Sub Panel Breaker Size Calculator – Find the Right Breaker for Your Electrical Sub Panel
Installing a sub panel is one of the best ways to extend electrical power safely in a home, workshop, or detached garage. But choosing the correct breaker size for that sub panel can be confusing. The right breaker protects both your wiring and equipment from overloads or faults. That’s where a sub panel breaker size calculator becomes extremely useful. It helps determine the right breaker size based on your load, wire size, and distance from the main panel.
Table of Contents
In this guide, we’ll explain how a sub panel breaker size calculator works, why it matters, and how to use it accurately. You’ll also learn how to estimate your electrical load, select proper wire sizes, and ensure your installation meets NEC (National Electrical Code) standards.
What is a Sub Panel Breaker Size Calculator?
A sub panel breaker size calculator is an online or manual tool that helps determine the right breaker size for connecting a sub panel to the main electrical panel. It considers key factors such as total load (in watts or amps), voltage, wire size, and distance.
For example, if you are installing a 60-amp sub panel in your garage, you must ensure that the feeder wire and breaker are properly rated for that load. Choosing a breaker that’s too small may cause nuisance tripping, while one that’s too large can risk overheating or fire.
The calculator automates this process by doing the math for you and recommending the correct breaker and wire size combination.
Sub Panel Breaker Size Calculator
Determine the correct breaker size for subpanels based on load, wire size, and installation conditions.
Enter the total connected load in watts, system voltage, and phase type. Add the distance to the subpanel and wire size if known. Select wire material, ambient temperature, and breaker type. Click “Calculate” to view the recommended breaker size based on NEC guidelines, including 125% safety margin and temperature derating. Use this to size subpanel feeders and ensure safe, code-compliant installations.
Why Do You Need a Sub Panel?
A sub panel acts as a smaller distribution panel connected to your main breaker panel. It is often installed in locations where additional circuits are needed, such as:
- Detached garages or workshops
- Outdoor kitchens or pool areas
- Home additions or second floors
- Commercial spaces with separate sections
Instead of running multiple long circuits from the main panel, a sub panel allows you to distribute power locally. It simplifies wiring, reduces voltage drop, and improves system safety.
Use our online tool Wire Size Calculator for Subpanels and Feeders – NEC Guidelines Included
How a Sub Panel Breaker Size Calculator Works
The sub panel breaker size calculator uses basic electrical formulas to determine the proper breaker size. The most common formula is:
Breaker Size (A) = Total Load (W) / Voltage (V)
Once you know the current (in amps), the next step is to select the nearest standard breaker rating, such as 30A, 40A, 50A, or 60A.
Let’s go through an example:
If your sub panel will supply a load of 9,600 watts on a 240V circuit:
Breaker size = 9600 / 240 = 40A
So, you would use a 40-amp breaker with suitable wire and distance considerations.
However, this simple calculation doesn’t account for voltage drop or long wire runs. That’s why modern calculators include additional parameters such as:
- Distance between panels (to account for voltage drop)
- Wire material (copper or aluminum)
- Ambient temperature
- Type of load (continuous or non-continuous)
Key Factors Affecting Sub Panel Breaker Size
Choosing the correct breaker size isn’t just about load. Several other factors play a major role.
1. Load Demand
Total load is the sum of all connected devices and appliances in the sub panel. This includes lighting, outlets, HVAC systems, and other equipment. You should calculate the expected load in watts and convert it to amps.
2. Voltage
Most sub panels are either 120/240V single-phase or 208V three-phase systems. Higher voltage reduces current draw, which affects breaker sizing.
Use our online tool Electricity Load Calculator in kVA – Accurate Power Load Estimation for Homes & Industries
3. Wire Size
The wire gauge determines how much current it can safely carry. Using a wire that’s too small for the breaker size can cause overheating.
4. Distance (Voltage Drop)
Long distances between the main panel and sub panel increase voltage drop. This requires upsizing the wire or adjusting the breaker rating.
5. Future Expansion
Always consider future load expansion. If you plan to add more circuits later, choose a slightly higher breaker size and wire capacity now.
Common Breaker and Wire Size Chart for Sub Panels
Here’s a quick reference table showing typical combinations used in residential and light commercial setups.
| Sub Panel Amperage | Recommended Wire Size (Copper) | Recommended Wire Size (Aluminum) | Breaker Size | Typical Voltage | Max Distance (ft)* |
|---|---|---|---|---|---|
| 30A | 10 AWG | 8 AWG | 30A | 240V | 50–75 |
| 40A | 8 AWG | 6 AWG | 40A | 240V | 75–100 |
| 50A | 6 AWG | 4 AWG | 50A | 240V | 100–125 |
| 60A | 6 AWG | 4 AWG | 60A | 240V | 125–150 |
| 100A | 3 AWG | 1 AWG | 100A | 240V | 150–200 |
*Approximate maximum distances before significant voltage drop occurs.
