How to Size a Panel Board

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Some large buildings have multiple service panels.

A panel board, otherwise known as an electrical service panel, is the central control center for the power to your home. Usually secured behind a metal door, the panel houses a fuse or circuit breaker for each circuit. Sometimes, you may want to add something to the panel. To ensure the panel can handle the load, if is important to size the panel board before you add anything additional. Calculating the load helps you determine the amperage needed and available. If you aren't comfortable calculating this on your own, it is best to call an electrician to complete the work to prevent injury or damage to your home.


Step 1

Determine the entire load to the board. This can be done by adding the maximum kilowatts of each input to the circuit board, including all appliances and power receptacles. Multiply this load by 1,000 to convert from kilowatts to watts.

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Step 2

Check the voltage of the service board. This is typically listed on the board. You will need to know the maximum voltage and the phasing. The maximum voltage is typically the number following the slash, with the number of phases immediately following the voltage. As an example, a circuit board may be rated 120/208v 3ø, 4w. The 208 indicated a maximum voltage of 208, and the 3ø shows it is a 3-phase board.

Step 3

Multiply the voltage by the square root of the number of phases. Multiply this number by the National Electrical Code safety factor, which is 125 percent or 1.25.

Step 4

Divide the load to the board as determined in Step 1 by the product in Step 3. This will give you the amperage needed for the electrical panel board.


Step 5

Determine the next size load available on an electrical panel board. They only come in certain increments, so you may need to round up to the nearest size. Always round up.

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