When wiring two lights and a switch, the direct answer depends entirely on your load profile and control method. For standard residential LED or incandescent fixtures drawing under 15A, a standard 15A single-pole electromechanical toggle (like the Leviton 5601) is the correct choice. However, if you are driving high-wattage commercial bays, magnetic low-voltage transformers, or integrating smart home automation, you must use a relay-driven switch (like the Aube RC840T or a DIN-rail contactor). In a relay setup, a low-current pilot signal triggers a high-current electromechanical contactor, separating the control circuit from the load circuit.
Electromechanical Ratings: Which Column Governs Your Load?
The most common mistake when wiring two lights and a switch is looking only at the "Amps" printed on the switch yoke. A switch rated for 15A resistive might fail catastrophically if used to switch a 10A inductive load. Electromechanical switches and relays are rated across multiple columns based on the physics of the arc generated when the contacts open.
| Component Type (Example) | Coil Voltage | Contact Rating (Resistive) | Contact Rating (Inductive/Ballast) | Motor Breaking Capacity |
|---|---|---|---|---|
| Standard Toggle (Leviton 5601) | N/A (Mechanical) | 15A @ 120V AC | 15A @ 120V AC (AC-1) | 1/2 HP @ 120V AC |
| Smart Relay (Aube RC840T-120) | 120V AC (Line) | 20A @ 2400W | 15A (Electronic Ballast) | 1/2 HP @ 120V AC |
| DIN Contactor (Schneider iCT) | 24V AC/DC | 20A (AC-1) | 20A (AC-1) / Utilization Cat. | N/A (Requires overload relay) |
Which rating column governs this load? For modern LED fixtures with internal electronic drivers, the Inductive/Ballast column governs your selection, not the pure resistive column. LED drivers present a high inrush current (often 10x to 50x the steady-state current for a few milliseconds) due to input capacitors charging. If your two lights draw a combined steady-state of 8A, but have an inrush of 60A, a standard 15A toggle switch will pit and weld its contacts shut over time. You must select a switch with a high making/breaking capacity or use a heavy-duty relay.
Selection Decision Path by Load Type
Use this decision tree to select the correct electromechanical component for your specific lighting load.
| Load Type | Characteristics | Recommended Component | Why? |
|---|---|---|---|
| Resistive (Incandescent, Halogen) | Inrush equals steady-state current. Arc is minimal. | Standard 15A/20A Toggle Switch | Contacts handle the pure thermal load without excessive pitting. |
| Inductive (Magnetic ballasts, MLV transformers) | High inrush current. Severe arcing on break (inductive kickback). | Heavy-Duty Relay or AC-3 Rated Contactor | Relays have wider contact gaps and stronger springs to snap open and extinguish arcs. |
| Electronic/LED (Modern recessed cans, strip drivers) | Extremely high capacitive inrush current (up to 100x steady state). | LED-Specific Smart Relay (e.g., Aube, Lutron) | Designed with tungsten-rated contacts or zero-cross switching to survive capacitive inrush. |
| Motor (Ceiling fans with integrated lights) | High starting torque, high inductive kickback. | Motor-Rated Switch (HP rated) | Must meet UL/CSA HP ratings to prevent contact welding during motor stall/start. |
Step-by-Step Wiring: Coil vs. Contact Side Execution
When using a standard toggle switch, wiring is straightforward: line in, load out. But when wiring two lights and a switch using a smart relay or contactor, you must separate the coil side (the control circuit) from the contact side (the load circuit).
1. The Contact Side (Load Circuit)
The contact side carries the actual current to your two lights.
- Connect your incoming hot (black) from the breaker panel to the Line (L) terminal on the relay/contactor.
- Connect the hot wires leading to your two lights to the Load (T) terminal. If wiring two lights, they are typically daisy-chained in parallel; the single switch leg feeds the first light's black wire, which then pigtails to the second light.
- Bundle all neutral (white) wires together with a wire nut, and all ground (bare/green) wires together. Standard electromechanical relays do not require a neutral for the load switching path, though smart switches with internal Wi-Fi/Zigbee radios do.
2. The Coil Side (Control Circuit)
The coil side is the electromagnet that physically pulls the contacts closed.
