This calculator provides an estimate for the total electrical load of a commercial building based on the National Electrical Code (NEC) standard calculation methods. Accurately determining the electrical load is a critical first step in designing a safe, reliable, and code-compliant electrical system for any commercial space, from offices and retail stores to restaurants.
208V
240V
480V
Three-Phase
Single-Phase
function calculateElectricalLoad() {
// 1. Get Input Values
var area = parseFloat(document.getElementById('buildingArea').value);
var receptacles = parseFloat(document.getElementById('receptacleCount').value);
var hvac = parseFloat(document.getElementById('hvacLoad').value);
var kitchen = parseFloat(document.getElementById('kitchenLoad').value);
var specialty = parseFloat(document.getElementById('specialtyLoad').value);
var voltage = parseFloat(document.getElementById('systemVoltage').value);
var phase = document.getElementById('systemPhase').value;
var resultDiv = document.getElementById('result');
// 2. Validate Inputs
if (isNaN(area) || isNaN(receptacles) || isNaN(hvac) || isNaN(kitchen) || isNaN(specialty)) {
resultDiv.innerHTML = 'Error: Please enter valid numbers in all load fields. Use 0 if a load is not present.';
return;
}
if (area <= 0 || voltage <= 0) {
resultDiv.innerHTML = 'Error: Building Area and Voltage must be greater than zero.';
return;
}
// 3. Calculate Individual Loads based on NEC principles
// General Lighting Load (NEC 220.12 – using 3 VA/sq.ft for office/store)
var lightingLoadVA = area * 3;
// Receptacle Load (NEC 220.14(I) – 180 VA per outlet)
var receptacleLoadVA = receptacles * 180;
// 4. Apply Demand Factors (NEC 220.42, 220.44)
// This is a simplified application.
var lightingDemandLoad = lightingLoadVA; // For simplicity, taking lighting at 100% as it's often continuous.
var receptacleDemandLoad;
if (receptacleLoadVA <= 10000) {
receptacleDemandLoad = receptacleLoadVA; // First 10kVA at 100%
} else {
receptacleDemandLoad = 10000 + (receptacleLoadVA – 10000) * 0.5; // Remainder at 50%
}
// Fixed appliance loads are typically taken at 100% or have their own factors. We'll sum them.
var fixedAppliancesLoad = hvac + kitchen + specialty;
// 5. Calculate Total Demand Load in VA
var totalDemandVA = lightingDemandLoad + receptacleDemandLoad + fixedAppliancesLoad;
// 6. Calculate Required Amperage
var calculatedAmps;
var sqrt3 = 1.732;
if (phase === 'single') {
calculatedAmps = totalDemandVA / voltage;
} else { // three-phase
calculatedAmps = totalDemandVA / (voltage * sqrt3);
}
// Continuous loads require a 125% multiplier (NEC 210.19(A)(1))
var finalAmps = calculatedAmps * 1.25;
// 7. Determine Standard Service Size
var standardSizes = [100, 125, 150, 175, 200, 225, 250, 300, 350, 400, 500, 600, 800, 1000, 1200, 1600, 2000, 2500, 3000];
var recommendedServiceSize = 0;
for (var i = 0; i = finalAmps) {
recommendedServiceSize = standardSizes[i];
break;
}
}
if (recommendedServiceSize === 0) {
recommendedServiceSize = "Over 3000A (Requires custom engineering)";
}
// 8. Display Results
var outputHTML = 'Total Calculated Load (after demand factors): ' + totalDemandVA.toFixed(0) + ' VA';
outputHTML += 'Minimum Required Amperage (at 125%): ' + finalAmps.toFixed(1) + ' Amps';
outputHTML += 'Recommended Standard Service Size:' + recommendedServiceSize + ' Amps';
resultDiv.innerHTML = outputHTML;
}
How to Use the Calculator
To estimate your commercial electrical service needs, simply fill in the fields above. Provide details about your building's size, the number of electrical outlets, and the power requirements (in Volt-Amperes or VA) for major systems like HVAC and kitchen equipment. Select your building's electrical service voltage and phase, then click "Calculate" to see the results.
Understanding the Inputs
Total Building Area (sq. ft.): This is used to calculate the general lighting load. The NEC specifies a minimum Volt-Ampere (VA) per square foot based on the building type. This calculator uses 3 VA/sq. ft., common for offices and retail stores.
Number of General Receptacles: This accounts for the power used by devices plugged into standard outlets. The NEC mandates a load of 180 VA for each receptacle.
Major Equipment Loads (VA): These are for large, fixed appliances. You should find the VA or Wattage rating on the equipment's nameplate. This includes HVAC (heating, ventilation, air conditioning), commercial kitchen equipment, large motors, or other specialty machinery. If the rating is in Watts (W), you can use it as a close approximation for VA for this estimation.
Service Voltage & Phase: This is the electrical system configuration provided by the utility. In the US, 208V 3-Phase is very common for commercial buildings, while smaller buildings might use 240V Single-Phase. 480V 3-Phase is used for larger industrial facilities. This is critical for converting total VA to Amperage.
Understanding the Results
Total Calculated Load (VA): This is the total power requirement in Volt-Amperes after applying NEC "demand factors." Demand factors account for the fact that not all lights and equipment will be running at full power simultaneously, preventing costly oversizing of the electrical service.
Minimum Required Amperage: This is the result of converting the total VA load into Amperes (Amps) based on your system's voltage and phase. It is multiplied by 125% to account for continuous loads, as required by the NEC for safety.
Recommended Standard Service Size: Electrical services are supplied in standard amperage ratings (e.g., 200A, 400A, 800A). This result rounds your minimum required amperage up to the next available standard size, which is the service you would need to install.
Example Calculation
Consider a 7,500 sq. ft. office building with 100 general receptacles, a 30,000 VA HVAC system, and no other major loads. The service is a standard 208V 3-Phase system.
1. Lighting Load: 7,500 sq. ft. * 3 VA/sq. ft. = 22,500 VA
2. Receptacle Load: 100 outlets * 180 VA/outlet = 18,000 VA
3. Receptacle Demand: 10,000 VA + (18,000 – 10,000 VA) * 50% = 14,000 VA
4. Total Demand Load: 22,500 VA (Lighting) + 14,000 VA (Receptacles) + 30,000 VA (HVAC) = 66,500 VA
5. Calculated Amps: 66,500 VA / (208V * 1.732) = 184.5 Amps
6. Final Amps (at 125%): 184.5 Amps * 1.25 = 230.6 Amps
7. Recommended Service: The next standard size up from 230.6A is 250 Amps.
Disclaimer: This calculator is for informational and educational purposes only. Electrical load calculations for actual projects are complex and must be performed by a qualified, licensed electrician or electrical engineer in accordance with the National Electrical Code (NEC) and all local regulations. Do not use these results for construction, permitting, or any real-world electrical system design.