How to Install a Supply Box: Complete Step-by-Step Installation Guide

Step 1. Choose the Right Location for Your Supply Box

The location of the supply box affects accessibility, pipe routing, and appliance placement. Select a spot that is easily accessible and positioned close to the fixture or appliance.

Best practices for supply box placement:

• Position the box between wall studs for secure mounting
• Ensure the location allows easy access for future maintenance
• Check the fixture manufacturer's specifications for recommended placement
• Verify the location does not interfere with electrical wiring or other plumbing
• Consider the appliance's water supply and drain hose lengths

Step 2. Measure and Mark the Height

Proper height placement helps avoid installation issues and supports proper appliance connections. Use a tape measure and pencil to mark the ideal height on the wall stud.

For most washing machine applications, supply boxes are installed 36-42 inches from the finished floor to the bottom of the box.

Always verify the appliance manufacturer's recommended height specifications before marking your installation point.

For additional washing-machine-specific selection considerations, see How to Choose a Washing Machine Outlet Box.

Step 3. Prepare the Supply Box for Installation

Preparation varies depending on whether you're installing a fire-rated MODA box or a standard supply box.

For fire-rated MODA boxes:

• Slide the large metal bracket onto the side of the box that will attach to the stud
• Insert the L-Bracket through the slots at the top of the box
• Position the L-Bracket at an upward-facing right angle for proper support

For non-fire-rated MODA boxes:

• Install plastic mounting brackets on the side of the box
• Ensure brackets are firmly attached and properly aligned

Pro Tip: If you're installing multiple MODA boxes side-by-side, they can be clipped together for a unified installation.

For more on fire-rated supply boxes, see Are Metal Supply Boxes Fire-Rated? What Plumbing Pros Need to Know. For more on MODA configurations, see Oatey 101: MODA Plumbing Supply Boxes and Modular Outlet Solutions.

Step 4. Align and Secure the Box to the Wall Stud

Proper mounting helps keep the supply box stable under the stress of connecting and disconnecting appliances.

Installation steps:

1. Carefully align the supply box with the marked measurement on the stud
2. Hold the box level against the stud
3. Use screws to firmly secure the box through the mounting brackets
4. Drive screws into the center of the stud for maximum holding power

Critical checkpoint: Thoroughly check the tightness of all screws and confirm there's no looseness in any part of the supply box. A loose supply box can place unnecessary stress on piping, valves, and connections.

Step 5. Connect the Drainpipe to the Supply Box

Note: Skip this step if you're installing a gas-only or valve-only MODA box.

For standard and fire-rated MODA boxes with drain connections, proper solvent welding creates a watertight seal.

Drainpipe connection process:

• Apply Oatey Purple Primer to both the box hub and the drainpipe end
• Apply Oatey Heavy Duty Clear PVC Cement to both primed surfaces
• Immediately insert the drainpipe into the box hub
• Hold firmly for 30 seconds to allow initial bonding
• Allow the solvent-welded connection to set and cure according to the solvent cement product instructions and application conditions

For complete solvent-welding guidance, see How to Solvent Weld with Oatey Products.

Step 6. Install the Drain Trap Assembly

Note: Skip this step for gas-only, valve-only, or other supply boxes that do not require a drain connection.

A drain trap helps prevent sewer gases and odors from entering the space. For washing machine installations, the trap is incorporated into the drain and standpipe rough-in. It is different from the tubular sink-style P-trap kits used beneath sinks.

Drain trap installation:

1. Confirm the trap assembly is properly positioned within the drain layout
2. Dry-fit the drainpipe, fittings, and trap assembly to confirm alignment
3. Solvent-weld the applicable PVC or ABS drain components using the appropriate primer and cement for the piping material and application
4. Maintain the required drain orientation and slope when tying into the drainage system

For detailed solvent-welding instructions, see How to Solvent Weld with Oatey Products.

Image/alt text note: Use wording such as “Washing machine standpipe and drain trap assembly installed inside wall framing.” Do not identify the component as an Oatey sink P-trap kit.

Step 7. Secure the Water or Gas Lines to the Supply Box

Secure connections help support safe, reliable appliance operation.

For water supply lines:

Connect the water lines according to the valve and piping system being used.

1. PEX: Crimp or clamp fittings.
2. Cold Expansion PEX: Use expansion fittings with an expansion tool; place the expansion ring/sleeve on the pipe, expand the tubing, insert the fitting, and allow it to contract for a secure seal.
3. CPVC: Solvent-weld with primer and cement.
4. Copper: Clean, flux, and solder.
5. Push-fit: Push onto properly prepared and deburred pipe.

Oatey Tip: Oatey supply boxes and valves are available in multiple connection types, such as PEX, CPVC, sweat, and push-fit. Select the valve style that matches your piping system for a seamless install.

For gas supply lines:

1. Use approved gas-rated flexible connectors or black iron pipe
2. Apply pipe thread sealant rated for gas applications
3. Tighten connections securely with appropriate wrenches
4. Follow all local codes and safety regulations for gas installations

Step 8. Test the Supply Box for Leaks

Test all applicable connections for leaks before closing the wall.

For water supply boxes:

1. Turn on the main water supply
2. Open the supply box valves fully
3. Inspect all connections for signs of leakage
4. Check for drips, moisture, or water accumulation
5. Tighten any connections showing leaks
6. Re-test after tightening

For gas supply boxes:

1. Turn on the gas supply
2. Apply Hercules MEGA BUBBLE leak detector to all connections
3. Watch for bubble formation, which indicates gas leaks
4. If bubbles appear, turn off gas immediately and tighten the connection
5. Re-test until no bubbles form
6. Ensure proper ventilation during testing

Safety Warning: Never use open flames to test for gas leaks. Always use approved leak-detection solutions.

Step 9. Connect the Appliance or Fixture to the Supply Box

After the supply box installation is complete, connect the appliance or fixture to the appropriate water, drain, or gas connection.

For washing machines and water-supplied appliances:

1. Connect the hot water supply hose to the red-marked valve
2. Connect the cold water supply hose to the blue-marked valve
3. Hand-tighten connections, then use pliers for an additional quarter-turn
4. Insert the drain hose into the drain opening of the supply box
5. Ensure the drain hose is secured but not creating an airtight seal
6. Run a test cycle and carefully check for leaks at all connection points

For gas appliances (dryers, etc.):

1. Connect the gas supply line from the appliance to the supply box valve
2. Ensure all connections are securely tightened with appropriate wrenches
3. Test the gas appliance for proper functionality
4. Use Hercules MEGA BUBBLE to check for leaks at the appliance connection
5. Follow all manufacturer safety precautions and local regulations