Section 220.40/Optional Calculation for Dwelling Units
NEC 220.40 through 220.42 permit an optional (simplified) load calculation method for individual dwelling unit services and feeders as an alternative to the standard method.
The optional method permits a simplified service and feeder calculation for individual dwelling units. The general loads (lighting at 3 VA/sq ft per Table 220.12, small appliance circuits at 1500 VA each, and laundry circuit at 1500 VA) are totaled. The first 10 kVA of this general load is taken at 100%, and the remainder at 40%. To this adjusted general load, the nameplate ratings of specific appliances are added at 100%: air conditioning, heat (the larger of heating or cooling), clothes dryer, cooking equipment, and any other fastened-in-place appliance rated over 1500 VA. The total determines the minimum service or feeder ampacity. This method is only permitted for individual dwelling units served by a single set of service-entrance conductors or a single feeder, not for feeders supplying multiple dwelling units.
Why this section exists
The standard calculation method in Parts III and IV of Article 220 uses demand factors applied to individual load categories, which can be conservative and time-consuming. The optional method recognizes that in a typical dwelling, not all loads operate simultaneously. The 40% demand factor on general loads above 10 kVA reflects the statistical diversity of residential usage patterns. The result is often a lower calculated load that more accurately reflects actual peak demand while maintaining adequate safety margins. For larger homes with many circuits, the optional method frequently produces a smaller required service size than the standard method.
What plan reviewers look for
Plan reviewers verify that the optional method is only applied to individual dwelling units, not to multi-unit feeders. They check that the general load includes all required loads: lighting at 3 VA/sq ft, two small appliance circuits, and one laundry circuit. They verify the 10 kVA breakpoint is applied correctly (first 10 kVA at 100%, remainder at 40%). They confirm all nameplate-rated appliances are listed and added at 100%. They check that the larger of heating or cooling is used (not both). They verify the final ampacity against the service conductor sizing requirements.