Single Light Switch Wiring Diagram

Single Light Switch Wiring Diagram — interactive diagram. Open it in the editor to customise components and wiring.

This is a free printable single light switch wiring diagram: download the diagram as SVG or open it and print to paper or PDF.

A single light switch wiring diagram shows one switch controlling one light fitting, illustrating how the live conductor is interrupted by the switch so current only flows to the lamp when the switch is in the on position, while neutral and earth remain continuously connected.

A single-pole single-throw (SPST) light switch is the simplest switching circuit in domestic electrical wiring, yet it is the basis for understanding all more complex switching arrangements. Getting the fundamental diagram right — particularly the principle that the switch must interrupt the live conductor, not the neutral — is essential before moving on to two-way, intermediate, or dimmer-switch wiring.

There are two main wiring methods used for single light switch circuits:

Method 1 — Loop at ceiling rose / light fitting (older UK method): In this arrangement, the supply cable runs to the ceiling rose first. Live, neutral, and earth are all at the rose. A switching cable (often called a switch drop or switch wire) runs from the rose down to the switch. The live conductor in this cable feeds the switch; the switched live returns from the switch back to the rose through the second core of the same cable; the neutral connects directly from the supply to the lamp within the rose. A single 2-core cable serves the switch, but both cores carry live voltage — one permanently (the feed) and one when the switch is on (the return). In modern wiring, both cores of this switch cable must be identified as live at both ends with brown sleeving or tape, even if the cable cores are different colours.

Method 2 — Loop at switch (modern method): The supply cable runs to the switch first. Live connects to the switch input terminal; the switched live output goes through a two-core cable to the light fitting, along with the neutral. This method requires less wiring effort when the switch is on the supply-side wall and is common in modern installations.

In both methods, the critical principle is identical: the switch contacts interrupt only the live conductor. The neutral is never switched. An RCD (residual current device) is required in circuits in high-risk locations (bathrooms, outdoors, kitchens) per modern wiring regulations.

For drawing the diagram, show the supply (consumer unit), the switch, and the lamp as three distinct nodes with conductors between them. Clearly label live (brown), neutral (blue), earth (green/yellow), and switched live (also brown, or brown-sleeved at both ends). The switch symbol (a line broken by a diagonal contact) sits in series with the live conductor path only.

How to wire single light switch wiring diagram

Isolate the lighting circuit at the consumer unit Switch off the MCB protecting the lighting circuit. Lock or tag it. Verify the circuit is dead at the switch and at the ceiling rose using a voltage tester or multimeter on all cores of both cables. Never rely on the light not lighting as proof the circuit is dead — a separate cable in the same back box could still be live.
Choose the wiring method for your installation Determine whether the supply cable arrives at the light fitting or at the switch. If the supply is at the ceiling, use the loop-at-fitting method. If the supply is at the switch location, use the loop-at-switch method. Your choice affects which cable cores are present at each location.
Run the cable(s) between supply, switch, and fitting For loop-at-fitting: one cable from supply to ceiling rose (L, N, E); one switch drop from ceiling rose to switch (switch feed and switch return, plus E). For loop-at-switch: one cable from supply to switch (L, N, E); one cable from switch to fitting (switched L, N, E). Secure cables per local code requirements for the installation method (concealed in wall, in conduit, clipped to surface).
Terminate the switch correctly At the switch, the permanent live connects to the Common (or L1) terminal. The switched live connects to the other switched terminal. For twin-and-earth cable with a grey or blue core used as the switch feed, sleeve that core with brown at both ends to indicate it is a live conductor. Connect the earth to the switch's earth terminal.
Terminate at the light fitting or ceiling rose At the fitting, connect the neutral (blue) to the neutral block/terminal, the switched live (brown or brown-sleeved) to the lamp terminal, and the earth to the earth terminal on the fitting or back plate. For a ceiling rose, the connection blocks are usually labelled for supply, switch, and lamp connections.
Check conductor identifications Every conductor that is live in normal operation — even only when the switch is on — must be identified as brown (or sleeved brown) at all accessible termination points. Any grey or blue cores used as switched live conductors must have brown sleeving applied at both ends.
Restore supply and test Re-energise at the consumer unit. Test that the switch controls only the intended lamp. Test that the lamp is fully off when the switch is open (no light, even faint — a faint glow indicates a neutral-switched or leakage fault). Test with a socket tester plugged into a nearby socket that the MCB is correct and the RCD (if fitted) operates within its trip parameters.

Specifications

Switch rating (domestic lighting)	6 A or 10 A, 250 V AC SPST
Cable size (standard domestic lighting)	1.0 mm² twin-and-earth; 1.5 mm² for longer runs or higher loads
MCB rating (domestic lighting circuit)	6 A Type B (UK); 15 A Type B for 15 A branch circuit (USA)
Live conductor colour	Brown (or brown-sleeved)
Switched live conductor colour	Brown (or brown-sleeved if using grey/blue core of T&E cable)
Neutral conductor colour	Blue
Earth conductor colour	Green and yellow stripes
RCD protection (BS 7671 requirement for lighting)	Required in high-risk locations (bathrooms, outdoors, potentially all socket circuits in current edition — verify against applicable edition of BS 7671)

