Wiring a three way switch with two lights in a standard residential setting typically involves running 14/3 or 12/3 NM-B cable between two mechanical Single-Pole Double-Throw (SPDT) switches. However, if those two lights are high-inrush commercial LEDs, magnetic ballasts, or draw near the 15A/20A circuit limit, mechanical switches will arc, pit, and weld shut prematurely. The professional, code-compliant solution is to wire your 3-way switches to the coil of an electromechanical lighting relay or latching contactor, allowing the relay's heavy-duty contacts to switch the actual light load. This guide breaks down the electromechanical component selection, wiring topology, and testing procedures required to do this correctly.
The Decision Path: Standard Switch vs. Electromechanical Relay
Before pulling wire, you must evaluate the load type. The National Electrical Code (NEC) Article 404 governs snap switches, but mechanical switches are rated primarily for resistive loads. Modern lighting is rarely purely resistive. Use the decision tree below to determine if a standard mechanical 3-way switch is sufficient or if you must step up to an electromechanical relay.
| Load Type | Characteristics | Component Selection |
|---|---|---|
| Resistive (Incandescent, Halogen, Space Heaters) | Inrush current equals steady-state current. No phase shift. | Standard 15A/20A mechanical 3-way switch is acceptable. |
| Inductive / LED (Commercial LED drivers, HID, Fluorescent) | Inrush current can be 10x to 100x the steady-state draw for the first half-cycle due to capacitor charging in LED drivers. | Electromechanical Relay/Contactor. Mechanical switches will suffer contact welding and early failure. |
| Motor (Integrated exhaust fans, motorized dampers in lighting rigs) | Locked Rotor Amperage (LRA) is 5x to 7x Full Load Amps (FLA). High inductive kickback on switch-off. | Motor-Rated Contactor. Must have a specific HP rating at the circuit voltage. |
A 100W LED high-bay light draws less than 1A at 120V steady-state. You might assume a 15A mechanical switch is plenty. But if the driver has poor inrush limiting, that 1A light can pull 80A for 2 milliseconds when switched on. Doing this twice a day from two locations will destroy a standard snap switch in under a year. Always use a relay with a specific 'Ballast' or 'LED' ampacity rating for commercial fixtures.
Coil vs. Contact Side Wiring Explained
When wiring a three way switch with two lights via a relay, you are essentially building two separate circuits: the low-energy control circuit (coil side) and the high-energy load circuit (contact side).
The Coil Side (Control Circuit)
The coil is an electromagnet. When energized, it pulls the mechanical contacts closed. For multi-location control, you can use a latching relay (such as the Functional Devices RL400 series). In this setup, your two standard 3-way switches are wired in series/parallel configurations to send a brief pulse to the coil, toggling the relay's state. Alternatively, you can replace the 3-way toggles with momentary push-button switches wired in parallel to a standard (non-latching) relay coil. The coil side typically operates at 24V AC or 24V DC, keeping low voltage in the switch boxes.
If your control circuit uses a 24V DC power supply (common in smart home and PLC integrations), you must wire a flyback diode (reverse-biased) directly across the relay coil terminals. When the DC circuit opens, the collapsing magnetic field generates a massive reverse voltage spike (inductive kickback). Without a flyback diode (e.g., 1N4007), this spike will arc across your switch contacts and fry your DC power supply or smart home controller. AC coils do not require this, as the AC sine wave naturally crosses zero, extinguishing the arc.
The Contact Side (Load Circuit)
The contacts are the heavy-duty metal pads that physically pass the current to your two lights. This side operates at your line voltage (120V/277V AC). The line hot enters the contactor's Line terminal, and the load hot (feeding the two lights) leaves the Load terminal. The neutrals and grounds are spliced in the enclosure, bypassing the relay entirely.
A Note on Overcurrent Protection: On the contact side, you protect the branch circuit wiring with a thermal-magnetic circuit breaker (e.g., 20A). On the coil side, control transformers are often protected by a glass fuse. Do not treat fuses and breakers as interchangeable without understanding their time-current curves. A standard breaker has an inverse-time thermal curve for moderate overloads and an instantaneous magnetic trip for dead shorts. A fast-acting fuse clears high short-circuit currents much faster (lower let-through I²t energy) but lacks a magnetic trip for moderate overloads. Never swap a specified control fuse for a breaker without verifying the let-through energy rating of the downstream coil and wiring.
Reading the Rating Table: Which Column Governs Your Load?
