Without demand factors, a typical 2,500 sq ft home would need a 400A service. With them, 200A is usually sufficient. NEC Article 220 provides generous demand factors that reflect real-world usage patterns — but applying them correctly requires understanding the rules.
What Are Demand Factors?
A demand factor is the ratio of maximum demand to total connected load. It recognizes that not all loads operate simultaneously.
NEC 220 provides specific demand factors for general lighting, cooking equipment, dryers, HVAC, and other load types. Applying them correctly can reduce your calculated service size by 30-50%.
General Lighting (Table 220.42)
General lighting load starts at 3 VA per square foot (dwelling units). For 2,500 sq ft: 2,500 × 3 = 7,500 VA.
Table 220.42 demand: First 3,000 VA at 100% = 3,000. Remaining 4,500 VA at 35% = 1,575. Total: 4,575 VA (39% reduction).
Don't forget the mandatory small appliance circuits (2 × 1,500 = 3,000 VA) and laundry circuit (1,500 VA) — these are added before applying the demand factor.
Electric Ranges (Table 220.55)
A 12 kW electric range has a demand of only 8 kW for a single unit — a 33% reduction. For multi-family buildings, the savings compound dramatically: 12 units of 12 kW ranges = only 27 kW demand (from 144 kW connected).
Note Column A, B, or C selection depends on the appliance rating and number of units. Column C (most common) applies to ranges rated 8.75-12 kW.
Dryers (NEC 220.54)
4 dryers at 5 kW each: connected = 20 kW. Demand per 220.54 = 20 kW (100% for ≤4 units). But 12 dryers: 60 kW × 47% = 28.2 kW.
The demand factor decreases as the number of units increases, reflecting the statistical improbability of all dryers running simultaneously.
HVAC vs. Electric Heat
NEC 220.60: Do not add both heating and cooling loads — use only the larger of the two. In most climates, the cooling load dominates.
However, if heat strips in the air handler are larger than the compressor load, use the heat strip value. This is a common oversight in load calculations.
Key Takeaways
- •General lighting demand factors reduce load by 35-65% depending on building size
- •Range demand (Table 220.55) provides the most dramatic reduction for multi-family buildings
- •Never add both heating and cooling loads — use only the larger per NEC 220.60
- •Demand factors are not optional — they're part of the NEC calculation method and should always be applied