Safety warnings

All electrical installation work on fixed wiring must comply with applicable wiring regulations: BS 7671 (IET Wiring Regulations) in the UK, NEC/NFPA 70 in the USA, AS/NZS 3000 in Australia/New Zealand, or IEC 60364 internationally. In many jurisdictions, adding new circuits or modifying fixed wiring requires notification to the building control authority and must be inspected or carried out by a qualified electrician.
Always isolate the lighting circuit MCB, verify the circuit is completely dead with a calibrated voltage tester at every cable in the back box (there may be more than one circuit present), and then begin work. Mains voltage is lethal.
Lighting circuits in bathrooms, outdoors, and other high-risk locations require RCD protection at 30 mA sensitivity per BS 7671 and equivalent international codes. Standard lighting circuits do not generally require an RCD in the UK, but this is changing in current editions of the wiring regulations — verify the current version applicable to your installation.
Never switch the neutral conductor with a single-pole switch. Interrupting the neutral leaves live voltage present at the lamp holder even when the switch is off. This is a safety violation under all wiring codes and creates a serious shock hazard during lamp replacement.

Tools needed

Two-pole voltage indicator (GS38 compliant in UK) or CAT II multimeter
Flat-blade and Pozidriv screwdrivers
Wire stripper (1.0–1.5 mm² capacity)
Side-cutter pliers
Electrician's knife (for outer sheath stripping)
Socket tester or multifunction installation tester (for final test)
Drill and flat bit (for cable routing through joists or walls)
Fish tape or rod (for drawing cable)

Common mistakes

Connecting the switch in series with the neutral conductor rather than the live, leaving live voltage present at the lamp holder when the switch is in the off position — an invisible and serious shock hazard.
Using grey or blue twin-and-earth cable cores as switch live conductors without applying brown sleeving at both ends, leaving the wire unidentified as live for anyone working on the circuit in future.
Connecting the switch drop's earth to the earth terminal on the light fitting only, without connecting it to the switch back box earth terminal — leaving the switch plate unprotected.
Allowing excess cable in the back box that is not neatly folded, causing conductors to be pinched by the switch faceplate screws when closed — which can cause insulation damage and intermittent or short-circuit faults.
Installing a dimmer switch rated for standard resistive loads on an LED lamp circuit without verifying LED-dimmer compatibility — resulting in flickering, buzzing, or premature LED driver failure.

Troubleshooting

Light does not turn on when switch is in the on position: Cause: Open circuit in the switch feed, the switch itself, or the switched live conductor; or blown lamp; or MCB tripped Fix: Check MCB is on. Use a voltage tester to confirm live voltage at the switch's input terminal. If present, check for switched live voltage at the output terminal when the switch is on. If that is also present, trace to the lamp terminals. Replace the lamp if voltage is present at the fitting.
Light stays on faintly even when switch is turned off: Cause: The switch is in series with the neutral rather than the live (wiring error); or an LED lamp is picking up stray voltage from capacitive coupling in long cable runs Fix: If the fault is dimly glowing when switched off, first verify that the switch interrupts the brown (live) conductor, not the blue (neutral). If wiring is confirmed correct and the issue is stray voltage with an LED lamp, fit a 'ghost eliminator' suppressor module across the lamp terminals.
MCB trips when the switch is turned on: Cause: Short circuit in the lamp, in the switched-live conductor, or at the fitting; or a lamp of excessive wattage overloading the circuit Fix: Disconnect the lamp fitting and test the cable insulation between the switched live and neutral. If good, the fault is in the lamp or fitting — replace. If the cable shows low insulation resistance, inspect for pinched insulation or moisture ingress at the back box or ceiling rose.

Frequently asked questions

Why must a light switch only break the live conductor, not the neutral?

If the switch interrupts the neutral while leaving live connected to the lamp, the lamp's metalwork, holder, and lampholder contacts remain at live voltage even when the switch is off. Anyone changing the bulb while the switch is off would touch live parts. The switch must break the live so that the entire lamp circuit is safe when switched off.

What is a switched live (switch return) conductor?

The switched live, also called the switch return, is the conductor that carries current from the switch output to the lamp. It is only energised (at live potential) when the switch is in the on position. In UK wiring, both the switch feed and switch return must be identified as live conductors — using brown insulation or brown sleeving — to warn anyone working on the circuit later.

What is the difference between loop-at-switch and loop-at-fitting wiring?

Loop-at-switch means the incoming supply arrives at the switch first; a switched cable then goes to the light fitting carrying switched live and neutral. Loop-at-fitting means the supply arrives at the fitting first; a switch cable drops to the switch and returns. Both achieve the same result but differ in where the junction point is and which cable carries what conductors.

Does a single light switch circuit require an earth conductor?

Yes, if the switch has a metal faceplate or if the light fitting has accessible metal parts (Class I fitting). The earth conductor must be connected to the earth terminal in the switch and at the light fitting. For double-insulated (Class II) plastic switches and fittings, an earth connection may not be required at those specific points, but the cable must still include the earth conductor.

What cable size should I use for a single light switch circuit?

In the UK, domestic lighting circuits typically use 1.0 mm² twin-and-earth cable protected by a 6 A MCB. For longer runs or higher loads (multiple fittings on one circuit), 1.5 mm² is common. The cable rating must be appropriate for the total load, installation method, and ambient temperature as per BS 7671 Table 4D2A or equivalent.