Lighting circuits differ from power circuits in three ways: commercial lighting is treated as a continuous load, so circuits are loaded to no more than 80% of the breaker; circuits follow switching and control zones rather than rooms; and commercial lighting usually runs at 277 V rather than 120 V. Egress lighting adds a separate emergency system.
Note: This overview is based on US practice — code references follow the model 2023 NEC (Articles 210, 215, 220, 410 and 700), the IBC and NFPA 101 for egress lighting, and ASHRAE 90.1 and California Title 24 for lighting controls. Wire sizes assume copper conductors with 75°C terminations. A companion guide covers power circuiting.
1. The lighting circuit path
Lighting power runs from the main panel through a feeder to a lighting panel, then on branch circuits through relays or switches to fixtures grouped by zone. Controls send signals to the relays or fixtures, and egress lighting follows its own emergency path.
Level | Residential | Commercial, office, school |
Source panel | Main load center or unit subpanel | Main switchboard, riser or distribution panel |
Lighting panel | None; lighting circuits share the load center | 480Y/277 V lighting panel per floor, wing or tenant |
Switching | Wall switches, dimmers, sensors on each circuit | Relay panels, contactors, room controllers, networked fixtures |
Branch circuit | 120 V, 15 A typical, AFCI | 277 V, 20 A typical |
Fixtures | Grouped by room | Grouped by control and daylight zone |
Emergency | Not required (smoke alarms excepted) | Separate emergency circuits, or fixtures with batteries |
2. Core lighting circuit rules
A lighting circuit is sized from its connected fixture wattage, held to 80% of the breaker where the load runs three hours or more, and kept within the zones the controls need.
Circuit capacity
Circuit | Breaker rating | 80% continuous limit | Typical design load |
120 V, 15 A, 14 AWG | 1,800 VA | 1,440 VA | 800–1,200 VA (residential) |
120 V, 20 A, 12 AWG | 2,400 VA | 1,920 VA | 1,200–1,600 VA |
277 V, 20 A, 12 AWG | 5,540 VA | 4,432 VA | 2,000–3,500 VA, leaving spare capacity |
277 V, 30 A, 10 AWG | 8,310 VA | 6,648 VA | Large high-bay or exterior circuits |
With LED fixtures drawing 20–40 W each, a 277 V, 20 A circuit can carry a hundred or more fixtures by load alone. In practice, control zones, voltage drop and the effect of a tripped breaker limit circuits well before load does.
Sizing and design rules
- Continuous load: size the breaker and wire for 125% of commercial lighting load (NEC 210.20, 215.2). Residential general lighting is usually not treated as continuous.
- Calculated load: use the larger of the fixture nameplate wattage (or driver input) and the NEC unit load for the occupancy (Article 220).
- Voltage drop: keep each circuit within about 3%; long 120 V runs drop quickly, which is one reason commercial lighting uses 277 V.
- Neutral at switch locations: required at most residential and occupiable-room switch boxes so electronic controls can be added (NEC 404.2(C)).
- Dimming compatibility: 0–10 V dimming adds a pair of low-voltage control wires per zone; phase-cut dimmers must be rated for the LED load.
- Fixture listing: wet or damp listing where required; IC rating in insulated ceilings (NEC 410).
Load calculation and energy-code limits
Occupancy | NEC unit load for sizing (approx.) | Energy-code design (typical LED) |
Dwelling units | 3 VA per sq ft (lighting and general receptacles together) | Often under 0.5 W per sq ft |
Offices | About 1.3 VA per sq ft | About 0.5–0.65 W per sq ft |
Schools | About 1.5 VA per sq ft | About 0.6–0.75 W per sq ft |
Retail | About 1.9 VA per sq ft | About 0.8–1.1 W per sq ft |
Warehouses | About 0.3 VA per sq ft | About 0.3–0.5 W per sq ft |
The NEC unit loads size panels and feeders; energy codes cap the installed lighting power, which is usually far lower. Non-dwelling NEC unit loads are from the 2026 NEC table, which already includes the 125% continuous-load factor (Open Exam Prep); energy-code values are typical ranges, not quotes from a specific edition.
