Helpful video reference. This UK tutorial, "How to wire a dimmer into a 2 way light switch", demonstrates the master and slave positions clearly and shows the terminal assignments that trip most people up the first time.
1. Confirm you have a two-way circuit
Two-way switching means two separate switches control the same light. It is common on stairs, in hallways and in rooms with two doorways. The give-away is that either switch will turn the light on or off regardless of what the other one is doing.
To be certain, unscrew one switch plate and count the conductors in the back box. A two-way position has three conductors going to the switch terminals: Common (C), L1 and L2. A one-way switch has just two.
2. Buy the right dimmer
Look for a dimmer marked "two-way compatible" in the product description. It will have three terminals labelled C, L1 and L2, matching the three conductors already in the back box. Most brands sell these as a kit: one master dimmer and one slave plate that completes the circuit at the second switch position.
While you are buying, check the dimmer's wattage range against your lamps and confirm it is compatible with LED loads if you have LED bulbs fitted. A trailing-edge dimmer is the safer choice for most LED fittings.
3. Isolate the circuit at the consumer unit
Find the MCB for the lighting circuit and switch it off. Take your voltage tester to both switch positions and confirm dead in all positions of the existing switches. If anything reads live, switch off the correct breaker before going any further.
4. Photograph both back boxes before touching anything
Undo both switch plates and ease them away from their back boxes. Take a clear photo at each position showing which conductor goes to which terminal. The three-core cable between the switches uses brown (Common), black (L1) and grey (L2) in current wiring colours, or red, yellow and blue in older installations. Your reference photos remove any guesswork when it comes to reconnecting.
5. Wire the master dimmer
At the position where you are fitting the dimmer, connect each conductor to the matching terminal on the new dimmer: C goes to C (Common), L1 to L1, L2 to L2. The labels on the dimmer body or its instructions will show the layout.
If the back box is metal, connect an earth fly lead from the back box earth terminal to the earth terminal on the dimmer plate. Plastic back boxes with no earth terminal need no earth on a plastic dimmer plate; metal plates always need one.
Fold the conductors gently back into the box and offer the plate up to the fixing holes. Do not trap any insulation under the plate edge. Tighten the two screws evenly.
6. Wire the slave plate at the second position
At the second switch position, fit the slave plate from the kit. Connect the conductors in the same way: C to C, L1 to L1, L2 to L2. The slave plate contains no electronics: it simply passes the conductors through to complete the circuit, so the dimmer at the master end can control the light from either position.
Fit the slave plate in the same way, checking for trapped insulation and tightening the screws evenly.
7. Restore power and test both positions
Switch the lighting circuit back on at the consumer unit. Test the dimmer from the master position first: press or rotate to switch on, then adjust to confirm the brightness changes. Then go to the slave position and confirm you can switch the light on and off from there too, with the dimming level holding as set.
If the light flickers at lower settings, check the dimmer's minimum load specification against the wattage of your lamps. Most two-way dimmers need at least 10 W of connected load to operate cleanly.
When to call us
The wiring itself is not complicated once you have the right dimmer and know which terminals to use. The tricky part is diagnosing an unusual wiring arrangement in an older property. If either back box contains more cables than expected, or the wiring dates from before the 2004 colour change, a quick visit from an electrician is cheaper than guessing.
Need a dimmer fitted in Sandwich?
Richard fits dimmers, smart switches and lighting controls on small jobs at the £10 per 10-minute rate.
Contact Richard