Electrical Load Calculation Guide
How to calculate electrical load for residential and commercial installations
What is Electrical Load Calculation?
Electrical load calculation determines the total power demand of a building's electrical system. It's required for sizing service panels, breakers, wire, and generators. The NEC provides standard methods in Article 220.
Residential Load Calculation (NEC 220.82)
Step 1: General Lighting and Receptacles
Calculate based on square footage:
General Lighting = Square Footage × 3 VA/sq ft
Example: 2,000 sq ft × 3 VA = 6,000 VA
Step 2: Small Appliance Branch Circuits
Minimum 2 small appliance circuits at 1,500 VA each = 3,000 VA
Step 3: Laundry Circuit
1 laundry circuit at 1,500 VA
Step 4: Major Appliances
| Appliance | Typical VA | Connected Load |
|---|---|---|
| Electric Range | 8,000-12,000 | Nameplate |
| Electric Dryer | 5,000 | Nameplate or 5,000 VA min |
| Water Heater | 4,000-4,500 | Nameplate |
| Dishwasher | 1,200 | Nameplate |
| Disposal | 800 | Nameplate |
| HVAC (3-ton AC) | 5,000-7,000 | Nameplate |
| EV Charger | 7,200-9,600 | 40A × 240V |
Step 5: Apply Demand Factors
The NEC allows demand factors to reduce the calculated load:
| Load Type | First | Remaining | Demand Factor |
|---|---|---|---|
| General lighting & receptacles | 3,000 VA | Remaining | 35% |
| Small appliance & laundry | — | All | 100% |
| Electric range | 8,000 VA | Remaining | 75% (or per table) |
| Electric dryer | — | All | 100% |
| HVAC (largest motor) | — | — | 125% |
Step 6: Sum and Size Service
Add all demand-adjusted loads and divide by 240V to get service amperage:
Service Amps = Total VA / 240V
Example: 38,400 VA / 240V = 160A → Use 200A service
Commercial Load Calculation
Commercial calculations follow NEC 220.82(B) for non-dwelling occupancies:
- General lighting: 1-3 VA/sq ft depending on occupancy type
- Receptacles: 180 VA per outlet, demand factor per NEC Table 220.42
- HVAC: Connected load × 125% for largest motor
- Cooking equipment: Per NEC Table 220.56
Generator Sizing from Load Calculation
Once you have your total connected load, size your generator:
- Running watts = Total connected load × 0.7 (diversity factor)
- Starting watts = Running watts × 1.5 (for motor loads)
- Generator size = Starting watts × 1.2 (safety margin)
| Home Size | Calculated Load | Generator Size | Recommended Model |
|---|---|---|---|
| 1,200 sq ft | 15-20 kW | 10-14 kW | Generac 7258 (14kW) |
| 2,000 sq ft | 25-30 kW | 18-22 kW | Generac 7324 (18kW) |
| 3,000 sq ft | 35-45 kW | 22-26 kW | Generac 7290 (26kW) |
| 4,000+ sq ft | 50+ kW | 26-30 kW | Generac 7282 (28kW) |