3. Residential lighting circuiting
The NEC requires a wall-switched lighting outlet in every habitable room, kitchen, bathroom, hallway, stairway, garage and outdoor entrance (NEC 210.70), but does not require dedicated lighting circuits. The key rules are what lighting may not share: kitchen countertop and laundry circuits may not serve lights, and a shared bathroom receptacle circuit may not either.
3.1 Kitchens
Rule | Code basis | Practice |
Kitchen lights may not be on the small-appliance (countertop) circuits | NEC 210.52(B)(2) | Put kitchen lighting on a general lighting circuit, often shared with the dining and family rooms |
AFCI protection on 120 V, 15–20 A kitchen circuits | NEC 210.12 | Use AFCI breakers on the lighting circuit |
Switched lighting outlet required | NEC 210.70(A) | Separate switching for ceiling, pendant and under-cabinet zones |
Under-cabinet and accent lighting | Low-voltage LED tape needs a listed driver | Feed the driver from the lighting circuit at an accessible location, not from a countertop receptacle circuit |
Range hood light | Part of the hood's circuit | Not counted as kitchen lighting |
A typical kitchen uses one 15 A lighting circuit for ceiling and pendant fixtures, with under-cabinet drivers on the same or an adjacent lighting circuit.
3.2 Bathrooms
Bathroom lighting depends on how the bathroom receptacles are circuited:
Bathroom receptacle arrangement | Where bathroom lighting goes | Code basis |
One 20 A circuit shared by receptacles in several bathrooms | Lighting and fans must be on a different circuit | NEC 210.11(C)(3) |
One 20 A circuit dedicated to a single bathroom | Lighting, fan and heater for that bathroom may share it | NEC 210.11(C)(3) exception |
- Fixtures within 3 ft horizontally and 8 ft vertically of a tub or shower must be listed for damp or wet locations; pendants, track and fans are not allowed in that zone (NEC 410.10(D)).
- Shower lights may need GFCI protection where the manufacturer requires it.
- Bathroom lighting circuits do not need AFCI protection unless they also serve AFCI-required rooms.
- Exhaust fans with lights and heat-lamp units often need their own circuit by nameplate.
3.3 Bedrooms and living areas
- AFCI required on all 15–20 A, 120 V circuits serving bedrooms, living, family and dining rooms, closets and hallways (NEC 210.12).
- Separate lighting from receptacles where practical: one lighting circuit can serve several bedrooms and the hall, so a tripped receptacle circuit does not leave rooms dark.
- Switched receptacles may substitute for a ceiling fixture in habitable rooms other than kitchens and bathrooms (NEC 210.70(A)(1)); in the 2026 NEC a switched receptacle no longer counts toward required receptacle spacing.
- Closets: fixtures must meet NEC 410.16 clearances; typically on the bedroom or hall lighting circuit.
- Smoke alarms are often placed on a bedroom-area lighting circuit so a tripped breaker is noticed quickly.
- Ceiling fans with lights run on the room's lighting circuit; a separately switched fan needs an extra conductor (a 3-wire cable) from the switch.
3.4 Laundry rooms
Rule | Code basis | Practice |
The 20 A laundry circuit may serve laundry receptacles only, not lights | NEC 210.11(C)(2) | Put laundry lighting on a general lighting circuit |
AFCI protection on laundry-area circuits | NEC 210.12 | Lighting circuit with an AFCI breaker |
GFCI protection for laundry-area receptacles | NEC 210.8(A) | Applies to receptacles, not hardwired lights |
Switched lighting outlet | NEC 210.70 | Switch at the door; vacancy sensors are common |
3.5 Other areas
Area | Lighting circuiting |
Garages | May share the garage receptacle circuit or a general lighting circuit; switched lighting outlet at each entry door |
Exterior | Porch, entry and landscape lighting on a lighting circuit with photocell or timer; low-voltage landscape lighting through a listed transformer |
Stairs and halls | 3-way and 4-way switching; stairways with six or more risers need switches at each level |
Attics, crawl spaces, basements | Switched lighting outlet at the point of entry where used for storage or containing equipment |
3.6 Example lighting circuit list
Lighting circuits for a three-bedroom, two-bath, single-story home (about 1,800 sq ft) with LED fixtures. Wattages are illustrative.
