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.
Last updated: September 2026
Formula below · 2 sources (ieee.org, Wikipedia) · Updated Sep 2026
Compare with similar
About this calculator
Total electrical load is calculated by adding the load categories and applying code adjustments. Motor loads are taken at 125% of their rating (NEC 430.24 adds 25% of the largest motor; applying it to all motor load is conservative). For a single-family dwelling, the NEC 220.82 optional method counts the first 10,000 VA of general loads (lighting, receptacles, appliances and motors) at 100% and the remainder at 40%, then adds heating and air-conditioning at 100% (the NEC allows some electric heat at 65% or 40%, so 100% is conservative). Formula (NEC method): Load = min(G, 10,000) + 0.4 × max(0, G − 10,000) + HVAC, where G = General Lighting + Appliance Load + Motor Load × 1.25. Choose "100% of connected load" for non-dwelling buildings or whenever everything may run at once; commercial demand factors in NEC Article 220 Part III apply only to specific load types, so a flat factor on the whole load is not an NEC method. Dividing by the system voltage gives the required amperage for panel and service sizing.
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, NEC method. Motor load adjusted: 1,500 × 1.25 = 1,875 W. General loads: 3,000 + 5,000 + 1,875 = 9,875 W, all within the first 10,000 VA, so counted at 100%. Add HVAC at 100%: 9,875 + 4,000 = 13,875 W. At 240 V single-phase, required current = 13,875 / 240 ≈ 57.8 A, well within a 100 A service (the NEC 230.79(C) minimum for a one-family dwelling). The defaults (3,000 + 5,000 + 2,000 × 1.25 = 10,500 W general, 8,000 W HVAC) give 10,000 + 200 + 8,000 = 18,200 W, about 76 A.
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 a system’s maximum demand to its total connected load. The NEC sets specific factors rather than one blended number: for dwellings, the 220.82 optional method takes the first 10,000 VA of general loads at 100% and the rest at 40%, with heating and cooling at 100%; the standard method takes the first 3,000 VA of general lighting at 100% and the next 117,000 VA at 35%. Applying a flat 70–90% to everything, including air-conditioning, can understate the load. For commercial or industrial facilities, use 100% here or have a licensed electrical engineer do the NEC Article 220 calculation.
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.