What is the 125% continuous load factor and how is it different from a demand factor?

The 125% factor is a conductor/overcurrent-protection sizing requirement for loads that operate continuously (3+ hours) — it addresses thermal derating for sustained operation. Demand factors, by contrast, address load diversity (not everything runs at once) for calculating the overall service size. Both can apply to the same load but serve different purposes in the calculation.

Why does commercial receptacle load get calculated separately from lighting, unlike residential?

Commercial buildings, especially large open office floors, have receptacle usage patterns and diversity characteristics genuinely different from a dwelling — Article 220 reflects this with separate unit loads and demand factor tables for commercial receptacle load rather than folding it into the general lighting calculation the way residential does.

Does every commercial load qualify for a demand factor?

No — some load categories in commercial occupancies are calculated at 100% with no demand reduction (fire pumps and certain life-safety loads are common examples), reflecting that these loads must be assumed fully available regardless of typical operating patterns. Always check which specific demand factor provisions apply to each load category.

Electrical·8 min read·August 14, 2026

🏢 Commercial Load Calculation: A Demand-Factor Worked Example

A step-by-step commercial building load calculation showing how Article 220's demand factors apply differently than residential — office lighting, receptacle load diversity, and continuous vs. noncontinuous load treatment.

By EngineersUniverse Editorial Team

Engineering Editorial & Technical Review Team

Published August 14, 2026

How Commercial Calculations Differ from Residential

Commercial load calculations follow the same Article 220 structure as residential — connected load by category, demand factors applied where the code specifies them — but with meaningfully different unit loads (offices, retail, and warehouse occupancies each have their own general lighting VA/sq ft value, distinct from dwelling units) and, critically, a different treatment of receptacle load diversity. Residential receptacle load is folded into the general lighting demand calculation; commercial receptacle load is often calculated and demand-factored separately, reflecting that a large open office floor's outlet usage pattern is genuinely different from a house's.

The Scenario

A representative example: a 20,000 sq ft, single-story office building, 480Y/277V three-phase service, with general office lighting, standard receptacle load, and a 40-ton packaged rooftop HVAC system (continuous load).

Step 1 — General Lighting Load

Using an office occupancy unit load (commonly cited around 3.5 VA/sq ft — always verify against the current edition's exact table): 20,000 sq ft × 3.5 VA/sq ft = 70,000 VA connected.

Step 2 — Continuous Load Treatment

A key commercial-calculation concept not usually relevant to residential work: continuous loads (operating at full load for 3 hours or more) require a 125% multiplier on the overcurrent protection and conductor sizing, distinct from the demand factor applied to the load calculation itself. Office lighting, running for the full workday, typically qualifies as continuous: 70,000 × 1.25 = 87,500 VA for conductor/OCPD sizing purposes (the demand-calculated value for service sizing may differ — this 125% factor specifically addresses sustained-operation derating, a separate consideration from demand diversity).

Step 3 — Receptacle Load

Using a general commercial receptacle unit load (commonly around 1 VA/sq ft baseline for office space, with specific circuits still counted at their code-minimum branch circuit value): 20,000 × 1 VA/sq ft = 20,000 VA connected. Article 220 permits a demand factor for receptacle loads above a certain threshold in commercial/other occupancies (distinct from the dwelling-specific table) — applying an illustrative reduced demand factor to the portion above the first several kVA reflects that not every outlet in a large office draws its full rated capacity simultaneously.

Step 4 — HVAC Load

A 40-ton packaged rooftop unit is a significant, generally continuous-operation load during occupied hours. Using an illustrative demand figure of roughly 1.2 kVA/ton for a packaged rooftop system: 40 × 1,200 VA ≈ 48,000 VA, also subject to the 125% continuous-load multiplier for conductor/OCPD sizing where applicable: 48,000 × 1.25 = 60,000 VA.

Step 5 — Totaling and Converting to Amperes

Summing the demand-calculated values (illustrative): lighting (87,500, continuous-adjusted) + receptacle (demand-factored portion) + HVAC (60,000, continuous-adjusted) gives a total in the range of roughly 160,000-180,000 VA depending on the exact receptacle demand factor applied. At 480Y/277V three-phase: Amps = VA ÷ (√3 × 480) — a total of, say, 170,000 VA yields roughly 170,000 ÷ 831 ≈ 205 A, which would inform selection of a standard service size (commonly 225A or 400A depending on available equipment increments and future load headroom).

What This Example Deliberately Simplifies

Real commercial projects have more load categories (elevators, specialized process equipment, exterior/site lighting, fire pump loads with their own special code treatment) and more nuanced demand factor application than this illustrative walkthrough covers. This example is meant to show the structural differences from residential calculation (continuous load treatment, different unit loads, separate receptacle demand factors) — always perform a complete, project-specific calculation against the current code edition for any real commercial project.

Topics covered

commercial demand factor examplecommercial load calculationNEC 220 commercial exampleoffice building electrical loadcommercial service sizing