Breaker Run Calculator

Determine the correct breaker size for your electrical circuit with our breaker run calculator. This tool helps electricians and DIYers select the appropriate circuit breaker based on voltage, amperage, and conductor size.

What is a breaker run?

A breaker run refers to the process of selecting and installing a circuit breaker that matches the electrical load requirements of a circuit. Proper breaker sizing ensures safety and prevents overheating of wires and equipment.

Key factors in breaker run calculations include:

Voltage of the electrical system
Amperage rating of the circuit
Conductor size (wire gauge)
Type of circuit (residential, commercial, industrial)
Local electrical codes and standards

Using the correct breaker size is crucial for electrical safety. An undersized breaker can cause overheating and fire hazards, while an oversized breaker won't provide adequate protection.

How to use this calculator

Our breaker run calculator provides a simple interface to determine the appropriate circuit breaker size. Follow these steps:

Enter the voltage of your electrical system (typically 120V or 240V)
Input the amperage rating of your circuit
Select the conductor size (wire gauge)
Choose the circuit type (residential, commercial, or industrial)
Click "Calculate" to get the recommended breaker size

The calculator will display the appropriate breaker size based on your inputs and electrical code requirements.

Formula used

The breaker size is determined using the following formula:

Breaker Size = (Amperage × 125%) / (Conductor Size × Voltage)

Where:

Amperage is the current rating of the circuit
Conductor Size is the wire gauge (in AWG)
Voltage is the system voltage (in volts)

This formula accounts for the 125% derating factor used in electrical calculations to ensure safety margins.

Worked example

Let's calculate the breaker size for a 20-amp circuit with 12-gauge wire at 120V:

Breaker Size = (20 × 1.25) / (12 × 120) Breaker Size = 25 / 1440 Breaker Size ≈ 0.01736

Since breaker sizes are typically whole numbers, we would select a 15-amp breaker for this circuit.

This example demonstrates how the calculator applies the formula to determine the appropriate breaker size.

Frequently Asked Questions

Why is the 125% derating factor used in breaker calculations?: The 125% factor accounts for potential increases in current due to voltage drops, temperature variations, and other factors to ensure safety margins.
What happens if I use a breaker that's too small for my circuit?: An undersized breaker can cause overheating, which may lead to fires or damage to electrical equipment. Always use a breaker that's properly sized for your circuit.
Can I use the same breaker size for both 120V and 240V circuits?: No, the voltage rating of the breaker must match the circuit voltage. A 120V breaker cannot be used on a 240V circuit and vice versa.
How often should I check my circuit breakers?: It's recommended to inspect circuit breakers annually to ensure they're functioning properly and to check for any signs of wear or damage.
What should I do if my breaker keeps tripping?: If a breaker trips frequently, it may indicate an overloaded circuit. Check for excessive power usage, faulty appliances, or wiring issues that may need repair or replacement.