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General Lighting Circuit Planning Guide

Lighting circuit planning has one distinction that trips up even experienced planners: the demand-factor-reduced load used for sizing a feeder or service is NOT the same figure used to determine how many branch circuits are needed. Branch circuits don't get diversity with each other — each one has to be sized for its own actual assigned load, while the demand factor only applies at the feeder/service level where many circuits are supplied together.

Last updated: August 29, 2026

The most consequential lighting circuit planning mistake isn't a wrong unit load figure — it's using the wrong TOTAL for the wrong purpose. A demand-factor-reduced load is correct for sizing a feeder or service, but using that same reduced figure to determine branch circuit count under-sizes the actual number of circuits a building needs.

This guide covers how connected lighting load is built up by occupancy, why feeder demand and branch circuit design load are different figures, show window/track lighting basics, and a worked example.

Two Different Totals, Two Different Purposes

Feeder/Service Demand = (Connected Load × Demand Factor) + Show Window/Track Lighting
Branch Circuit Design Load = Connected Load (undiminished) + Show Window/Track Lighting

Notice the demand factor appears in the first formula only — this is the single most important distinction in lighting circuit planning:

Load TypeHow It's CalculatedDemand Factor?
General lightingFloor area × occupancy-specific unit load (VA/ft²)Feeder/service: yes, for certain occupancy types. Branch circuit count: no — uses connected load
Show window lightingFixed VA per linear foot along the window's base, calculated separatelyNever reduced by the general lighting demand factor at either level
Track lightingFixed VA per length of track, calculated separatelyNever reduced by the general lighting demand factor at either level

Branch circuits don't get diversity with each other — each one is sized for its own actual assigned load. The demand factor is a feeder/service-level concept only.

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Which Occupancy Types Get a Demand Factor Reduction

OccupancyFeeder-Level Demand Factor
Dwelling unitTiered demand factor for feeder sizing (a portion at 100%, remainder at a reduced percentage)
Office / retail / restaurant / school / gymnasiumNo demand factor reduction — full connected load used at both levels
HospitalTiered demand factor for feeder sizing only
Hotel / motel (guest rooms)Tiered demand factor for feeder sizing only
Storage warehouseTiered demand factor for feeder sizing only, based on a connected-load threshold

Notice most common commercial occupancy types (office, retail, restaurant, school, gymnasium) receive NO reduction at either level — the full connected load applies throughout for these.

Worked Example — Retail Store With a Show Window

3,000 sq ft Retail Store, 40 ft Show Window, 120V/20A Circuits

Illustrative example

StepResult
Connected general lighting load (retail — no demand factor)Full value, used at both feeder and branch circuit level identically
Show window load (calculated separately)Added to both totals at its own calculated value — never demand-reduced
Branch circuit countConnected load ÷ circuit capacity, rounded up

Because retail receives no demand factor reduction at all, this particular example's feeder demand and branch circuit design load actually come out identical — a useful sanity check, but not the case for an occupancy type that DOES receive a reduction, like a dwelling or hospital.

Common Mistakes

Using the Demand-Reduced Feeder Load to Determine Branch Circuit Count

This is the single most consequential lighting circuit planning mistake — demand factors are a feeder/service-level tool recognizing diversity across many circuits, not a basis for reducing how many branch circuits are actually needed, since each branch circuit must carry its own full assigned load.

Applying a Demand Factor to an Occupancy That Doesn't Receive One

Only specific occupancy categories receive a feeder-level demand factor reduction — applying one to an occupancy type that's calculated at 100% of connected load understates the actual feeder/service sizing requirement.

Reducing Show Window or Track Lighting by the General Lighting Demand Factor

These loads are calculated entirely separately from general lighting and are never subject to the general lighting demand factor at any occupancy — they're added to the total at their own calculated value.

Double-Applying (or Skipping) the Continuous-Load Margin

Whether a separate continuous-load multiplier needs to be applied depends on whether it's already built into the specific unit-load table figure being used — applying it a second time on top of a table that already includes it overstates the load, while skipping it where it's genuinely not yet included understates it.

Using Usable/Carpet Area Instead of Outside Building Dimensions for Floor Area

General lighting load calculations are based on outside building dimensions, not the smaller usable or carpet area a leasing document might reference — using the wrong area figure produces an incorrect connected load from the very first step.

Relevant Standards and References

RegionRelevant Codes / Guidance
United StatesNEC 220.12/Table 220.42 covers general lighting unit loads by occupancy; Table 220.45 covers feeder/service demand factors; NEC 220.46 covers show window and track lighting
Europe / UKBS 7671 and CIBSE guidance cover lighting circuit design and load assessment for building electrical installations
IndiaIS 732 and National Building Code provisions cover lighting load allowances for building wiring
Australia / New ZealandAS/NZS 3000 covers lighting circuit design requirements for electrical installations
General guidanceUnit load tables, section numbering, and demand factor structures have been revised across successive code editions in most jurisdictions — always confirm figures against the current, locally-adopted code rather than a general reference.

Final Verdict

Correct lighting circuit planning means keeping feeder/service demand load and branch circuit design load as two genuinely separate figures — using the demand-reduced total for the wrong purpose is the single most consequential mistake in this calculation.

  • Use the CONNECTED (undiminished) lighting load for branch circuit count — never the demand-reduced feeder figure.
  • Only certain occupancy types (dwelling, hospital, hotel/motel, warehouse) receive a feeder-level demand factor reduction at all.
  • Show window and track lighting are calculated separately and are never reduced by the general lighting demand factor.
  • Check whether continuous-load margin is already built into the specific unit-load table before applying it again.
  • Use outside building dimensions for floor area, not usable/carpet area.
  • Confirm circuit voltage matches your actual electrical system — 277V circuits carry meaningfully more capacity than 120V for the same breaker size.

Related calculators

Use these calculators when you need to turn this reference information into project quantities:

Related resources

  • Circuit Breaker Types & Protection Guide

    Complete guide to circuit breaker types — standard thermal-magnetic breakers vs GFCI, AFCI, and dual-function breakers, what each one actually protects against, and how to think about where each applies.

  • Home Electrical Panel Sizing Guide

    Complete guide to sizing a home's electrical service panel — general lighting load, small appliance/laundry circuits, the demand factor that reduces the total, HVAC's largest-of rule, common service sizes, and signs you've outgrown your panel.

FAQ

The connected lighting load is calculated by multiplying floor area by an occupancy-specific unit load, expressed in VA per square foot — different occupancy types (office, retail, restaurant, warehouse, hospital, and so on) are assigned different unit loads reflecting typical lighting density for that kind of space. This connected load is the starting point for two different downstream calculations that serve two different purposes — sizing branch circuits, and sizing the feeder/service that supplies them.
A demand factor recognizes that not every circuit in a large building draws its full load simultaneously, from the perspective of a feeder or service supplying many circuits together — this diversity is a real, legitimate basis for reducing the feeder/service sizing figure below the raw connected total for certain occupancy types (dwelling units, hospitals, hotels/motels, and storage warehouses commonly receive this reduction; many other occupancy types don't get any reduction at all). Branch circuits, however, don't get this same diversity treatment with each other — each individual branch circuit must still be sized for its own actual, undiminished assigned load, since it's a single circuit serving its own specific portion of the lighting, not a shared resource across many circuits the way a feeder is. Using the demand-reduced feeder figure to determine branch circuit count under-sizes the actual number of circuits needed.