Circuit | Breaker | Areas served | Approx. connected LED load | Protection |
L1 | 15 A | Kitchen ceiling, pendants and under-cabinet drivers | 250 W | AFCI |
L2 | 15 A | Living, dining and entry | 300 W | AFCI |
L3 | 15 A | Bedrooms 2 and 3, hall, closets, smoke alarms | 250 W | AFCI |
L4 | 15 A | Primary bedroom, closet and ceiling fan | 200 W | AFCI |
L5 | 15 A | Laundry room and hall bath (if bath receptacles are on a shared circuit) | 150 W | AFCI |
— | 20 A | Primary bath lights and fan, on that bath's dedicated receptacle circuit | 150 W | GFCI |
L6 | 15 A | Garage and exterior lighting | 300 W | — |
Five or six lighting circuits with LED fixtures is typical for a home this size; with incandescent fixtures the same house once needed eight to ten.
Circuit rules for kitchens, bathrooms and laundry, and AFCI and GFCI locations: Open Exam Prep.
4. Circuiting fixtures to lighting panels
Lighting circuits are laid out zone first: decide which fixtures switch, dim and respond to sensors together, then group zones onto circuits, then route each circuit back to the panel.
Grouping strategies
Strategy | How it works | Where it fits |
One circuit per control zone | Each switched or sensor zone has its own circuit | Simple relay and switch systems |
Several zones per circuit | Zones share a circuit; each fixture or zone dims independently through a networked control or relay pack | Networked lighting controls, luminaire-level controls |
By daylight zone | Fixtures near windows and skylights on separate circuits or control channels | Offices, classrooms and retail under energy codes |
By schedule | Fixtures that switch together (sales floor, exterior, signage) on circuits fed through a contactor | Retail, parking, gyms |
Alternating rows | Adjacent rows on different circuits | Large open spaces, so a tripped breaker leaves half the lights on |
Normal vs emergency | Egress fixtures on separate emergency circuits or with integral batteries | All commercial buildings |
Switch legs and wiring
- A switch leg runs from the fixture box down to the switch; the circuit's line and neutral are usually brought to the fixture box first.
- 3-way and 4-way switching adds traveler conductors between switch locations.
- 0–10 V dimming adds a gray and purple pair from the dimmer or control module to each fixture in the zone, kept as Class 1 wiring in the same raceway or as Class 2 wiring separated from power.
- Networked and wireless controls often remove switch legs entirely: line power runs fixture to fixture, and control is by radio or a low-voltage bus.
- Relay packs and room controllers mount above the ceiling or at the room entry and switch or dim each zone from sensor and keypad inputs.
Homeruns
- Bring each circuit's homerun from the fixture nearest the panel; label it with panel and circuit number on plans (for example, "LP-3-7").
- Combine up to three 277 V circuits on a shared neutral only with handle-tied or common-trip breakers.
- More than three current-carrying conductors in a raceway requires ampacity derating; this often limits how many lighting circuits share a homerun conduit.
- Keep circuits within about 150–200 ft at 277 V or 75–100 ft at 120 V before upsizing wire for voltage drop.
Emergency lighting separation
- Emergency (life-safety) circuits must run in their own raceways, boxes and panels, separate from normal wiring (NEC 700.10).
