How to size a breaker

Sizing a breaker, also known as a circuit breaker, is a crucial step in ensuring the safe and efficient operation of an electrical circuit. Here's a step-by-step guide to help you size a breaker:

Step 1: Determine the Load

Identify the load you want to protect with the breaker. This includes the total current draw of all devices connected to the circuit, such as lights, appliances, and motors.

Step 2: Calculate the Total Current Draw

Add up the current draw of each device in amperes (A). You can find this information on the device's nameplate or in the manufacturer's documentation.

Step 3: Determine the Voltage

Identify the voltage of the circuit, which is usually 120/240 volts for residential applications.

Step 4: Choose the Correct Breaker Size

Use a breaker sizing chart or consult with a licensed electrician to determine the correct breaker size. The chart will provide the recommended breaker size based on the total current draw and voltage.

Breaker Sizing Chart:

Example:

Let's say you have a circuit with a total current draw of 30A, consisting of:

• 2 x 15A lights = 30A
• 1 x 10A refrigerator = 10A
• 1 x 5A TV = 5A

Total current draw: 30A + 10A + 5A = 45A

Using the breaker sizing chart, you would choose a 40A breaker to protect this circuit.

Additional Considerations:

• Inrush Current: Some devices, like motors or transformers, may have a high inrush current when they start up. You may need to add 10-20% to the total current draw to account for this.
• Voltage Drop: If the circuit is long or has a high resistance, you may need to reduce the breaker size to account for voltage drop.
• Overcurrent Protection: Make sure the breaker is rated for the maximum overcurrent protection required by the National Electric Code (NEC) or local building codes.

Remember to always follow local building codes and consult with a licensed electrician if you're unsure about sizing a breaker for your specific application.