The Standard Calculation Method
NEC Article 220 provides the standard method for calculating residential service load. This calculation determines the minimum service size (100A, 200A, 400A) and the main breaker rating. Most single-family homes require 200A service. The calculation uses demand factors that reduce the total connected load to reflect the statistical improbability of all loads operating simultaneously.
The alternative: NEC 220.82 provides an optional calculation method that is simpler but may result in a larger service size. Most engineers use the standard method (220.40-220.60) for accuracy.
General Lighting and Receptacles
NEC Table 220.12: dwelling units = 3 VA per square foot of habitable space. Include all finished areas — living rooms, bedrooms, kitchens, hallways, closets, bathrooms. Exclude open porches, garages, and unfinished spaces.
Example: 2,400 sq ft home = 2,400 × 3 = 7,200 VA for general lighting.
Small Appliance Branch Circuits (NEC 220.52(A)): minimum 2 circuits × 1,500 VA = 3,000 VA. These cover kitchen countertop and pantry receptacles.
Laundry Circuit (NEC 220.52(B)): 1 circuit × 1,500 VA = 1,500 VA.
Total before demand: 7,200 + 3,000 + 1,500 = 11,700 VA.
Demand Factors (NEC Table 220.42)
The general lighting/small appliance/laundry total gets demand factors applied: first 3,000 VA at 100%, remainder at 35%.
Example: 11,700 VA total → first 3,000 at 100% = 3,000 VA + remaining 8,700 at 35% = 3,045 VA. Demand total = 6,045 VA.
These factors reflect the reality that not all lights and receptacles operate at full load simultaneously in a residence.
Fixed Appliances and Equipment
Fixed Appliances (NEC 220.53): If 4 or more fixed appliances (dishwasher, disposal, water heater, etc.), apply 75% demand factor to their total nameplate VA. Fewer than 4 = 100%.
Electric Dryer (NEC 220.54): Use 5,000 VA or nameplate rating, whichever is larger. For multiple dwelling units, Table 220.54 provides demand factors.
Electric Range (NEC Table 220.55): A single household range ≤ 12 kW: use 8 kW demand. Ranges over 12 kW: 8 kW + 400W per kW over 12 kW. Example: 16 kW range = 8,000 + (4 × 400) = 9,600 VA.
Largest Motor (NEC 220.50): Add 25% of the largest motor FLC to the total. If the A/C compressor is the largest motor, include 25% of its FLC in addition to the full A/C load.
Heating vs Cooling (NEC 220.60)
NEC 220.60 permits using only the LARGER of heating or cooling load — not both — since they don't operate simultaneously. This significantly reduces the calculated load for most homes.
Central A/C: Use nameplate VA (or voltage × MCA for compressor units).
Electric Heat: total kW of all heating elements at 100% demand (or per NEC 220.51 for large heating loads).
Heat Pump: More complex — include both heating elements (strip heat/auxiliary) and compressor. During defrost, both may operate simultaneously, so use the larger of: (a) compressor + supplemental heat, or (b) total heating load.
Putting It All Together
Complete example for a 2,400 sq ft home: General lighting demand = 6,045 VA. Dishwasher (1,500 VA) + disposal (900 VA) + water heater (4,500 VA) = 6,900 VA × 75% = 5,175 VA. Dryer = 5,000 VA. Range (12 kW) = 8,000 VA. A/C (5 ton) = 7,200 VA. Largest motor (A/C compressor) × 25% = 1,800 VA.
Total = 6,045 + 5,175 + 5,000 + 8,000 + 7,200 + 1,800 = 33,220 VA.
Service amps (240V single-phase) = 33,220 / 240 = 138.4A → 200A service required.