the disposal. This prevents dirty water from the sink from backflowing into the dishwasher.

With all connections tightened, it's time to test your work. Remove the bucket and rags. Go to the circuit breaker panel and restore power to the garbage disposal. Place the stopper in the non-disposal (right-hand) sink and fill it halfway with water. Remove the stopper and watch all the drain connections closely for any drips or leaks. Repeat the process for the disposal (left-hand) sink. While the water is running, turn on the garbage disposal to check for any leaks caused by vibration. If you find a leak, turn off the water, identify the leaking joint, and try tightening the slip nut slightly. If it still leaks, you may need to disassemble the joint and check that the washer is seated correctly. Once all is dry, run the dishwasher on a rinse cycle to check the dishwasher drain connection for leaks.