Global Electrical Load Calculator

Commercial-grade sizing for Residential (Homes) and Commercial (Offices) worldwide. Support for US (NEC), European, and Asian voltage standards (120V - 415V). Use the appliance library to quickly estimate your total load.

Habitable space (Base Lighting Load)

Base Loads (NEC 220.52)

Kitchen/Dining (1500VA each)

Washing Area (1500VA each)

Interactive Appliance List

Total Appliance Load: 0 Watts

Non-Coincident Loads (Compare Heat vs AC)

Commercial Loads

Computers, Printers, Desk equipment

Signage, Exterior Lighting (125%)

Motors, HVAC (100%)

Engineering Guide: Electrical Load Calculation

1. Fundamentals of Load Calculation

Load calculation is the critical first step in electrical design. It determines the size of the service entrance conductors, the main breaker panel, and the transformer required to supply power to a building. The primary goal is to balance safety (preventing overloads and fires) with economic efficiency (avoiding vastly oversized equipment).

There are two types of loads to consider:

  • Connected Load: The simple arithmetic sum of the nameplate ratings of every single electrical device in the building. If you turned on every light, plugged in every appliance, and ran every motor simultaneously, this would be the connected load. Sizing a service for this value is usually unnecessary and prohibitively expensive.
  • Demand Load: The statistically adjusted load that accounts for "diversity." It is highly improbable that the oven, dryer, AC, water heater, toaster, and every light bulb will be active at the exact same moment. Codes like the NEC and IEC provide "Demand Factors" to reduce the connected load to a realistic peak value.

2. Global Voltage Standards & Phase Systems

Understanding your local voltage system is crucial for accurate amperage calculation ($Amps = Watts / Volts$).

North America (Split-Phase & Wye)

  • Residential (120/240V): Homes receive two 120V "legs" and a neutral. Large appliances (dryers, ranges, AC) connect across both legs for 240V, while lights and outlets use one leg for 120V.
  • Commercial (208Y/120V): A 3-phase system common in offices. It provides 208V for motors and 120V for outlets. Note that 208V is lower than residential 240V, meaning heaters will produce 25% less heat unless rated for 208V.
  • Industrial (480Y/277V): Used for heavy machinery and commercial lighting. Lighting often runs on 277V.

Europe, Asia, Australia (IEC Standards)

  • Residential (230V): Most homes receive a single phase of 230V (Line to Neutral). Unlike the US split-phase, all appliances run on this single voltage.
  • Commercial/Industrial (400V/415V): A 3-phase system providing 400V (Line-to-Line) and 230V (Line-to-Neutral). This allows powerful motors to run efficiently while supplying standard office power.

3. NEC Article 220: The "Standard Method"

The National Electrical Code (NEC) Article 220 outlines the prescriptive method for calculation used in the US. Key sections include:

  • 220.12 (General Lighting): Assigns a VA/sq ft value based on occupancy (e.g., 3 VA for homes, 3.5 VA for offices). This covers all general lights and convenience outlets.
  • 220.52 (Small Appliance & Laundry): Specific to homes. Requires 1500 VA circuits for kitchens and laundry areas to handle high-draw devices like toasters and irons.
  • 220.53 (Appliance Derating): If a home has 4 or more fixed appliances (disposal, water heater, etc.), you can apply a 75% demand factor, recognizing they likely won't all cycle on at once.
  • 220.55 (Electric Ranges): Electric cooking allows for massive diversity. A 12kW range is calculated at only 8kW because elements cycle on and off to maintain temperature.
  • 220.60 (Non-Coincident Loads): Sizing for the "worst case" scenario between heating and cooling. You don't need capacity for both simultaneously.

4. Commercial Considerations

Commercial calculations differ significantly from residential ones due to the nature of the loads.

Continuous Loads (NEC 215.2)

A continuous load is defined as a load expected to run for 3 hours or more (e.g., store lighting, server room cooling). The service conductors and overcurrent protection must be sized at 125% of the continuous load rating. This extra 25% buffer prevents the breaker from tripping thermally under long-duration load.

