Electrical Load Calculator
Calculate total electrical load for residential or commercial buildings per NEC Article 220.
Results
How to Use This Calculator
Enter the total square footage of the dwelling. The calculator applies the NEC general lighting and receptacle load of 3 VA per square foot. Add specific appliance loads: enter the nameplate ratings for the range (typically 8000W to 12000W), dryer (5000W), water heater (4500W), dishwasher (1200W), and any other fixed appliances. Enter the air conditioning load and heating load separately, as the NEC allows you to omit the smaller of the two per NEC 220.60. The calculator applies all applicable demand factors from NEC Article 220, including the general lighting demand factor from NEC Table 220.42 and the range demand factor from NEC Table 220.55. For a 2000 square foot home with a 12 kW range, 5 kW dryer, and 4.5 kW water heater, expect a calculated demand of approximately 25 to 30 kVA.
Understanding the Concept
An electrical load calculation determines the total power demand of a building to properly size the service entrance, main breaker, and feeder conductors. The NEC requires load calculations for all new construction and service upgrades per Article 220. The calculation is not simply adding up every load in the building, because not all loads operate simultaneously. The NEC provides demand factors that statistically reduce certain loads to reflect actual usage patterns. For example, the general lighting load uses a declining demand factor: the first 3000 VA at 100%, and the remainder at 35% per NEC Table 220.42 for dwellings. Range loads for household cooking equipment follow NEC Table 220.55, which can reduce a 12 kW range to as little as 8 kW for a single unit. The dryer load uses a demand factor from NEC Table 220.54. These demand factors prevent oversizing the electrical service, which saves the homeowner money on equipment and utility connection fees while still providing adequate capacity for real world usage.
The Formula Explained
The standard NEC dwelling service load calculation per Article 220 Part III follows this sequence. General lighting and receptacle load equals the floor area in square feet multiplied by 3 VA per square foot per NEC 220.12, plus 1500 VA for each 20A small appliance branch circuit (minimum two per NEC 220.52(A)), plus 1500 VA for the laundry circuit per NEC 220.52(B). Apply the demand factor from NEC Table 220.42: 100% of the first 3000 VA and 35% of the remainder. Add fixed appliances at 100% if three or fewer, or at 75% if four or more per NEC 220.53. Apply NEC Table 220.55 for ranges and cooking equipment. Apply NEC Table 220.54 for dryers. Add the larger of heating or air conditioning load per NEC 220.60. The total gives the minimum service demand in VA. Divide by the system voltage (240V for single phase residential) to get the minimum service amperage.
Frequently Asked Questions
What size electrical service do I need for my house?
Most modern homes require a minimum 200A service. The NEC requires a load calculation per Article 220 to determine the actual minimum. For homes under 2000 square feet with gas appliances, a 100A or 150A service may suffice. Homes with all electric appliances, including an electric range, dryer, water heater, and heat pump typically need 200A. Larger homes over 3000 square feet or homes with multiple HVAC systems, pools, or workshops may require a 320A or 400A service.
What is the 3 VA per square foot rule in the NEC?
NEC 220.12 Table 220.12 requires a minimum general lighting load of 3 VA per square foot for dwelling units. This covers general illumination and general use receptacles throughout the home. The square footage is calculated from the outside dimensions of the building, not including open porches, garages, or unfinished spaces not adaptable for future use. This base load is then subject to the demand factors in NEC Table 220.42.
Do I count both heating and air conditioning in a load calculation?
No. NEC 220.60 allows you to omit the smaller of the heating load or the air conditioning load, because they do not typically operate simultaneously. You include only the larger of the two in your calculation. For example, if the air conditioning load is 5000W and the heating load is 8000W, you include only the 8000W heating load. This provision applies to both the standard and optional calculation methods.
What are demand factors and why are they used?
Demand factors are reduction percentages applied to connected loads to reflect the fact that not all electrical loads in a building operate at full capacity at the same time. Without demand factors, every electrical service would be massively oversized. The NEC establishes demand factors based on statistical analysis of actual power usage. For example, the general lighting demand factor in NEC Table 220.42 recognizes that a homeowner never turns on every light and plugs in every appliance simultaneously. Demand factors vary by load type and are only used for service and feeder calculations, not for individual branch circuits.
What is the difference between the standard and optional load calculation methods?
The NEC provides two methods for dwelling unit load calculations. The standard method in Article 220 Part III calculates each load category separately with individual demand factors. The optional method in NEC 220.82 is simpler and often yields a smaller result. It takes the first 10 kVA of general loads at 100% and applies a 40% demand factor to everything above 10 kVA, then adds heating or air conditioning at 100%. The optional method can only be used for existing dwellings served by a 120/240V or higher service. Both methods are accepted by inspectors.