To wire a 120V relay to a standard wall switch, route the switch’s load wire to the relay’s coil terminal (A1), connect the coil neutral (A2) to the panel, feed the relay’s common terminal (COM) with line voltage, and route the normally open (NO) terminal to your load. For a standard 15A residential circuit, use 14 AWG THHN copper wire. This setup isolates your control circuit from the high-draw load, protecting smart switches and low-voltage timers from contact welding.
The Real-World Scenario: Why Use a Relay?
Homeowners and DIYers frequently run into a specific problem: they want to control a heavy 120V load—like a 9A Panasonic WhisperCeiling bathroom exhaust fan, a bank of LED grow lights, or a shop dust collector—using a smart switch or a remote timer. Most standard smart switches (like the Lutron Caseta PD-6WCL) are rated for lighting loads and can fail prematurely if subjected to the high inrush current of an electric motor.
By learning how to wire a relay to a switch, you use the wall switch only to energize a low-draw relay coil (typically under 0.1A). The relay’s heavy-duty mechanical contacts then handle the actual motor load. For this guide, we are wiring a 120V, 9A exhaust fan using a 15A single-pole toggle switch and an industry-standard Omron LY2N-AC120 relay seated in a PYF08A-E 8-pin socket.
This procedure involves 120V AC mains voltage. Before opening any junction box or panel, de-energize the circuit at the breaker. Lock out or tag the breaker if you are not the only person in the building. Verify the circuit is dead using a known-working non-contact voltage tester (NCVT) and a multimeter on the line and load wires. NEC-style guidance requires that panel work and hardwired appliance connections comply with local codes; if you are uncomfortable working inside an electrical panel, hire a licensed electrician.
Tools, Materials, and Device Ratings
Using the correct wire gauge and device ratings is critical. Per NEC 240.4(D), 14 AWG copper wire is limited to 15A overcurrent protection. Because our load is 9A and our breaker is 15A, 14 AWG THHN is the correct, code-compliant choice. The Omron LY2N-AC120 is rated for 10A at 120VAC, giving us a safe margin for the 9A motor load, though motor inrush currents should always be considered.
- Relay: Omron LY2N-AC120 (120V AC coil, 10A DPDT contacts)
- Socket: Omron PYF08A-E (8-pin, screw-terminal, DIN or panel mount)
- Wire: 14 AWG THHN stranded copper (Black, White, Red, Green)
- Switch: Standard 15A 120V single-pole toggle switch
- Connectors: Ideal wire nuts or Wago 221 lever nuts (for splices)
- Tools: Non-contact voltage tester, digital multimeter, wire strippers (set to 14 AWG), Phillips #1 screwdriver, torque screwdriver (optional but recommended)
Omron PYF08A-E Terminal Mapping (The Core Data)
The 8-pin octal socket layout confuses many beginners. The pins are numbered on the socket face. Pins 7 and 8 control the electromagnetic coil. Pins 1 and 4 are the Common (COM) inputs for the two internal switches. Pins 2 and 5 are Normally Open (NO), and pins 3 and 6 are Normally Closed (NC). For a single 120V load, we only use one pole of the relay.
| Terminal Label | Pin Number | Function | Wire Color Landing Here | Destination / Source |
|---|---|---|---|---|
| A1 | Pin 8 | Coil Hot | Black 14 AWG | From Switch Load Screw |
| A2 | Pin 7 | Coil Neutral | White 14 AWG | To Panel Neutral Bar |
| COM | Pin 1 | Line In (Load Feed) | Black 14 AWG | From 15A Breaker |
| NO | Pin 2 | Load Out | Red 14 AWG | To Exhaust Fan Hot |
| NC | Pin 3 | Unused | None | Leave empty |
Note: The Omron PYF08A-E socket screws require a tightening torque of 0.5 N·m (approx. 4.4 in-lbs). Stripped socket screws are a primary cause of high-resistance connections and melted relay bases.
