Electrical Load Calculator
An electrical load calculator helps you estimate the total electrical demand of a building or circuit group to determine the minimum panel size and service entrance rating required. The National Electrical Code (NEC) Article 220 specifies the standard and optional methods for calculating residential and commercial service loads. This tool allows you to enter multiple electrical loads by wattage and voltage, applies a demand factor, and calculates the total connected load in amps and kilowatts. The result helps determine if an existing panel is adequate or what size new service to specify. Always have a licensed electrician perform final load calculations for permit applications.
Electrical load formula
Total Connected Load (W) = sum of all loads
Demand Load (W) = Total Load x (Demand Factor / 100)
Current (A) = Demand Load (W) / Voltage (V)
Min panel = standard sizes: 100, 150, 200, 400A (next size above current)
Frequently asked questions
How do I calculate total electrical load?
Total connected load (VA) = sum of all load voltages x amperages. For a 120V circuit with a 1,500W load: 1,500 / 120 = 12.5 amps. Add all circuit loads to get total connected amps. Apply demand factors from NEC Table 220.42 to get demand load. Service size must be at least 125% of the largest motor plus sum of remaining loads.
What is demand factor in electrical load calculation?
Demand factor accounts for the fact that not all loads operate simultaneously at full capacity. NEC Article 220 specifies demand factors: for example, the first 3,000 VA of lighting at 100%, the next 117,000 VA at 35%, and remaining at 25%. Applying demand factors reduces the calculated load below the total connected load.
What service panel size do I need for a house?
Most new single-family homes use 200A service. Smaller homes under 1,500 sq ft with gas appliances may use 100A. Homes with electric vehicles, pools, hot tubs, or large HVAC systems may need 200A to 400A. Calculate the NEC 220.82 optional method load to determine actual requirements.
What is the NEC optional method for dwelling unit loads?
NEC 220.82 permits calculating the dwelling service size as: (general loads at 100%) + (largest HVAC or heating load). General loads include 100% of first 10 kVA + 40% of remaining load. This method typically yields a smaller service size than the standard method and is commonly used for 200A service verification.
How is load measured in VA versus watts?
Watts (W) measure real power. Volt-amperes (VA) measure apparent power. For resistive loads (heaters, incandescent lights), VA = W. For motors and electronics, VA is greater than W due to power factor (PF). Electrical panels and conductors are sized in VA or amps; motors and equipment nameplate ratings are in watts or horsepower.
Official sources
- NFPA: NFPA 70 National Electrical Code (NEC) - Article 220: Branch-Circuit, Feeder, and Service Load Calculations.
- International Code Council: ICC International Residential Code (IRC) - Part VII: Electrical.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.