Dishwasher Plumbing: How to Hook Up the Water Supply and Drain
Dishwasher plumbing comes down to three connections plus backflow protection: a hot water supply line under the sink, a drain hose tied into the garbage disposal or a sink tailpiece, and either a high loop or an air gap that keeps dirty water from siphoning back. Get those right, remove the disposal knockout plug, and the machine fills, washes, and drains cleanly.
What does plumbing a dishwasher involve?
Plumbing a dishwasher means making a water connection, a drain connection, and a power connection, then adding code-required backflow protection. The supply feeds the fill valve, the drain hose carries wash water out, and the high loop or air gap stops waste from flowing back in. Most under-counter models use the same fittings regardless of brand.
Four things have to be in place before the machine slides into the cabinet:
- Water supply: a 3/8 inch line off the hot side, controlled by a shutoff valve you can reach.
- Drain: a hose to the disposal inlet or a dishwasher tailpiece at the sink drain.
- Backflow protection: a high loop or a countertop air gap, set by local code.
- Power: a dedicated 15 or 20 amp circuit, hardwired or on a receptacle.
On a new kitchen where no dishwasher line or drain branch exists yet, the branch and circuit get run during kitchen plumbing rough-in, before cabinets and countertop go in.
How do you connect the dishwasher water supply?
Connect the dishwasher to the hot water supply under the sink using a 3/8 inch braided line and a brass dishwasher 90 elbow. The machine heats incoming water to about 120 degrees during the cycle, but a hot feed cleans better and runs faster. Feed it from a dedicated shutoff or a dual-outlet angle stop so you can isolate the appliance later.
- Shut off the hot angle stop under the sink, or the main if there is no stop yet.
- Wrap the threads of the brass dishwasher 90 elbow with PTFE thread tape (clockwise), then thread it into the water inlet at the base of the dishwasher and point the 3/8 inch outlet toward the rear.
- Connect one end of a 3/8 inch braided stainless supply line to the elbow. This is a compression fitting, so use no tape here. Hand-tighten, then add a quarter turn with a wrench.
- Route the other end to the hot shutoff valve under the sink and connect it the same way, hand-tight plus a quarter turn.
- Do not overtighten. Crushed washers and crimped braided lines are a common source of slow drips.
How do you hook up the dishwasher drain?
Route the drain hose from the dishwasher to the garbage disposal drain nipple or to a sink tailpiece with a threaded side inlet. Keep the run free of kinks and tight bends, add the code-required high loop or air gap, and clamp every slip-on connection. The hose ships attached at the pump, so most of the work is at the sink end.
- Feed the drain hose through the cabinet side panel or the gap behind the base cabinet toward the sink.
- Slide the hose onto the disposal drain nipple, or onto the side inlet of a dishwasher branch tailpiece if there is no disposal, then secure it with a hose clamp.
- Form the high loop, or connect the hose to the inlet leg of a countertop air gap and run a second hose from the air gap down to the disposal or tailpiece.
- Confirm the hose has continuous downward slope after its high point so water drains by gravity.
The tailpiece and trap that receive this hose are part of how the under-sink assembly fits together. If you are dropping in a new basin at the same time, our guide to installing a kitchen sink covers the drain and supply hookups that share this space.
High loop vs air gap: what does your code require?
Both a high loop and an air gap prevent sink or sewer water from siphoning back into the dishwasher, but they work differently and code treats them differently. A high loop uses elevation. An air gap uses an open break in the line. The table shows how each performs and where each is accepted, because the wrong choice can fail inspection.
| Method | How it works | Where required by code | Pros | Cons |
|---|---|---|---|---|
| High loop | Drain hose is fastened as high as possible under the counter (often 32+ inches above the floor) so water must climb before it can flow back. | Accepted in many areas on the International Plumbing Code (IPC) as the minimum backflow method. | Free, hidden in the cabinet, no extra parts, nothing to clog. | Not a true air break; a strap can sag; not accepted everywhere. |
| Air gap | A countertop fixture creates a physical open gap between the dishwasher hose and the drain hose, so backflow is impossible. | Required on the Uniform Plumbing Code (UPC): California, Washington, Minnesota, Hawaii, and others; a high loop alone is not compliant. | Positive, code-proof backflow prevention; visible sign of a backup. | Needs a hole in the counter or sink deck; can spit water at the cap if the line downstream clogs. |
Requirements vary by state and city, and inspectors on the UPC will fail a high-loop-only install. Confirm with your local building department before you close up the cabinet.
