Electrical Load Calculator — Free Online Calculator
Calculate your home's total electrical load to determine the correct service panel size per NEC Article 220.
How to Use This Calculator
Enter home square footage, total 240V appliance wattage (range, dryer, water heater), and largest HVAC load.
The Formula Explained
NEC 220 standard calculation: General lighting = sqft × 3 VA + 3000 (small appliance) + 1500 (laundry). Apply demand: first 10kW at 100%, remainder at 40%. Add 240V appliances and largest motor load at 100%.
Load Calculations: The Engineering Foundation
Load calculations are the starting point for any electrical service design. Without accurate load calculations, you cannot size the service entrance, the feeders, the transformer, or the main breaker. You cannot predict voltage drop, evaluate power factor, or design protective devices. Every legitimate electrical installation begins with a load calculation, and every NEC-compliant project documents the calculation in writing for review by the AHJ (authority having jurisdiction).
The NEC provides three main methods for residential calculations and occupancy-specific methods for commercial and industrial. The choice of method affects the final number; the optional method typically gives 20-30% lower values than the standard method for new construction. For existing buildings, the load can also be verified by actually measuring demand over a representative period — for 200A service upgrade decisions, 30 days of demand logging can show the real numbers.
Worked Example: Small Restaurant Service Sizing
A 2,500 sq ft restaurant with commercial kitchen: 5 refrigerators/freezers, 2 commercial ovens, 1 fryer, dishwasher, 2 HVAC units (rooftop), commercial lighting, exhaust hood with makeup air, and 30 receptacles.
General lighting (NEC Table 220.12 restaurants): 2,500 × 2 = 5,000 VA. Receptacles: 30 × 180 = 5,400 VA.
Kitchen equipment (NEC 220.56 allows demand factors): 2 ovens at 16,000 VA each, fryer 10,000 VA, dishwasher 8,000 VA, 5 refrigeration 2,000 VA each. Total nameplate: 60,000 VA. Demand factor for 4+ pieces: 65%. Kitchen load: 39,000 VA.
HVAC: 2 × 10,000 = 20,000 VA. At 100% for cooling (summer-dominant in this climate).
Total: 5,000 + 5,400 + 39,000 + 20,000 = 69,400 VA. At 208V three-phase: 69,400 / (1.732 × 208) = 193 amps. Service size: 200A minimum, though 225A or 250A gives better future margin.
Worked Example: Industrial Motor Circuit
A machine shop feeder serving: 25 HP motor (FLA 34A from NEC Table 430.250), 15 HP motor (FLA 21A), 10 HP motor (FLA 14A), and 7.5 HP motor (FLA 11A). All three-phase 460V.
Per NEC 430.24: feeder sized at 125% of the largest motor plus 100% of remaining motors. Largest: 25 HP at 34A × 1.25 = 42.5A. Remaining: 21 + 14 + 11 = 46A. Total feeder: 42.5 + 46 = 88.5 amps.
Feeder conductor: 3 AWG copper (100A at 75°C) or larger. Main feeder breaker: next standard size above largest motor inrush protection. Short-circuit protection sized per NEC 430.52 (typically 250% of largest motor FLA plus 100% of others) = 34 × 2.5 + 46 = 131A, round up to 150A feeder breaker (standard size).
Compare to pure sum without the 125% rule: 34 + 21 + 14 + 11 = 80A, leading to 90A or 100A feeder breaker. The 125% motor rule adds about 10% capacity above simple sum to accommodate starting surges.
Five Load Calculation Mistakes
1. Using wrong occupancy type for general lighting VA/sq ft. Residential is 3 VA/sq ft, offices are 3.5, warehouses are 0.25, restaurants 2. Getting this wrong throws off the whole calculation.
2. Forgetting demand factors. Summing every nameplate without applying demand factors yields 50-100% over-calculation. Always check for applicable demand factors.
3. Missing the largest motor rule. NEC 430.24 is easy to miss. For feeders with motors plus other loads, the 125% motor factor and 125% continuous rule may both apply.
