Switch Loop Wiring Diagram: Power-at-Light Configuration and NEC 200.7 Compliance

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A switch loop brings the hot conductor from a ceiling fixture down to a wall switch and back, re-identifying the returning white conductor as live per NEC 200.7(C).

A switch loop is the traditional method used in residential wiring when the electrical supply arrives at the light fixture before it reaches the wall switch — known as the 'power-at-light' configuration. Instead of running a three-conductor cable from the panel to the switch box and then up to the fixture, the electrician runs a two-conductor cable (usually 14/2 or 12/2 NM-B) from the fixture box down to the switch location.

In this configuration, the cable's black conductor carries unswitched hot down from the fixture box to the switch. The white conductor, which normally serves as the neutral, is repurposed as the switched-hot return path from the switch back up to the fixture. Because white is used as a hot conductor in this loop, NEC 200.7(C) requires that the white conductor be re-identified at both ends — at the switch box and at the fixture box — using black electrical tape, black paint, or a black permanent marker. This re-identification signals to future electricians that the white wire is live, not neutral, and must not be confused with a true grounded neutral.

At the switch: the black conductor and the re-identified white conductor are both connected to the switch terminals. The switch simply opens and closes the circuit between them — no neutral is present at the switch box in a classic switch loop.

At the fixture: the unswitched hot from the panel connects to the black conductor going down to the switch. The re-identified white (switched hot returning from the switch) connects to the black fixture lead. The true neutral from the panel connects directly to the white fixture lead without passing through the switch.

From 2011 onward, NEC 404.2(C) introduced the requirement for a neutral conductor to be present in switch boxes for most residential applications, to support smart switches and dimmers that require a neutral path. This means new installations increasingly use three-conductor cable (14/3 or 12/3) even to switch boxes, carrying a true neutral in addition to the two switch conductors. Always verify the applicable code edition enforced in your jurisdiction before wiring.

A switch loop is a wiring method where power arrives at the light fitting first, and a loop of cable runs down to the switch and back rather than routing power through the switch directly. It is common in older Australian and New Zealand installations and in ceiling-fed North American circuits. In a traditional switch loop only two conductors run to the switch, so the white wire is re-purposed as the hot feed — it must be taped or marked red/black to indicate it is not a neutral. Draw and annotate switch loop diagrams for free at CircuitDiagramMaker.com.

How to wire switch loop wiring diagram

Verify power-at-light configuration and plan the route Confirm that the supply cable (hot and neutral from the panel) terminates at the ceiling fixture box, not at the switch box. Plan the cable route from the ceiling box down through the wall to the switch box. Measure the cable length and add 300 mm at each box for connections.
Isolate the circuit at the breaker and verify dead Switch off the circuit breaker feeding the fixture. Lock it out with a lockout clip or tag and place a 'Do not switch on' notice. Use a non-contact voltage tester at the fixture box to verify that all conductors are de-energised before touching any wires. Test all conductors, not just the one you expect to be live.
Run the switch loop cable Fish or surface-run 14/2 or 12/2 NM-B cable from the fixture box down to the switch box location. At the fixture box, leave 300 mm of free conductor. At the switch box, leave 300 mm of free conductor. Staple or secure the cable within 300 mm of each box and at intervals not exceeding 1.4 m (1350 mm) per NEC 334.30.
Re-identify the white conductor at both ends At the switch box and at the fixture box, wrap the last 25–50 mm of the white insulated conductor with black electrical tape or colour it solidly with a black permanent marker. This re-identification is a code requirement (NEC 200.7(C)) — it is not optional and must be done at every accessible location.
Make connections at the switch box Connect the black conductor to one switch terminal and the re-identified white conductor (now acting as switched-hot return) to the other switch terminal. The bare copper ground wire connects to the switch's green grounding screw or ground terminal. In a standard single-pole switch, terminal polarity does not matter — either conductor goes to either switch terminal.
Make connections at the fixture box At the fixture box: connect the hot (black) from the supply cable to the black conductor of the switch loop cable — this feeds unswitched hot down to the switch. Connect the re-identified white conductor (switched hot returning from the switch) to the black lead of the light fixture. Connect the neutral (white) from the supply cable directly to the white lead of the fixture. Connect all bare copper ground wires together and to the fixture's ground lead and the box ground screw.
Test operation and inspect Restore power at the breaker. Test the switch in both positions: fixture should be on with switch up (or closed) and off with switch down (or open). Use a non-contact tester to confirm that the switch loop white conductor at the switch box reads live when the switch is on — confirming re-identification is correct. Have the installation inspected before closing the wall if required by local permit conditions.

Specifications

Cable type (USA residential)	NM-B (Non-Metallic sheathed, Type B insulation)
Wire gauge for 15 A circuits	14 AWG (1.5 mm²)
Wire gauge for 20 A circuits	12 AWG (2.5 mm²)
Cable voltage rating (NM-B)	600 V
NEC white conductor re-identification requirement	NEC 200.7(C) — black tape, paint, or marker at every accessible location
Neutral at switch box requirement (new construction USA)	NEC 404.2(C) (2011 edition onward) — neutral required in switch boxes in most residential applications
Maximum NM-B cable staple spacing	Every 1350 mm (4.5 ft) and within 300 mm (12 in) of each box (NEC 334.30)
Minimum box fill per 14 AWG conductor	2.0 cu in (NEC 314.16(B)) per conductor and per device counted

