Load Calculations: The NEC 220 Basics Students Keep Getting Wrong

Regulation: NEC Article 220

Keywords: NEC 220 load calculation, service load calculation NEC, demand factor NEC, general lighting load NEC, dwelling unit load calculation, NEC feeder sizing calculation, optional calculation NEC 220, small appliance circuits NEC

Core Concepts

• General Lighting Load: Dwelling units: 3 VA per square foot of floor area using outside dimensions, excluding open porches, garages, and unfinished spaces not adaptable for future use.
• Small Appliance and Laundry Circuits: Minimum 2 small appliance circuits (1500 VA each = 3000 VA total) and 1 laundry circuit (1500 VA) added to general lighting load.
• Demand Factors: NEC Table 220.42: first 3000 VA at 100%, next 3001–120,000 VA at 35%, over 120,000 VA at 25%.
• Largest Motor Load: Add 25% of the largest motor load to the total calculated load for feeder/service calculations.
• Optional Calculation (220.82): Simplified dwelling method: 100% of first 10 kVA of total load, then 40% of remainder. Often yields a smaller service size than the standard method.

Exam Traps

• Square footage uses outside dimensions — not interior usable square footage; excludes garages and unfinished spaces.
• The 3000 VA small appliance load and 1500 VA laundry load are added BEFORE applying demand factors.
• Demand factors apply to the total lighting load — not to individual circuits.
• The 25% largest motor adder applies to feeders and services — not branch circuit calculations.
• The optional calculation method (220.82) cannot be mixed with the standard method in the same calculation.
• Include only the larger of heating or cooling loads — do not add both unless they operate simultaneously.

Memory Aid

Lighting (3 VA/ft²) + small appliance (3000 VA) + laundry (1500 VA) → apply demand factors → add fixed appliances + largest motor × 125%.