To wire a 120V AC relay switch circuit using a standard 8-pin DPDT relay (like the industry-standard Omron LY2N-AC120 or MY2N), you route your 120V control switch to the coil pins (2 and 7), and pass your high-current load through the common (4, 5) and normally open (3, 6) contacts. This setup allows a low-current toggle switch, thermostat, or PLC output to safely command a high-amperage 120V or 240V load without running heavy-gauge wire through your control panel.
Below is a complete textual trace, physical terminal mapping, and meter-verification sequence for building this circuit in a residential subpanel or industrial control enclosure. All wire sizing assumes copper conductors, 60°C ampacity ratings, and standard NEC-style color codes.
Tracing the 120V AC Relay Switch Circuit Node-by-Node
Before stripping any wire, you must understand the schematic. A standard relay control diagram splits into two isolated circuits: the control (coil) side and the load (contact) side.
Understanding the Symbols: On your schematic, the relay coil is drawn as a rectangle or circle with two connection lines, often labeled "CR" (Control Relay) followed by a number. The contacts are drawn as lines with a hinge point; a line parallel to the contact indicates a Normally Open (NO) state, while a line crossing the contact indicates Normally Closed (NC).
Node-by-Node Trace (Control Circuit):
- Source (L1): 120V AC Hot leaves a 15A single-pole breaker (14 AWG black THHN).
- Node A (Switch): Hot enters your control switch (toggle, pushbutton, or smart relay dry contact).
- Node B (Coil Input): Switched hot exits the switch and lands on Relay Pin 2.
- Node C (Coil Output): Current travels through the electromagnetic coil and exits at Relay Pin 7.
- Return (L2): Pin 7 connects to the 120V AC Neutral bus (14 AWG white THHN).
Node-by-Node Trace (Load Circuit):
- Source (L1): 120V AC Hot leaves a 20A single-pole breaker (12 AWG black THHN).
- Node D (Common): Hot lands on Relay Pin 4 (Common terminal for Pole 1).
- Node E (NO Contact): When the coil energizes, the internal armature pulls the contact closed, allowing current to flow from Pin 4 to Pin 3 (Normally Open).
- Load Hot: Pin 3 connects to the Hot terminal of your load (e.g., a 120V receptacle or lighting bank).
- Load Neutral & Ground: The load neutral returns directly to the L2 bus. The equipment grounding conductor (EGC) bypasses the relay entirely, bonding the panel enclosure directly to the load chassis.
Because this is an AC coil, polarity does not matter—swapping Pins 2 and 7 will not affect operation. However, if you adapt this diagram for a DC coil, Pin 2 must be positive and Pin 7 negative to prevent reverse-EMF damage to switching transistors. Furthermore, the EGC (bare/green) must never be switched by the relay contacts; it requires a continuous, unbroken path per NEC Article 250.
Terminal and Pin Mapping (Omron LY2N-AC120 & PYF08A Socket)
Reading a wiring diagram relay switch schematic is only half the battle; you must translate those logical nodes to the physical 8-pin octal base. The mapping below applies to standard 8-pin DPDT relays and their corresponding DIN-rail sockets (like the PYF08A).
| Relay Pin | Socket Screw | Function | NEC Wire Color (120V) | Physical Location on Base |
|---|---|---|---|---|
| 2 | 7 | Coil Input (A1) | Black (Switched Hot) | Top Right |
| 7 | 2 | Coil Output (A2) | White (Neutral) | Bottom Left |
| 4 | 5 | Common 1 (COM) | Black (Load Hot In) | Center Right |
| 3 | 6 | Normally Open 1 (NO) | Black/Red Stripe (Load Hot Out) | Top Center |
| 1 | 8 | Normally Closed 1 (NC) | Black/Blue Stripe (Alt Load) | Bottom Right |
| 5 | 4 | Common 2 (COM) | Red (2nd Pole In) | Center Left |
| 6 | 3 | Normally Open 2 (NO) | Red/Black Stripe (2nd Pole Out) | Bottom Center |
| 8 | 1 | Normally Closed 2 (NC) | Red/Blue Stripe (2nd Alt Load) | Top Left |
Note: Socket terminal numbering is often reversed or mirrored compared to the relay pin numbering. Always wire to the socket screw numbers, not the relay base pins, when the relay is unplugged.