Electromechanical relays have multiple rating columns on their data sheets. Misreading these is the most common cause of field failures. Below is a standard rating table for a 30A enclosed lighting contactor.
| Specification | Rating Value | What It Governs |
|---|---|---|
| Coil Voltage | 24V AC/DC, 50/60Hz | The control circuit voltage required to pull the contacts in. Exceeding this burns the coil; under-voltage causes chattering. |
| Contact Rating (Resistive) | 30A at 120/277V AC | Maximum steady-state current for purely resistive loads (heaters, incandescent). |
| Contact Rating (Ballast/LED) | 15A at 120/277V AC | Maximum steady-state current for inductive/capacitive loads with high inrush. |
| Breaking Capacity (kAIC) | 10 kA at 277V | The maximum fault current the relay can safely interrupt or withstand without exploding before the upstream breaker trips. |
Which rating column governs this load?
For lighting circuits, the Contact Rating (Ballast/LED) column governs your load, not the Resistive column. If your two lights draw a combined 18A of steady-state LED current, a 30A resistive-rated relay will still fail because it is only rated for 15A of ballast/LED load. The Breaking Capacity (kAIC) governs fault survival; ensure the relay's kAIC rating meets or exceeds the available fault current at the panel (typically 10kA for residential, up to 65kA for commercial services).
Testing, Troubleshooting, and Replacement
When a 3-way relay circuit fails, you must isolate whether the fault is on the coil side (switches/control wire) or the contact side (line voltage/contacts).
How to Test It Dead (Power Off)
Safety: De-energize the breaker, lock/tag out, and verify dead with a tested non-contact voltage tester and multimeter.
- Test the Coil: Set your multimeter to Ohms (Ω). Place probes across the coil terminals (usually labeled A1 and A2). A healthy 24V AC coil will read between 10Ω and 50Ω. If it reads OL (open), the internal coil wire is broken. If it reads 0.0Ω, it is shorted.
- Test the Contacts: Set the meter to continuity or low Ohms. Place probes across the Line and Load terminals. With the relay de-energized, it should read OL. Manually press the contactor's mechanical plunger with an insulated tool; the meter should read < 0.5Ω. If it reads higher, the contacts are pitted with carbon buildup.
How to Test It Live (Power On)
Safety: Mains voltage is present. Use properly rated CAT III/IV test leads and keep fingers clear of terminals.
- Verify Coil Voltage: Set the meter to AC Volts. Have a helper toggle the 3-way switches. Measure across A1 and A2. You should read your nominal control voltage (e.g., 24V AC). If you read 0V, the fault is in your 3-way switch wiring or control transformer.
- Measure Contact Voltage Drop: With the relay energized and the lights on, measure the AC voltage across the Line and Load terminals (probe on Line, probe on Load). A perfect closed contact reads 0.0V. A healthy contact reads < 0.1V. If you read > 0.5V, the contacts are degrading and generating heat (P = I²R). Plan for replacement.
When to Repair vs. Replace
For enclosed, epoxy-potted lighting relays (like the Functional Devices RIB or RL series), always replace the entire unit. They are sealed against dust and moisture; attempting to open them destroys the housing. For large, open-frame industrial contactors (e.g., Square D or Eaton 30A+ units), you can unbolt and replace just the coil or the contact pads. However, for units under 30A, the labor cost of sourcing parts and cleaning the arc chute exceeds the cost of a new $40-$80 replacement unit.
Frequently Asked Questions
How do I wire a three way switch with two lights and a smart relay?
To integrate smart control, use a smart relay module (like a Shelly 1 or a Zigbee dry-contact relay) wired in parallel with your mechanical 3-way switches. Wire the smart relay's dry contacts across the momentary push-button switches that trigger your main lighting contactor's coil. This allows the smart home hub to pulse the coil just like a human pressing the physical switch, keeping the high-voltage load isolated from the sensitive smart electronics.
Can I use a standard 15A three way switch for two 100W LED high bay lights?
Technically, yes, if the combined steady-state draw is under 15A (two 100W LEDs at 120V draw roughly 1.6A total). However, per NEC guidelines and best practices for equipment longevity, you must check the LED driver's inrush current specification. If the inrush exceeds the switch's rated make/break capacity (often not published on cheap residential switches), the contacts will degrade rapidly. For commercial high-bays, a 20A motor-rated or ballast-rated snap switch, or a lighting contactor, is strongly recommended.
What size wire do I need for wiring a three way switch with two lights on a 20A breaker?
For the contact (load) side on a 20A breaker, you must use a minimum of 12 AWG copper wire (THHN in conduit or 12/2 and 12/3 NM-B cable). The 12 AWG wire has an ampacity of 20A in the 60°C column (which governs most residential terminations). For the coil (control) side, if you are using a 24V AC/DC control circuit protected by a 2A or 5A fuse, 18 AWG or 16 AWG control wire is permissible inside the enclosure, but 14 AWG is standard for mechanical durability when pulling through walls.
