What is the Electrical Panel Calculator used for?

The Electrical Panel Calculator is designed to help electricians, engineers, and homeowners accurately size electrical panels based on total connected load, demand factors, and applicable electrical codes. It simplifies the complex process of calculating the required breaker capacity, bus bar rating, and feeder conductor sizes to ensure safety and code compliance.

Does the calculator account for both continuous and non-continuous loads?

Yes, the calculator distinguishes between continuous loads (operating for three or more hours, such as lighting or HVAC) and non-continuous loads, applying the standard 125% derating factor to continuous loads per the National Electrical Code (NEC). This ensures that the panel’s total capacity is not exceeded under prolonged peak usage, preventing overheating and nuisance tripping.

Can I input multiple subpanels or branch circuits into the same calculation?

Absolutely, the calculator allows you to add unlimited branch circuits, subpanels, and individual loads, each with its own voltage, phase, and load type. It then aggregates these values, applies demand factors (like those for residential ranges or dryers), and outputs a comprehensive summary of total demand, minimum panel size, and spare capacity, helping you plan for future expansions.

Does the tool provide code-compliant recommendations for main breaker and bus bar ratings?

Yes, after you enter your load details, the calculator automatically recommends a minimum main breaker size and panel bus bar rating based on the calculated total demand plus the required safety margin. It references common NEC tables (such as 310.16 for conductor ampacity and 240.6 for standard breaker sizes) and flags any potential undersizing, offering a clear warning if your panel would exceed 80% of its rated capacity for continuous loads.

Electrical Panel Calculator

Last updated: 2026-09-09

Electrical Panel Calculator — Electrical panel design based on total load and supply type.

Inputs

Result

Enter values and press Calculate

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Common Examples — Click to Fill

	Contracted power (kW)	Voltage	Diversity factor
Single-family home, all-electric	9.2	230	0.8
Apartment with gas heating	5.75	230	0.6
Small commercial workshop	25	400	0.9
Office floor with cooling	18.4	400	0.75

TL;DR: To calculate an electrical panel, divide your total connected load (in watts) by the supply voltage to find the current in amps, then apply a simultaneity factor (typically 0.6) to size the main protection breaker (IGA) and select an input cable cross-section that safely carries this current.

What Is the Electrical Panel Calculator?

An electrical panel calculator is a practical tool used by electricians, installers, and homeowners to determine the correct specifications for a building's main electrical distribution panel. It translates a total connected load (the sum of all potential power consumption in watts) into the required protective device rating (measured in amps), the input cable size (measured in square millimetres), and the effective simultaneous power draw. This is not a theoretical exercise; it is a mandatory step in designing any safe and legal electrical installation, from a small apartment to a large commercial workshop.

The calculator addresses a core problem in electrical design: not every appliance runs at full power at the same time. If you size your panel based on the theoretical maximum load, you will over-specify the equipment, leading to unnecessary costs and oversized components. Conversely, if you undersize the panel, you risk overheating cables, tripping breakers, and creating a fire hazard. This tool uses a simultaneity factor to model real-world usage, ensuring the panel is both safe and economical.

This calculator is specifically designed for a single-phase or three-phase supply. It takes your total power demand and voltage, calculates the base current, applies a standard simultaneity coefficient (0.6 in this version), and then recommends the main switch (IGA) rating and the minimum cross-section for the input cable. The final output provides a clear specification that can be used to purchase the correct components or to verify an existing installation design.

How to Use the Calculator

Using this tool is straightforward and requires only three pieces of information about your planned or existing installation. Follow these steps to get your panel specifications.

Enter Total Power (kW): Input the sum of all electrical loads you intend to connect. This is the total wattage of every light, socket, appliance, and machine, divided by 1000 to convert to kilowatts. For an existing installation, this is the sum of the circuit breakers' ratings.
Enter Supply Voltage (V): Specify the line-to-neutral voltage of your electrical supply. For residential single-phase systems, this is typically 230V (as used in Europe, Australia, and parts of Asia) or 120V (in North America). For three-phase commercial systems, you will typically enter 400V (line-to-line) or 230V (line-to-neutral), depending on how you have calculated your loads.
Set Simultaneity Factor: This is a number between 0 and 1 that represents the fraction of the total load that will realistically be used at the same time. The calculator uses a default of 0.6, which is a common standard for residential installations. For a house with electric heating you might use 0.8, while for a warehouse with intermittent machinery, 0.5 might be more appropriate.

After entering these three values, the calculator performs the calculation and displays the output. The results show the simultaneous power in kilowatts, the main switch (IGA) rating in amps, and the recommended minimum cross-section for the input cable in square millimetres. The main switch rating is selected based on the contracted power capacity (which is sometimes higher than the calculated current), ensuring the panel is compatible with the utility provider's service.

Formula and Calculation Method

The calculation follows a three-step process that is standard in electrical engineering. The core principle is Ohm's Law and the power equation, which links power (watts), voltage (volts), and current (amps).

Step 1: Calculate the Total Base Current. The first step is to determine the maximum current that would flow if all loads were operating simultaneously. The formula for a single-phase system is:

Intensity (A) = Power (W) ÷ Voltage (V)

For example, if you have a connected load of 9.2kW (9200W) on a 230V supply, the theoretical maximum current is 9200W ÷ 230V = 40A. This is the current that would flow if every appliance were turned on at once.

Step 2: Apply the Simultaneity Factor. Because it is unrealistic to have all loads running at full power, we multiply the base current by the simultaneity factor. The formula is:

Simultaneous Intensity (A) = Base Intensity × Factor

Using our example: 40A × 0.6 = 24A. This 24A represents the realistic expected current draw. The simultaneous power is also calculated by multiplying the total power by the factor: 9.2kW × 0.6 = 5.52kW.