Step-by-Step Wiring Procedure
Ensure your 4x4x2 inch metal junction box is securely mounted and the 14 AWG THHN wires are pulled through the conduit with at least 6 inches of slack. Strip exactly 3/8 inch (10mm) of insulation from the ends of all THHN wires. If using stranded wire, twist the strands tightly or apply a pin-type ferrule to prevent stray strands from shorting under the socket screws.
- Wire the Switch to the Relay Coil (A1): Take the Black 14 AWG wire originating from the wall switch's load (brass) screw. Land this black wire under the A1 (Pin 8) screw terminal on the relay socket. Tighten to 0.5 N·m. This wire carries 120V only when the wall switch is flipped ON.
- Wire the Relay Coil Neutral (A2): Take a White 14 AWG wire and land one end under the A2 (Pin 7) screw terminal. Route the other end of this white wire back to the panel and terminate it under the neutral bar screw. This completes the 120V control circuit for the coil.
- Feed the Relay Common (COM): Take a Black 14 AWG wire originating directly from the 15A breaker in the panel (line voltage). Land this black wire under the COM (Pin 1) screw terminal. This provides the constant 120V feed that will eventually power the fan.
- Wire the Relay Load Out (NO): Take a Red 14 AWG wire and land it under the NO (Pin 2) screw terminal. Route the other end of this red wire to the exhaust fan and splice it to the fan's black (hot) wire using a Wago 221 lever nut. We use red here to visually distinguish the switched load hot from the constant line hot.
- Complete the Load Neutral: Take a White 14 AWG wire from the panel neutral bar and splice it directly to the fan's white (neutral) wire using a wire nut. The relay only switches the hot leg; the neutral must be continuous.
- Bond the Grounds: Splice all Green 14 AWG (or bare copper) equipment grounding conductors together, including the panel ground, the fan ground, and a Green 14 AWG pigtail landing on the green grounding screw inside the metal junction box.
- Seat the Relay: Align the 8 pins of the Omron LY2N-AC120 relay with the socket holes. Push down firmly until the locking clip snaps over the top of the relay casing.
Verify and Test: Expected Meter Readings
Never assume a circuit is functional just because the wire colors match the diagram. Before restoring power, perform a visual inspection to ensure no stray wire strands are bridging terminals. Once cleared, turn the 15A breaker ON, but leave the wall switch OFF.
Set your digital multimeter to AC Voltage (V~). Place the black probe on the bare ground wire and the red probe on the Black wire at Pin 1 (COM). You should read 120V (±5V). Next, probe the Red wire at Pin 2 (NO). With the switch OFF, you must read 0V.
Now, flip the wall switch ON. You should hear a distinct mechanical click from the relay. Probe the Red wire at Pin 2 (NO) again; your meter should now read 120V (±5V). Finally, verify the coil voltage by probing across A1 (Pin 8) and A2 (Pin 7) with the switch ON. You should read 120V. If you read 120V at the coil but the relay does not click, the coil is internally open, or the relay is not fully seated in the socket.
The Most Common Botch (And How to Fix It)
The most frequent error when figuring out how to wire a relay to a switch is landing the load hot wire on the Normally Closed (NC) terminal (Pin 3) instead of the Normally Open (NO) terminal (Pin 2).
The Symptom: The exhaust fan runs continuously as soon as the breaker is turned on. When you flip the wall switch to the ON position, the fan turns OFF. When you turn the switch OFF, the fan turns back ON. This happens because the NC contact passes current when the coil is de-energized (switch OFF), and breaks the circuit when the coil pulls the armature in (switch ON).
The Fix: De-energize the breaker. Verify dead. Move the Red 14 AWG load wire from Pin 3 to Pin 2. A secondary botch involves mixing up the coil neutral (White on A2) with the load neutral. While both ultimately land on the panel neutral bar, keeping them as distinct, separate wires prevents confusion during future troubleshooting and ensures the coil circuit remains isolated from load-side voltage drops.
For deeper reading on electromechanical relay theory and contact ratings, refer to the All About Circuits textbook chapter on relays. Always consult Mike Holt Enterprises or your local Authority Having Jurisdiction (AHJ) for specific NEC code interpretations regarding junction box fill limits and conductor derating in your region.