- Where an emergency fixture shares a room with normal fixtures and is switched with them, a listed UL 924 transfer device lets the switch control it normally and forces it on when normal power fails.
- Fixtures with integral battery packs stay on normal circuits but need an unswitched hot conductor to sense power loss and keep the battery charged.
5. Lighting panels to the main panel
In homes, lighting circuits come straight from the main panel or a unit subpanel. In commercial buildings, lighting has its own 480Y/277 V panels, fed by feeders from the main switchboard, with emergency lighting on a separate feeder path.
Feeder sizing
- Size the lighting panel feeder for 125% of the continuous lighting load plus other loads on the panel (NEC 215.2), using the NEC unit load or the actual connected load, whichever is larger.
- Add 20–25% spare capacity and spare breaker spaces for future tenant changes.
- Keep feeder voltage drop to about 2% so feeder plus branch circuit stay under 5%.
- 480Y/277 V feeders of 1,000 A or more need ground-fault protection at the feeder disconnect (NEC 215.10).
Typical lighting panel arrangements
Building | Lighting panel | Feeder |
Single-family home | No separate panel; lighting circuits in the main load center | Service entrance |
Apartment | Lighting circuits in each unit's load center; common-area lighting on a house panel | 100–150 A unit feeders from a meter stack |
Small commercial tenant | One 208Y/120 V panel for lighting and receptacles, or a 480Y/277 V lighting panel in larger spaces | 100–225 A feeder from the building distribution |
Office floor | 480Y/277 V lighting panel in each floor's electrical room | 100–225 A from a riser or busway tap |
School wing | 480Y/277 V lighting panel per wing | 100–225 A feeder from the main switchboard |
Exterior and site | Lighting circuits through a contactor in the main electrical room | From the main switchboard or house panel |
Lighting contactors and control panels
- A lighting contactor is a relay that switches several lighting circuits at once from a time clock, photocell or building automation signal. It is installed between the panel breakers and the circuits.
- A lighting control panel (relay panel) combines individually controlled relays with a schedule controller; it may be a separate enclosure beside the panelboard or built into it.
- Energy codes require automatic shutoff and exterior lighting controls, which these devices provide.
Transformers
- 277 V lighting needs no transformer. Where some lighting runs at 120 V (decorative fixtures, track, task lighting), it is fed from a 208Y/120 V panel behind a 480–208Y/120 V step-down transformer.
- Each transformer establishes a new neutral-to-ground bond and grounding electrode connection.
Emergency lighting feeders
- Emergency lighting panels are fed from the emergency side of an automatic transfer switch, from a central lighting inverter, or by fixtures with integral batteries.
- Emergency feeders and branch circuits stay in dedicated raceways and must be selectively coordinated so a fault trips only the nearest device (NEC 700.10, 700.32).
- On emergency power, egress paths need at least 1 fc average and 0.1 fc minimum for 90 minutes (IBC 1008.3, NFPA 101 7.9).
Feeder sizing and grounding: Open Exam Prep. Egress lighting levels: CSE Magazine.
6. Commercial and retail lighting circuiting
Retail lighting is circuited by schedule: what turns on before opening, what runs during business hours, and what stays on after close.
Lighting | Typical circuiting | Control |
Sales floor ambient | 277 V, 20 A circuits; alternate rows on different circuits | Lighting control panel on the store schedule |
Accent and track | 120 V track circuits from a 208Y/120 V panel, or 277 V track with step-down in the track; one circuit per track run or display zone | Scene or schedule control; dimming where specified |
Show windows | Separate circuits; NEC load at 200 VA per linear ft | Time clock, often later than store hours |
Signs | At least one 20 A circuit for each entrance sign outlet, serving nothing else | Time clock or photocell (NEC 600.5) |
Back of house | 277 V circuits by room | Occupancy sensors |
Refrigerated cases | Case lighting usually on the case's own circuit | Case controller |
Exterior and parking | 277 V or 480 V circuits through a contactor | Photocell plus time clock; reduced output after hours |
Emergency and exit | Unit batteries in small stores; emergency panel in large ones | UL 924 devices where switched with normal lighting |
Panel strategy. Small tenants put lighting and receptacles in one 208Y/120 V panel; larger stores add a 480Y/277 V lighting panel with a contactor or relay panel beside it. Site lighting is often on the landlord's house panel, not the tenant's.