Step-by-Step Wiring and Meter Verification
Working inside a panel requires strict adherence to safety protocols. De-energize the main breaker, apply a lockout/tagout device, and verify the bus bars are dead using a non-contact voltage tester and a CAT III multimeter before touching any conductors.
- Mount the Socket: Snap the PYF08A socket onto the 35mm DIN rail. Ensure the retaining clip clicks securely.
- Wire the Coil: Terminate a 14 AWG black pigtail from your control switch to Socket Terminal 7 (Pin 2). Terminate a 14 AWG white pigtail from the neutral bus to Socket Terminal 2 (Pin 7). Torque the screw terminals to 0.5 Nm (4.4 in-lbs).
- Wire the Load Contacts: Land your 12 AWG load hot wire on Socket Terminal 5 (Pin 4, Common). Connect the wire running to your load on Socket Terminal 6 (Pin 3, NO).
- Bond the Ground: Run a continuous 12 AWG bare copper or green THHN from the panel ground bus directly to your load enclosure. Do not pass this through the relay.
- Verify Coil Integrity (De-energized): Set your multimeter to Ohms (Ω). Place probes across Socket Terminals 2 and 7. For a 120VAC Omron LY2N coil, you should read a DC resistance between 3,600 Ω and 4,400 Ω. If you read OL (open) or near 0 Ω (short), the coil is defective.
- Verify Contact Mechanics (De-energized): Switch your meter to Continuity (beep mode). Place probes on Terminals 5 (COM) and 8 (NC). It should beep. Place probes on Terminals 5 (COM) and 6 (NO). It should remain silent. Manually press the relay's test button; the NO circuit should now beep, and the NC circuit should open.
- Live Voltage Test: Remove locks, re-energize the panel, and turn on the control switch. Measure across the load wires at the device. You should read 120V AC (±5%). If you read 0V, check for a tripped breaker or a loose terminal screw.
For advanced diagnostics, such as checking for voltage drop across closed contacts under load, refer to Fluke's relay testing guidelines. A healthy closed contact should show less than 50 millivolts of drop at rated current.
Frequently Asked Questions
What does a wiring diagram relay switch symbol look like?
In North American (NEMA) schematics, a relay coil is typically drawn as a circle with two wires, labeled with a device tag like "1CR". The associated contacts are drawn as two parallel lines with a diagonal line indicating the movable armature. In European (IEC) diagrams, the coil is a rectangle labeled "A1" and "A2", and the contacts are drawn as a hinge mechanism with a slash for NO and an X for NC. Always check the legend on your specific schematic to confirm the standard being used.
Can I use a DC relay switch wiring diagram for an AC coil?
While the logical wiring path (switch to coil, power to common) remains identical, the physical components are not interchangeable. An AC relay coil relies on a copper "shading ring" on the armature to prevent the magnetic field from collapsing to zero 120 times a second (at 60Hz). If you wire a DC coil into an AC circuit, the coil will rapidly overheat and burn out due to eddy currents and lack of inductive impedance. Conversely, an AC coil wired to DC will draw excessive current and fail because it lacks the AC inductive reactance that normally limits current flow.
How do I wire a relay switch to toggle between two different loads?
Use the SPDT (Single Pole, Double Throw) nature of the relay. Wire your main power source to the Common pin (Pin 4). Wire Load A to the Normally Closed pin (Pin 1) and Load B to the Normally Open pin (Pin 3). When the relay is at rest, Load A is powered. When the control switch energizes the coil, the internal armature snaps over, cutting power to Load A and instantly routing power to Load B. This "break-before-make" action ensures the two loads are never powered simultaneously, which is critical for applications like reversing motor directions or switching between utility and inverter power.
