Why Load Calculations Matter

Electrical load calculations determine the size of your conductors, overcurrent protection, and service entrance. Undersize them and you risk overloaded circuits and fire hazards. Oversize them and you waste money on unnecessary copper and equipment. The NEC (Article 220) provides the methodology.

Residential Load Calculation (Standard Method)

For a typical single-family home, the standard method under NEC 220.82 follows these steps:

  1. General lighting and receptacle load: 3 VA per square foot of living space
  2. Small appliance circuits: 1,500 VA per circuit (minimum 2 required in kitchen)
  3. Laundry circuit: 1,500 VA
  4. Apply demand factors: First 3,000 VA at 100%, remainder at 35%
  5. Fixed appliances: Add 100% of nameplate rating for each (dishwasher, disposal, etc.)
  6. Dryer: 5,000 VA minimum or nameplate, whichever is larger
  7. Cooking equipment: Use Table 220.55 for demand factors on ranges
  8. HVAC: Add the larger of heating or cooling load (not both)
  9. EV charger: Add per NEC 220.57 if present

Total the VA, divide by the service voltage (240V for residential), and you get the ampere demand. Most homes need a 200A service.

Commercial Load Calculation

Commercial buildings use a different approach. The NEC provides unit loads by occupancy type in Table 220.12. An office building, for example, is calculated at 3.5 VA/sq ft for general lighting.

Commercial calculations must also account for:

  • Motor loads (Article 430) — size at 125% of full-load current
  • HVAC equipment — largest motor at 125%, others at 100%
  • Elevator feeders
  • Panel and switchboard demand

Feeder and Service Sizing

Once you have the calculated load in amperes, select a conductor from NEC Table 310.16 (for 60°C or 75°C rated conductors). The conductor must have an ampacity at or above the calculated load, and the overcurrent protective device must not exceed the conductor's ampacity.

Using a Load Calculator App

Apps like Electrical Load Calculator and Commercial Electrical Design walk you through these calculations step by step, helping you avoid errors and producing code-compliant results for permit applications and design documentation.