Step 3: Select the Main Switch and Cable. The final step involves choosing the appropriate protective devices. The Main Switch (IGA) is selected based on the contracted power from the utility company. In our example, even though the calculated simultaneous current is 24A, the IGA is selected as 40A to match the utility's contracted capacity (which was implied by the original 9.2kW load). The cable cross-section is then determined based on this IGA rating. A 10mm² copper cable is selected because it can safely carry up to 50A under standard installation conditions, providing a safety margin above the 40A IGA rating.

Practical Examples

The following table illustrates how different inputs affect the final output of the calculator. These are realistic scenarios that highlight the relationship between power, voltage, and the simultaneity factor.

Scenario	Total Power (kW)	Voltage (V)	Factor	Simultaneous Power (kW)	IGA Rating (A)	Cable Section (mm²)
Small Apartment	5.75	230	0.6	3.45	25	6
Detached House (All-Electric)	13.8	230	0.6	8.28	63	16
Small Workshop (Single-Phase)	11.5	230	0.6	6.9	50	10
Commercial Unit (Three-Phase)	27.6	400	0.7	19.32	40 (per phase)	10

In the first example, a small apartment with 5.75kW of connected load requires a 25A IGA and a 6mm² cable. In the second example, a large house with electric underfloor heating and an induction cooktop has a 13.8kW load, which requires a 63A main switch and a 16mm² cable. The workshop example shows an 11.5kW load, which needs a 50A IGA and a 10mm² cable. The commercial example demonstrates that for a 400V three-phase supply, the current per phase is much lower, allowing for a smaller cable despite the higher total power.

Tips for Accurate Results

To ensure your electrical panel calculation is correct and safe, you must avoid common pitfalls that lead to undersized or oversized installations. The following tips are based on the most frequent errors made during manual and calculator-based panel design.

Always Apply the Simultaneity Factor: Neglecting this factor is the single most common mistake. It leads to a massively oversized panel with a larger IGA and thicker cable than necessary. This not only wastes money but can also be a problem because an oversized IGA might not protect a smaller downstream cable. The 0.6 factor is a good starting point, but for specific installations, you must consider actual usage patterns.
Check Cable Length for Voltage Drop: The calculator provides a cable cross-section based on current-carrying capacity (thermal limit). However, if your meter or main panel is located a long distance from the utility connection point, you must check for voltage drop. A cable that is too thin for a long run will cause a voltage drop, leading to poor performance of appliances and potential overheating. As a rule of thumb, for runs exceeding 30 meters, increase the cable cross-section by one size.
Verify the Diferencial Protection: The calculator sizes the main switch (IGA) and the input cable, but it does not design the entire protection scheme. You must always include a Residual Current Device (RCD), commonly called a differential switch, in the panel. The standard requirement is a 30mA differential with a minimum rating of 40A. This protects against electric shock and must be sized to be compatible with your new IGA and cable.
Distinguish Between kW and kVA: For single-phase resistive loads, kW and kVA are effectively the same. However, if you have inductive loads (motors, transformers, fluorescent lighting), the power factor is less than 1. This means the actual current drawn will be higher than the calculation suggests. If your load includes significant inductive equipment, you should multiply the final current by 1.25 to account for the power factor and starting currents.
Use the Correct Voltage: Confirm whether your supply is single-phase (230V) or three-phase (400V). Mixing these up will result in a current calculation that is off by a factor of 1.73. If you are unsure, check your main fuse or utility meter. Entering the line-to-line voltage instead of the line-to-neutral voltage for a three-phase system is a common data-entry error.

Frequently Asked Questions

Q: What is the difference between the calculated current and the IGA rating?

The calculated current (after applying the simultaneity factor) is the expected operational current under normal conditions. The IGA rating is the maximum current the main switch can safely handle and is also the rating of the utility's contracted power. In the provided example, the calculated current was 24A, but the IGA is rated at 40A. This discrepancy exists because the IGA protects the cable and the installation against short circuits and overloads up to the contracted limit, not just the expected simultaneous load. The 40A rating provides a buffer for temporary overloads (e.g., starting a motor) and matches the utility's service capacity, which is based on the total connected load (9.2kW), not the simultaneous load.

Q: Can I use a lower simultaneity factor to save money on cable costs?

While using a lower factor (like 0.5) will result in a smaller calculated current and potentially a thinner cable, doing so is dangerous unless you are absolutely certain about your load profile. The simultaneity factor is a design standard, not a precise measurement. If you arbitrarily lower it, you risk creating a situation where the cable is undersized for the real-world current draw, leading to overheating and fire risk. The 0.6 factor is considered a safe minimum for residential use. For commercial or industrial settings, you must conduct a load analysis to justify a lower factor. The cost savings from a thinner cable are minimal compared to the cost of a potential electrical fire or the hassle of rewiring.

Q: Why does the calculator suggest a cable size, but is that all I need for the panel?

No, this calculator provides the specifications for the input section of the panel: the main cable coming from the meter and the main switch (IGA). A complete electrical panel includes several other critical components that are not calculated here. You will still need to include an RCD (differential switch) of 30mA and at least 40A. You will also need to size and install individual circuit breakers for each final circuit (lighting, sockets, oven, etc.). These branch circuits have their own cable sizes and breaker ratings, which must be calculated separately based on the expected load of each specific circuit. This calculator is the first step in the panel design process, not the final blueprint.