4. Using single-phase formulas on three-phase services. Commercial services are usually three-phase. Use I = VA / (1.732 × V_LL), not I = VA / V.
5. Not documenting the calculation. The AHJ wants to see how you arrived at your numbers. Keep calculation sheets for every project, with code references for each step. "Because I said so" does not pass inspection.
Common Occupancy Lighting Load (NEC Table 220.12)
Unit VA per square foot for general lighting: Armories and auditoriums: 1. Banks: 3.5. Barber shops: 3. Churches: 1. Clubs: 2. Court rooms: 2. Dwelling units (residential): 3. Garages: 0.5. Hospitals: 2. Hotels and motels: 2. Industrial buildings: 2. Lodge rooms: 1.5. Office buildings: 3.5. Restaurants: 2. Schools: 3. Stores: 3. Warehouses: 0.25.
These values are the minimum for general lighting assuming standard fixture density. Specific fixture loads (task lighting, display lighting, etc.) add on top. For energy code compliance, LPD (lighting power density) limits may further constrain allowed lighting load, but the NEC minimum still governs wiring sizing.
NEC Code References
NEC 220.10 — branch circuit load calculation requirements. NEC 220.40 — feeder load calculation general. NEC 220.84 — service calculations for dwelling units. NEC 220.87 — existing installation load determination using actual measured maximum demand.
NEC 220 Part III — Standard method for dwelling units. NEC 220 Part IV — Optional method for dwelling units, schools, farms. NEC 220 Part V — Farm loads. NEC Table 220.12 — General lighting loads by occupancy. NEC Table 220.14 — Other loads all occupancies. Work through these systematically for any commercial project.
Electrical load: building it up circuit by circuit
Calculating electrical load from the bottom up means adding individual loads (appliances, circuits, equipment) and applying demand factors that account for the unlikelihood of simultaneous operation. Used for service sizing, generator sizing, and transformer sizing in residential and small commercial contexts.
The formula and what it does
List each load with its watts/VA rating. Apply demand factors per NEC 220 tables (35 percent on general lighting above 3000 VA, 75 percent on appliances if four or more, specific factors for ranges and dryers). Add largest motor at 125 percent of FLA. Sum gives total connected and demand-corrected load.
Worked example
Scenario: Small office, 1200 sq-ft, 3 PCs at 300 W each, server with 600 W UPS load, central AC at 4 kW, water heater 4.5 kW, two 1500 W space heaters (winter only).
General lighting: 1200 sq-ft x 3.5 VA (NEC office) = 4200 VA. Receptacles per 220.14(I): 1 VA/sq-ft x 1200 = 1200 VA. AC: 4000 VA at 125 percent largest motor = 5000 VA. Water heater: 4500 VA. Space heaters: 2 x 1500 = 3000 VA (heat at 100 percent because non-coincident with cooling). Total: 17,900 VA. At 240 V: 75 A. 100 A service sufficient.
Common mistakes to avoid
undefinedFrequently asked questions
Why apply demand factors?
Real-world simultaneous operation is rare. NEC has decades of data showing that not all loads run at once. Demand factors avoid massive over-sizing of service and transformers, saving cost without compromising reliability.
How is residential different from commercial?
Different demand factor tables. NEC 220 Part III for dwellings, 220.42-220.44 for non-dwellings. Commercial generally has higher per-sq-ft lighting loads but lower factors on appliance-style equipment.
What about peak demand on commercial bills?
Utility demand charges (commercial) are based on actual peak 15-minute kW, not calculated load. Load calculation sets minimum infrastructure size; demand charge is what you actually pay for peaks.
How do I handle a kitchen with multiple cooktops?
NEC 220.55 Table for ranges and cooktops by number and total kW. The demand factor decreases as count increases.
Where do EV chargers fit in load calculations?
NEC 625.41 requires 100 percent of EV charger rating in the load calculation. No demand factor allowed.
How precise are demand factors?
They are statistical, not exact. Real load can briefly exceed calculated load. The 125 percent largest-motor adder and round-up to next standard service size give margin.