Sub Panel Breaker Size Calculator

How to Use This Calculator

Using our sub panel breaker size calculator is simple:

1. Enter the total load in amperes (A) that your sub panel will serve
2. Select the voltage of your electrical system (typically 120V or 240V)
3. Click "Calculate" to determine the recommended breaker size
4. Review the result and any additional recommendations

The calculator will provide the appropriate breaker size based on standard electrical sizing practices and safety margins.

What Is a Sub Panel?

A sub panel, also known as a subcircuit breaker panel, is an electrical distribution board that connects to the main electrical panel. It's used to distribute power to specific areas or circuits within a building.

Common uses for sub panels include:

• Serving multiple circuits in a single area (e.g., a garage with outlets and lights)
• Providing power to outdoor circuits
• Creating separate circuits for different electrical loads
• Adding capacity to an existing electrical system

Calculating the Correct Breaker Size

The proper breaker size for a sub panel is determined by several factors:

1. The total load in amperes (A) that the sub panel will serve
2. The voltage of the electrical system (120V or 240V)
3. Local electrical codes and regulations
4. Manufacturer recommendations for the sub panel

The general rule is to size the breaker at least 125% of the total load, rounded up to the nearest standard breaker size.

Standard breaker sizes typically follow the sequence: 15, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 125, 150, 200, 225, 250, etc. amperes.

Common Mistakes to Avoid

When sizing a sub panel breaker, avoid these common errors:

• Using the same breaker size as the main panel - this can overload the sub panel
• Ignoring voltage requirements - different voltages need different breaker sizes
• Not accounting for future electrical needs - always leave some capacity for expansion
• Using non-standard breaker sizes - these may not provide proper protection
• Skipping proper grounding and bonding - essential for safety

Always consult with a professional electrician for complex electrical installations.

Example Calculation

Let's calculate the proper breaker size for a sub panel serving a total load of 45 amperes at 120V:

1. Calculate 125% of the total load: 45 × 1.25 = 56.25 amperes
2. Round up to the nearest standard breaker size: 60 amperes
3. Verify with local electrical codes and manufacturer recommendations

Therefore, a 60-amp breaker would be appropriate for this sub panel.