TryBuildCalcCommercial Lighting Circuits Calculator (277V Branch Circuit Sizing)
Calculate your commercial lighting circuits.
🕒 Last updated: August 29, 2026
Inputs
ℹ️Sets the NEC Table 220.42 unit lighting load (VA/ft²) and whether a demand factor reduction applies.
ℹ️Measured from outside building dimensions per NEC 220.12/220.42, not usable/carpet area.
Show Window Lighting
Track Lighting
Circuit Sizing
Cost
Branch Circuits Needed: 7 × 20A @ 277V
For 20,000 ft² of Retail Store — branch circuit design load 38,000 VA
General Lighting Load
Unit load: 1.90 VA/ft²
Connected load: 38,000 VA
Already includes the 125% continuous-load margin, per NEC Table 220.42(A)'s own note.
Branch Circuit Sizing
Circuit design load: 38,000 VA
Capacity per circuit: 5,540 VA (20A × 277V, full rating)
Uses the connected load, not the demand-reduced feeder load below — branch circuits don't get diversity with each other.
Feeder / Service Demand Load (Informational)
After Table 220.45 demand factor: 38,000 VA
Total feeder/service demand: 38,000 VA (137.2 A)
This is the load a feeder/service supplying this space would need to carry — not the same figure as the branch circuit design load above, and not used for the circuit count.
Assumptions Used
NEC 2023 220.12/Table 220.42(A) (general lighting unit loads — default method, not the optional 220.42(B) energy-code alternate), Table 220.45 (feeder/service demand factors), 220.46(A)/(B) (show window/track lighting). Branch circuit count uses the connected (non-demand-factor-reduced) load, since demand factors apply to feeder/service sizing only. Single-phase VA/voltage estimate — a genuine 3-phase panel schedule needs per-phase balancing beyond this calculator's scope. This is a reference estimate — confirm with a licensed electrician before installation.
Checking your panel has enough total capacity for this plus everything else? Panel Size Calculator →
Need to size the wire and breaker for one of these circuits? Wire Size Calculator →
Looking for the verification checklist, reference tables, tips, or common mistakes?See the complete Lighting Load Calculator.
277V commercial lighting circuit count
Larger commercial buildings are commonly served by a 480Y/277V system, making 277V the standard lighting circuit voltage rather than 120V.
This page is pre-set to a 20,000 sq ft retail space on a 277V/20A circuit configuration — edit the occupancy type and floor area above to match your actual building.
- 20,000 ft² × 1.9 VA/ft² (retail) = 38,000 VA connected load, used in full.
- At 277V on 20A circuits (5,540 VA capacity each), this needs 7 branch circuits.
- 277V circuits allow more fixtures per circuit and less voltage drop than 120V on the same load.
Lighting Load Formula: How Is It Calculated?
This calculator computes two different totals: a feeder/service demand load (informational) and a branch circuit design load (what actually sets the circuit count) — they are not the same figure, and using the wrong one under- or over-sizes the result.
Step 1 — Connected Lighting Load
Connected Load (VA) = Floor Area (ft²) × Unit Load (VA/ft²)
The unit load comes from NEC Table 220.42(A) (or 220.12 for dwelling units) based on your occupancy type. Per Table 220.42(A)'s own note, this figure already includes the NEC 210.20(A) 125% continuous-load multiplier — no additional factor is applied to it anywhere below.
Step 2 — Feeder/Service Demand Load (Informational Only)
Feeder Demand = (Connected Load × Table 220.45 demand factor) + Show Window Design VA + Track Lighting Design VA
(demand factor applies to dwelling, hospital, hotel/motel, and warehouse only — all others: 100%)
Only certain occupancies receive a demand factor reduction, applied in tiers to different VA brackets — this reduction reflects that not every circuit in a large building draws its full load at once, from the perspective of the feeder/service supplying all of them together.
Step 3 — Branch Circuit Design Load & Circuit Count
Circuit Design Load = Connected Load + Show Window Design VA + Track Lighting Design VA
Circuits Needed = ceil(Circuit Design Load ÷ (Breaker A × Voltage))
Circuit count uses the CONNECTED load, not the demand-reduced feeder figure — demand factors are a feeder/service tool only; individual branch circuits don't get diversity with each other and must each be sized for their own actual assigned load (NEC 210.19). Show window (200 VA/ft) and track lighting (150 VA per 2 ft) come from NEC 220.46, are never demand-factor-reduced, and — since no note exempts them the way Table 220.42(A) is exempted — get their own explicit ×1.25 continuous-load factor (their "Design VA") before being added in. Because both totals are already fully design-value-inclusive, circuit capacity uses the breaker's full rating with no further 80%/125% adjustment.
Worked Example
This example walks through your current inputs above, using the same steps as the Formula section.
Input Values Used
| Input | Value | Why it is used |
|---|---|---|
| Occupancy | Retail Store (1.90 VA/ft²) | Sets the unit lighting load and demand factor category |
| Floor Area | 20,000 ft² | Multiplied by the unit load for connected VA |
| Circuit | 20A @ 277V | Sets each circuit's usable capacity |
Step 1 — Connected Load
| Calculation | 20,000 × 1.90 |
| Connected load (125% already included) | 38,000 VA |
Step 2 — Feeder/Service Demand Load (Informational)
| Demand factor category | None — 100% of connected load |
| General lighting demand load | 38,000 VA |
| Total feeder/service demand (not used for circuit count) | 38,000 VA (137.2 A) |
Step 3 — Branch Circuit Design Load & Circuit Count
| Circuit design load (connected load, not demand-reduced) | 38,000 VA |
| Capacity per circuit | 20 × 277 = 5,540 VA |
| Circuits needed | ceil(38,000 ÷ 5,540) = 7 |
Therefore, for 20,000 ft² of Retail Store, you need 7 circuits at 20A / 277V.
Related Calculators
Disclaimer: This calculator provides approximate results for planning and estimation purposes only. Actual requirements may vary based on site conditions, materials, workmanship, and local building regulations. Always consult a qualified engineer, architect, or construction professional before making final decisions.