Electrical Load Calculator
Compute the total electrical demand of a building by summing lighting, appliance, HVAC, and motor loads, then applying an NEC-compliant demand factor. Used by electricians and engineers to size service panels and main breakers.
About this calculator
Total electrical load is calculated by adding individual load categories and applying code-required adjustments. Motors must be derated at 125% of their nameplate current to account for starting surges, per NEC Article 430. A demand factor (typically between 0.5 and 1.0) reflects the statistical reality that not all loads operate simultaneously at full capacity. The formula is: Total Load (W) = (General Lighting + Appliance Load + HVAC Load + Motor Load × 1.25) × Demand Factor. The result represents the maximum expected demand the electrical service must supply. Dividing by the system voltage gives the required amperage for panel and service sizing. Accurate load calculations prevent nuisance tripping, overloaded conductors, and costly panel upgrades after installation.
How to use
Example home: general lighting = 3,000 W, appliances = 5,000 W, HVAC = 4,000 W, motor load (pool pump) = 1,500 W, demand factor = 0.8. Motor load adjusted: 1,500 × 1.25 = 1,875 W. Sum before demand factor: 3,000 + 5,000 + 4,000 + 1,875 = 13,875 W. Apply demand factor: 13,875 × 0.8 = 11,100 W. At 240 V single-phase, required current = 11,100 / 240 ≈ 46.25 A. A 60 A service panel section would comfortably cover this load with headroom for future additions.
Frequently asked questions
What is a demand factor and how do I choose the right value for my electrical load calculation?
A demand factor is the ratio of the maximum demand of a system to its total connected load, expressed as a decimal between 0 and 1. It acknowledges that not every circuit is running at full capacity at the same time. NEC Table 220.42 provides standard demand factors for lighting loads — for example, the first 3,000 VA is taken at 100%, the next 117,000 VA at 35%. For a conservative residential estimate, 0.8 is commonly used. For commercial or industrial facilities, consult NEC Article 220 or a licensed electrical engineer for the applicable factor.
Why is motor load multiplied by 1.25 in an electrical load calculation?
Electric motors draw significantly more current during startup than during normal running — often 6–8 times the full-load current for a brief moment. NEC Article 430.24 requires that the largest motor in a group be sized at 125% of its full-load current to ensure the circuit can handle this inrush without nuisance tripping or conductor overheating. Applying the 1.25 multiplier to the motor's nameplate wattage in a load calculation provides the equivalent safety margin at the planning stage, before individual motor circuits are designed in detail.
How do I convert total wattage from a load calculation into panel amperage requirements?
For a single-phase system, divide total watts by the system voltage: A = W / V. For a 240 V single-phase service, 12,000 W requires 50 A. For a three-phase system, the formula is A = W / (V × √3 × power factor), where √3 ≈ 1.732. Once you have the required amperage, select the next standard breaker or service size above that value — common sizes are 100, 125, 150, 200, and 400 A. Always include a minimum 20–25% safety margin above your calculated demand to allow for future load growth.