Last updated: October 4, 2026
NEC 220.42 sets the demand factors for general lighting loads in commercial buildings. You calculate the base lighting load from the occupancy's VA-per-square-foot value, then apply the permitted demand factor above set thresholds — for example, the first 50,000 VA at 100% and the remainder at 50%. The result is the lighting load you carry into feeder and service sizing.
Load calculation is not just a code requirement — it is the foundation for safety, cost efficiency, and passing inspections. New to the Code? Start with our NEC overview, and see Services and Feeders Explained for how service entrance conductors and feeder systems are structured. NEC 220.42 builds on that by telling you how much lighting load actually needs to be counted when sizing those systems.
What does NEC 220.42 require?
NEC 220.42 requires three things: a base lighting load set by occupancy type, demand factors applied above defined thresholds, and separate treatment for occupancies where reductions are not allowed.
1. Lighting loads based on occupancy type. Office buildings, schools, retail, warehouses, and other occupancies each have prescribed load values in VA per square foot. For example:
- Offices: 3.5 VA/sq ft
- Retail: 3 VA/sq ft
- Warehouses: 1 VA/sq ft
2. Demand factors. NEC 220.42 allows load reductions above certain thresholds — for example, the first 50,000 VA at 100% and the remainder at 50%. This drastically reduces conductor and panel size requirements on large projects.
3. Special conditions. Multi-occupancy buildings may require separate calculations for each tenant space. Some occupancies, like hospitals or theaters, may have no demand factor allowance because reliability is critical.
Why does NEC 220.42 matter for contractors?
Because lighting is one of the largest contributors to commercial building load, getting 220.42 wrong is expensive in both directions.
Oversizing leads to:
- Unnecessary cost — larger conductors, oversized panels, higher labor and materials.
- Reduced efficiency — excessive copper and oversized transformers.
Undersizing leads to:
- Voltage drop problems.
- Overheating and breaker trips.
- Code violations and failed inspections.
Applied correctly, NEC 220.42 balances compliance with efficiency — a safe system without unnecessary expense.
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How do you calculate a lighting load under NEC 220.42?
A 20,000 sq ft commercial office building works out to a 60,000 VA calculated lighting load. Here is the math:
- Step 1 — Calculate the base lighting load: 20,000 sq ft × 3.5 VA = 70,000 VA.
- Step 2 — Apply the demand factors: first 50,000 VA at 100% = 50,000 VA; remaining 20,000 VA at 50% = 10,000 VA.
- Step 3 — Final calculated lighting load: 50,000 + 10,000 = 60,000 VA.
That 60,000 VA — not the raw 70,000 VA — is the number carried forward when sizing feeders and service conductors under NEC Article 215. See our guide on Services and Feeders for how loads like this are applied to conductor and OCPD sizing.
What are the most common NEC 220.42 mistakes?
The three mistakes inspectors and estimators see most often:
- Ignoring occupancy classification — using office values for a warehouse can oversize a system by 300%.
- Not applying demand factors — skipping the permitted reductions inflates conductor sizes, panelboards, and transformer capacity.
- Mixing lighting with receptacle loads — the NEC requires separate calculations; combining them too early creates errors.
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How do you stay compliant with NEC 220.42?
Four habits keep lighting load calculations clean:
- ✅ Start with occupancy type — verify square footage and VA/sq ft values.
- ✅ Apply demand factors carefully — double-check thresholds to maximize cost savings without sacrificing compliance.
- ✅ Cross-reference NEC 215.2 — your calculated loads flow directly into feeder conductor sizing.
- ✅ Document everything — AHJs often ask for load calcs during plan review.
NEC 220.42 frequently asked questions
What is the general lighting load for a commercial office?
Offices are calculated at 3.5 VA per square foot, retail at 3 VA, and warehouses at 1 VA per square foot. The occupancy type sets the base load before any demand factors are applied.
What demand factor does NEC 220.42 allow?
Above set thresholds the calculated lighting load can be reduced — for example, the first 50,000 VA counted at 100% and the remainder at 50%. On large projects this materially reduces conductor and panel sizes.
Do all occupancies qualify for lighting demand factors?
No. Some occupancies, such as hospitals and theaters, may have no demand factor allowance because reliability is critical, and multi-occupancy buildings may require separate calculations for each tenant space.
Can lighting and receptacle loads be combined in one calculation?
No. The NEC requires separate calculations for lighting and receptacle loads; combining them too early is a common source of errors and failed plan reviews.
Final word
NEC 220.42 is more than a math exercise — it is a cost-saving tool when applied correctly. Combine it with a solid understanding of Services and Feeders and NEC 215.2 feeder conductor sizing, and you can design compliant systems that are lean, safe, and inspection-ready.