7. Office lighting circuiting
Office lighting circuits follow the energy-code control zones: each enclosed room, each open-office zone, and each daylight zone must be controllable on its own, so circuits and control channels are laid out to match.
Space | Typical circuiting | Control |
Open office | 277 V, 20 A circuits covering roughly 2,000–4,000 sq ft each; daylight zones on separate circuits or control channels | Ceiling occupancy and daylight sensors; zone keypads |
Private offices | Several offices per circuit; each office a separate control zone | Vacancy sensor with dimmer per office |
Conference rooms | Room fixtures may share a corridor or office circuit; presentation-wall fixtures on a separate zone | Multi-scene keypad, often tied to AV |
Corridors and lobbies | Separate circuits from tenant spaces; egress fixtures on emergency circuits | Schedule with after-hours override |
Restrooms | Shared circuit with nearby core spaces | Occupancy sensors |
Stairs | Emergency circuits or battery-backed fixtures | Often occupancy-dimmed to a low level, full on when occupied |
Networked controls. With networked or luminaire-level controls, every fixture is addressable, so one 277 V circuit can serve several rooms and zones; the control system, not the circuit, defines the zones. This cuts homeruns and switch legs, but circuits are still split by area so a tripped breaker affects a limited part of the floor.
Panel strategy. One 480Y/277 V lighting panel per floor, beside the receptacle panels in the floor's electrical room, fed from the riser. Many owners meter lighting separately from receptacles and HVAC to track energy use by end use.
8. School lighting circuiting
School lighting is circuited by wing and by room, with each classroom's zones (general, teaching wall, daylight) controlled separately and egress lighting on emergency circuits.
Space | Typical circuiting | Control |
Classrooms | 277 V; one circuit for two to four classrooms, with each room's general, teaching-wall and daylight zones on separate relays or channels | Vacancy sensor plus multi-button keypad |
Science labs and shops | Room lighting on normal circuits; not interrupted by the bench or machine emergency-off | Keypad at the door |
Gyms | 277 V or 480 V high-bay circuits split by court zone; alternate rows on different circuits | Key switch or keypad; occupancy sensors |
Auditoriums | House lighting on its own circuits; stage lighting from a dedicated dimming system and panel | House-light presets; theatrical console |
Corridors and stairs | Separate circuits from classrooms; egress fixtures on emergency circuits | Schedule with key-switch override |
Cafeterias and kitchens | Circuits by area; kitchen fixtures listed for damp locations | Keypad or schedule |
Exterior and fields | 277 V or 480 V circuits through a contactor; sports lighting on its own panel | Photocell, time clock, school calendar |
Panel strategy. One 480Y/277 V lighting panel per wing keeps classroom circuits short. Schools used as emergency shelters often put more lighting on generator-backed emergency or standby circuits than code requires.
Sources
- Required branch circuits and GFCI/AFCI protection, Open Exam Prep (NEC 210.11, 210.12, 210.52)
- Feeders and outside branch circuits, Articles 215 and 225, Open Exam Prep
- Commercial lighting and receptacle load, Open Exam Prep
- Luminaires and clothes closet clearances (NEC Article 410), Open Exam Prep
- Illuminating the means of egress, Consulting-Specifying Engineer
- NEC requirements for switches, EC&M
Circuit capacities, fixtures and areas per circuit, energy-code lighting power ranges and building-type practice are from general industry knowledge, not these sources. Size actual circuits from the fixture schedule and adopted codes, and have a licensed engineer or electrician confirm them.