Kitchen Equipment (NEC 220.56)

Commercial kitchens are energy-intensive. However, fryers, ovens, and warmers are thermostatically controlled. The code allows aggressive demand factors:

  • 3 Units: 90%
  • 4 Units: 80%
  • 5 Units: 70%
  • 6+ Units: 65%

5. Three-Phase Math Explained

For 3-phase systems, calculation errors are common if the $\sqrt{3}$ factor is ignored. The relationship between Total Power (kVA) and Line Current (Amps) is:

$$ I = \frac{kVA \times 1000}{V_{L-L} \times 1.732} $$

For example, a 50 kVA load at 208V 3-phase draws 139 Amps. The same 50 kVA load at 240V 1-phase would draw 208 Amps. This efficiency is why 3-phase is standard for commercial buildings.

6. Frequently Asked Questions (FAQ)

1. What is the difference between Amps, Watts, and Volts?
Think of electricity like water in a pipe. Volts (V) is the pressure pushing the water. Amps (A) is the volume of water flowing (current). Watts (W) is the total power or work being done. The relationship is $Watts = Volts \times Amps$. Service panels are sized in Amps, but loads are often rated in Watts.
2. Why do we use Demand Factors instead of total load?
It is economically wasteful to size a service for a scenario that will likely never happen (everything ON at once). Demand factors are based on decades of statistical data showing peak usage patterns. Using them allows for smaller wires, smaller transformers, and lower installation costs without compromising safety.
3. What is the "3 VA per square foot" rule (NEC 220.12)?
For dwelling units, the NEC mandates a baseline load calculation of 3 Volt-Amperes (roughly Watts) for every square foot of habitable space. This calculation covers general illumination and general-use receptacle outlets, ensuring enough capacity for lamps, TVs, vacuums, etc., without listing every single bulb.
4. How do I calculate load for a 3-Phase commercial building?
First, sum all loads in kVA (kilovolt-amperes). Then, convert the total kVA to Amps using the 3-phase formula: $Amps = (kVA \times 1000) / (Voltage \times 1.732)$. Simply dividing by Voltage (like in single-phase) will give an incorrect, oversized result.
5. Does a 100 Amp panel mean I can use 100 Amps continuously?
Generally, no. Standard breakers are rated for 80% continuous loading. A 100A service can handle momentary peaks of 100A, but should not exceed 80A for periods longer than 3 hours. "100% rated" equipment exists but is specialized and more expensive.
6. How does the calculator handle Air Conditioning vs. Heating?
Per NEC 220.60, non-coincident loads (loads that cannot run simultaneously) do not need to be added together. The calculator compares your input for AC and Heating and automatically adds only the larger of the two to the total service demand.
7. What is the difference between Single Phase and Three Phase?
Single-phase power (common in homes) has two power wires (L1, L2) and a neutral, creating a pulsating power wave. Three-phase power (common in commercial) has three power wires (L1, L2, L3) offset by 120 degrees, creating constant, smooth power ideal for efficient motor operation and heavy loads.
8. How do I size the Neutral Conductor?
The neutral carries the unbalanced load. In a 240V system, if L1 has 50A and L2 has 40A, the neutral carries 10A. NEC 220.61 allows demand factors for the neutral (e.g., 70% for ranges) and excludes 240V-only loads (like AC units) since they don't use the neutral.
9. Can I use this for a subpanel or just the main service?
Yes, this tool can calculate the load for a subpanel. Treat the area served by the subpanel (e.g., "Second Floor" or "Garage") as the total area and input only the specific appliances fed by that panel to determine the required feeder size.
10. Why is the Kitchen load calculated differently?
Kitchens have high power density due to heating elements. In homes, the NEC mandates fixed 1500VA small appliance circuits. In commercial kitchens, NEC 220.56 provides aggressive demand factors because chefs rarely have every fryer, oven, and mixer running at 100% power simultaneously.