How to Use a Sub Panel Breaker Size Calculator
Using a sub panel breaker size calculator is simple. Most tools ask for the following information:
- Input your total load in watts or amps.
- Select voltage (120V, 208V, or 240V).
- Choose wire material (copper or aluminum).
- Enter the distance between main and sub panel.
- Click “Calculate.”
The calculator will return the ideal breaker size, wire gauge, and sometimes even voltage drop percentage.
This process helps you avoid guesswork and ensures compliance with NEC standards.
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Example Calculation for a 60A Sub Panel
Suppose you are installing a 60-amp sub panel in your garage located 100 feet from the main panel.
- Total load: 12,000 watts
- Voltage: 240V
- Wire type: Copper
- Distance: 100 ft
Current = 12,000 ÷ 240 = 50A
At 100 ft, voltage drop is around 3%. To stay within safe limits, it’s wise to use a 60A breaker and 6 AWG copper wire.
This setup ensures enough capacity for your load and future expansion without tripping the breaker.
NEC Guidelines for Sub Panel Breaker Sizing
The National Electrical Code provides specific rules to ensure safety:
- The feeder breaker must match or be slightly higher than the sub panel rating.
- Continuous loads should not exceed 80% of the breaker capacity.
- Ground and neutral wires must be isolated in sub panels.
- The wire size must be chosen based on temperature rating and installation method.
Always verify your installation with a licensed electrician or local building inspector.
Common Sub Panel Breaker Size Options
Sub panels come in different amperage ratings depending on the application.
| Sub Panel Rating | Ideal Application |
|---|---|
| 30A Sub Panel | Small shed, outdoor lights, small workshop |
| 50A Sub Panel | Garage with moderate tools, small apartment |
| 60A Sub Panel | Medium workshop, pool equipment, small home addition |
| 100A Sub Panel | Large workshop, secondary house, large HVAC systems |
| 125A–150A | Commercial or heavy residential use |
Selecting the right size depends on both your present and future power requirements.
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Safety Tips When Installing Sub Panel Breaker
Always prioritize safety when working with electricity.
- Turn off the main breaker before making connections.
- Use proper gauge wires as per your calculated values.
- Label circuits clearly in both panels.
- Ensure neutral and ground are isolated in the sub panel.
- Check all terminations for tightness to avoid overheating.
If you’re unsure about electrical wiring, always hire a licensed electrician.
Benefits of Using a Sub Panel Breaker Size Calculator
Using a sub panel breaker size calculator saves time, ensures safety, and reduces installation errors. It helps you:
- Avoid undersized or oversized breakers
- Select correct wire gauge for your setup
- Minimize voltage drop issues
- Comply with NEC and local codes
- Estimate installation cost and material requirements
It’s an essential tool for electricians, homeowners, and DIY enthusiasts alike.
Troubleshooting Common Issues
If your breaker trips often, it could indicate:
- The load is exceeding breaker capacity
- Incorrect wire size causing heat buildup
- Loose connections or grounding problems
Using the calculator helps identify whether your setup is appropriately sized or needs adjustment.
Use our online tool Electricity Load Calculator (kVA) According to IEC and NEC
Frequently Asked Questions
What size breaker do I need for a 100-amp sub panel?
You should use a 100A breaker and #3 AWG copper or #1 AWG aluminum wire, depending on distance.
Can I run a 60-amp sub panel off a 50-amp breaker?
No. The breaker must match or exceed the sub panel rating for safe operation.
Do I need a ground rod for a sub panel?
If it’s located in a detached building, yes. A ground rod or grounding electrode system is required.
Can I use aluminum wire for my sub panel?
Yes, aluminum wire can be used if properly sized and installed with antioxidant compound.
Conclusion
A sub panel breaker size calculator is more than just a convenience—it’s a safety tool that helps ensure your electrical system runs efficiently and within code. Whether you’re powering a garage, workshop, or backyard office, selecting the correct breaker size and wire gauge is crucial for performance and protection.
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By understanding your total load, distance, and voltage, you can use a calculator to get precise results without the risk of overloading your system. Always double-check your results with local regulations and consult a qualified electrician when in doubt.
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