Residential NEC Load Calculator (Standard Method)
Calculate the required electrical service size for a single-family dwelling based on NEC Article 220.
General Lighting & Small Appliances
Fixed Appliances & Equipment
HVAC (Largest Load Only)
*Calculations follow NEC Table 220.42 and 220.55 guidelines.
Understanding the NEC Load Calculation for Residential Service
Calculating the electrical load for a home is a critical step for electricians and engineers to ensure the main service entrance panel, conductors, and grounding systems are sized correctly according to the National Electrical Code (NEC). Using the "Standard Method" (NEC Article 220, Part III), we determine the Volt-Amperes (VA) required and then convert that to Amperage.
Key Components of the Calculation
1. General Lighting and Receptacles
The NEC requires a calculation of 3 Volt-Amperes (VA) per square foot of living space. This covers general lighting and common-use wall receptacles. It is important to use the outside dimensions of the dwelling when determining square footage.
2. Small Appliance and Laundry Circuits
Per NEC 220.52, a dwelling must have at least two 20-amp small appliance branch circuits (1,500 VA each) and one 20-amp laundry circuit (1,500 VA). These are added to the general lighting load before applying demand factors.
3. Demand Factors (Table 220.42)
Because not every light and outlet in a house is used simultaneously, the NEC allows for a "Demand Factor":
- The first 3,000 VA is calculated at 100%.
- The next 3,001 to 120,000 VA is calculated at 35%.
- Anything above 120,000 VA is calculated at 25%.
Fixed Appliances and HVAC
Appliances like dishwashers, water heaters, and garbage disposals are added. If there are four or more fixed appliances, a 75% demand factor may be applied to the total of those specific appliances (NEC 220.53). For heating and cooling, the NEC allows you to omit the smaller of the two loads since they rarely run at the same time (non-coincident loads).
Example Calculation
Imagine a 2,000 sq. ft. home with a 12kW range and a 5kW dryer:
- Lighting: 2,000 * 3 = 6,000 VA
- Circuits: (2 * 1,500) + 1,500 = 4,500 VA
- Subtotal: 10,500 VA. Apply factors: (3,000 * 100%) + (7,500 * 35%) = 5,625 VA.
- Add Range: 8,000 VA (Standard demand for 12kW range).
- Add HVAC: Larger of AC or Heat.
Total VA divided by 240 Volts gives you the minimum service amperage required (e.g., 100A, 150A, or 200A).