Safety warnings

Electrical wiring in dwellings must comply with NFPA 70 (National Electrical Code) in the USA, or the applicable national standard (BS 7671 in the UK, AS/NZS 3000 in Australia/NZ, CEC in Canada, SANS 10142 in South Africa). In most jurisdictions, fixed electrical installation work must be performed by or under the supervision of a licensed electrician and may require a permit and inspection.
Always isolate the circuit at the breaker before commencing work and verify dead with an approved voltage tester on every conductor in the box. The switch loop white conductor will be live when the switch is on — both conductors in a switch loop box can be live simultaneously depending on switch position. Test before touching.
Re-identification of the white conductor as a hot is mandatory, not optional. Failure to re-identify is a code violation and creates a genuine safety hazard for future electricians or homeowners who may assume white is always neutral and receive an electric shock.
Ensure box fill calculations comply with NEC 314.16. Overcrowded boxes generate heat, damage insulation, and are an inspection failure. Add conductor count, device count, and clamp count per the NEC table.
For any wiring near water, wet locations, or in outdoor boxes, use weatherproof boxes, GFCI protection, and WR-rated devices as required by NEC 210.8 and local amendments.

Tools needed

Non-contact voltage tester (essential — use before touching any wire)
Circuit breaker lockout clip or padlock
Fish tape or wire-fishing rods
Cable staple gun and NM-B staples
Flat-blade and Phillips screwdrivers
Needle-nose pliers and linesman's pliers
Wire strippers (sized for 12 AWG and 14 AWG)
Pencil or permanent marker for box opening layout

Common mistakes

Failing to re-identify the white conductor with black tape or marker at both the switch box and the fixture box — a code violation and a safety hazard for future workers.
Using a switch loop white conductor as the neutral to the fixture and connecting only the black as switched-hot — reverses polarity through the fixture, leaving the shell of a screw-base lamp socket live even when the switch is off.
Installing a smart switch or neutral-required dimmer on a classic switch loop without first verifying that a neutral is present — these devices require a neutral conductor to operate and will not function correctly on a switch loop.
Under-sizing the wire — using 14/2 on a 20 A circuit is a serious code violation and fire hazard. Match wire gauge to circuit breaker size.
Not accounting for box fill — adding the switch loop cable to an already-full fixture box can overload the box and damage conductor insulation through heat buildup.

Troubleshooting

Light is always on regardless of switch position: Cause: Switch loop conductors have been wired incorrectly — the returning switched-hot has been connected to the neutral at the fixture box instead of the fixture hot, creating a permanent circuit bypass Fix: Isolate, verify dead, and re-examine the fixture box wiring. The unswitched hot (from supply) must connect to the switch loop black going down; the switched-hot return (re-identified white from switch) must connect to the fixture's hot lead. Reconfigure and retest.
Light is always off even with switch in the on position: Cause: Open circuit — broken conductor in the cable, a loose wire connector, or the switch terminals are not making contact with the conductors (wire slipped from push-in terminal or not seated in wire nut) Fix: Isolate and verify dead. Check each wire connector at both boxes for secure conductor seating. Pull gently on each conductor to detect slippage. Use a multimeter to trace continuity from fixture box to switch and back.
Breaker trips when switch is turned on: Cause: A short circuit — likely the switch loop black and re-identified white have contacted each other inside the wall or at a box connection, creating a direct hot-to-switched-hot short Fix: Isolate the circuit. Inspect both boxes for pinched conductors or bare wire contacts. Inspect the cable run for damage from staples driven through the cable. Repair or replace the damaged cable section.

Frequently asked questions

Why is the white wire in a switch loop taped black?

NEC 200.7(C)(1) requires that when a white conductor is used as an ungrounded (hot) conductor — as it is in a switch loop return path — it must be permanently re-identified with black or another non-grey, non-white colour at each location where it is accessible. This warns future electricians and inspectors that the wire is live, preventing dangerous misidentification.

Is a switch loop still legal under current NEC editions?

Switch loops in existing installations remain legal. In new construction, NEC 2011 and later editions (404.2(C)) require a neutral conductor to be present at switch locations in most single-family dwelling wiring, to support smart devices. Many jurisdictions adopted this requirement between 2012 and 2017, so check which NEC edition your local authority has adopted.

What cable size should I use for a switch loop?

The switch loop cable must match the branch circuit's ampacity. A 15 A circuit uses 14/2 NM-B (14 AWG); a 20 A circuit uses 12/2 NM-B (12 AWG). Using undersized cable is a fire hazard. The switch itself must also be rated for the circuit amperage — do not use a 15 A switch on a 20 A circuit.

Can I install a smart switch or dimmer on a switch loop?

Most smart switches and dimmers require a neutral conductor to power their internal electronics. A traditional switch loop provides no neutral at the switch box, so most smart switches cannot be installed without running new cable or using a smart switch specifically designed for neutral-free operation (these exist but have limited compatibility). Neutral-required dimmers must not be used.

What is the difference between a switch loop and a switch leg?

The terms are often used interchangeably in the trade. Strictly, a switch leg (or switch drop) is a sub-run of cable from a junction or fixture box down to the switch, carrying hot and switched-hot — the same topology as a switch loop. Some electricians use 'switch loop' specifically for the power-at-light configuration and 'switch leg' for a power-at-panel configuration where the switch cable is run as a tap from an existing circuit. Confirm the terminology with your jurisdiction's inspection authority.

What is a light switch loop wiring diagram?

A light switch loop wiring diagram shows the circuit where mains power feeds into the ceiling rose or light fitting first, and a two-core cable drops down to the wall switch and returns. The incoming live wire connects to the live terminal of the ceiling rose and also runs down to the switch as the feed; the return conductor from the switch carries the switched live back up to the fitting to energise the lamp. The neutral from the mains connects directly to the light, never travelling to the switch. In modern wiring a three-core cable with earth is preferred so a true neutral is available at the switch position for smart or dimmer switches.

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