Knock out the disposal plug or the dishwasher will not drain
If the drain ties into a garbage disposal, the single most common new-install failure is the knockout plug. Every disposal ships with a plastic plug sealing the dishwasher inlet, meant for homes with no dishwasher. Knock it out from inside the disposal and fish out the disc before you attach the hose, or wash water has nowhere to go and pools in the tub.
Tap the plug loose with a screwdriver and mallet through the inlet, then remove the freed disc so it cannot lodge in the disposal. If the disposal itself hums or stays dead after that, that is a separate issue covered in garbage disposal not working.
How do you connect the power?
A built-in dishwasher needs its own 15 or 20 amp circuit, either hardwired into a junction box on the machine or plugged into a receptacle in the adjacent cabinet. Never share the circuit with the disposal unless local code and the wiring allow it. Wire colors match: black to black, white to white, ground to the green screw.
Where the appliance is hardwired, install a strain-relief connector at the junction box and cap each pair with a wire nut. Where it plugs in, the outlet is usually in the sink base or the next cabinet, not directly behind the dishwasher. Power and water requirements vary by code, so check what your jurisdiction allows.
How do you test the hookup for leaks?
Before pushing the dishwasher fully under the counter, open the water, restore power, and run a short cycle while you watch every joint. Testing early means you can still reach the supply elbow and drain clamp if something weeps. Check the supply, the drain, and the pump housing at the same time.
- Open the hot shutoff valve slowly and watch the supply elbow and both compression nuts for drips.
- Start a cycle and let the dishwasher fill, then pause it and look for water at the fill valve and the tub floor.
- Let it drain and confirm water moves through the hose, over the high loop or air gap, and into the disposal or tailpiece with no backup into the sink.
- Run one full empty cycle, then re-check every connection before you screw the dishwasher brackets to the underside of the counter.
Frequently asked questions
Does a dishwasher connect to hot or cold water?
In most US homes a dishwasher connects to the hot water supply under the sink, not cold. The machine still heats incoming water to roughly 120 degrees during the cycle, but a hot feed shortens run time and improves cleaning. Tap the hot angle stop or a dual-outlet stop valve so you can shut the dishwasher off on its own.
Can a dishwasher drain into the sink tailpiece?
Yes. If there is no garbage disposal, the drain hose connects to a dishwasher tailpiece, a sink tailpiece with a threaded side inlet installed between the strainer and the P-trap. The hose still needs backflow protection, either a high loop fastened under the counter or a countertop air gap, depending on your local code.
Why is my new dishwasher not draining?
The most common cause after a new disposal install is the knockout plug. Disposals ship with a plastic plug sealing the dishwasher inlet, and it must be knocked out before the hose is attached, or wash water has nowhere to go. Check that first. A kinked hose or a missing high loop or air gap can also block drainage.
How high should the dishwasher drain loop be?
Fasten the high loop as high as possible under the countertop, generally at least 32 inches above the floor and above the sink flood rim. Secure it there with a bracket or strap so it cannot sag. The loop uses elevation to stop sink water and sewer gas from siphoning back into the dishwasher between cycles.
Do I need an air gap for my dishwasher?
It depends on local code. States and cities on the Uniform Plumbing Code, including California, Washington, Minnesota, and Hawaii, require a countertop air gap and do not accept a high loop alone. Many areas on the International Plumbing Code allow a properly fastened high loop instead. Confirm with your local building department before you finalize the drain.
Can you install a dishwasher without a garbage disposal?
Yes. Without a disposal the drain hose ties into a dishwasher branch tailpiece at the sink drain instead of the disposal inlet. Everything else stays the same: hot water supply, backflow protection through a high loop or air gap, and a dedicated circuit for power. The branch tailpiece slips in above the P-trap during the sink hookup.
Last reviewed: September 2026.