- For a 120V AC coil (like the Aube RC840T), wire a standard low-current pilot switch (a basic 15A toggle) to interrupt the hot wire feeding the relay's coil terminal (A1). The other coil terminal (A2) connects to neutral.
- Flyback Protection Note for DC Coils: If your smart relay or DIN-rail contactor uses a DC coil (e.g., 24V DC from a smart home hub or PLC), you must wire a flyback diode (like a 1N4007) in reverse bias across the coil terminals (A1 and A2). When the DC circuit opens, the collapsing magnetic field generates a high-voltage spike that will fry your hub's output transistor. The diode recirculates this kickback safely. AC coils rely on internal shading rings and do not use diodes.
Diagnostics: Testing, Repairing, and Replacing
Suspect a faulty switch or relay? According to Fluke's electrical testing guidelines, you should test both dead and live to isolate the failure.
How to Test It Dead (Continuity)
- Turn off the breaker and verify zero voltage.
- Disconnect the load and line wires from the switch/relay contacts.
- Set your multimeter to Ohms (Ω) or continuity mode.
- Place probes on the Line and Load terminals. Toggle the switch or energize the coil manually.
- Pass: Reading drops to < 1 ohm (closed) and OL/infinite (open).
- Fail: Reading fluctuates, stays above 5 ohms when closed (indicating carbon buildup/pitting), or shows continuity when open (welded contacts).
How to Test It Live (Voltage Drop)
- Reconnect wires, turn on the breaker, and turn the lights on.
- Set your multimeter to AC Volts.
- Place one probe on the Line terminal and the other on the Load terminal while the switch is ON.
- Pass: Voltage drop is < 0.5V. The contacts are clean.
- Fail: Voltage drop is > 2V. The contacts are degrading, generating heat, and wasting power. (See NFPA 70 NEC guidelines on terminal temperature limits).
When to Repair vs. Replace
Always replace standard wall switches, dimmers, and sealed smart relays. They are riveted or ultrasonically welded shut; attempting to file pitted contacts inside a sealed 15A switch compromises its arc-chute geometry, creating a severe fire hazard. For large, open-frame industrial contactors (like a 60A 3-phase unit), you can sometimes inspect and clean contacts, but if pitting exceeds 1mm or the contact spring tension feels weak, replacement of the contact block or the entire unit is mandatory.
Frequently Asked Questions (FAQ)
Can I use one dimmer switch for wiring two lights and a switch setup?
Yes, but you must check the dimmer's total wattage rating and compatibility. A standard 600W incandescent dimmer can handle two 100W bulbs easily. However, if you are using LED lights, you must use an "ELV" (Electronic Low Voltage) or "LED-rated" dimmer. Furthermore, two LED fixtures with cheap internal drivers on the same dimmer circuit often cause a phenomenon called "popcorning" or flickering due to phase-cut waveform interference. If flickering occurs, upgrade to a 0-10V DC dimming setup or use smart bulbs bypassing the dimmer entirely.
What wire gauge is required when wiring two lights and a switch on a 20A breaker?
If the branch circuit is protected by a 20A breaker, NEC guidelines mandate a minimum of 12 AWG copper wire (THHN in conduit or 12/2 NM-B Romex) for the entire circuit, including the switch legs feeding the two lights. You cannot use 14 AWG wire anywhere on a 20A circuit. Additionally, ensure the switch itself is rated for 20A. While a 15A duplex receptacle is legally allowed on a 20A circuit, a 15A switch controlling a 20A load is a code violation if the load exceeds 15A.
Why do my LED lights flicker when wiring two lights and a switch with a smart relay?
Flickering with smart relays or Wi-Fi switches usually stems from a missing neutral wire or "ghost voltage." Many older smart switches leak a tiny amount of current (1-5mA) through the load to power their internal radios when the light is "off." Because LED drivers require very little current to ignite, this leakage causes the bulb's internal capacitor to charge and discharge repeatedly, resulting in a strobe effect. The fix is to either install a smart switch that requires a dedicated neutral wire (which provides a complete return path, bypassing the load) or wire a bypass resistor/capacitor (often included in the box) in parallel with the LED fixture to absorb the leakage current.
