Sub Panel Wire Size Calculator

When installing a sub panel, choosing the correct wire size is one of the most important safety and performance decisions you’ll make. An undersized wire can lead to overheating, voltage drop, or even fire hazards, while an oversized wire may increase project costs unnecessarily.

🔌 What Is a Sub Panel?

A sub panel is a secondary electrical panel that receives power from the main breaker panel and distributes it to circuits in a specific area, such as a garage, shed, basement, or guest house. It’s an extension of your home’s electrical system.

To connect it safely, the wire gauge must be adequate to handle the current (amps) over the given distance without causing excessive voltage drop or overheating.

🛠️ What the Sub Panel Wire Size Calculator Does

This calculator helps you determine the minimum recommended wire size (AWG) by evaluating:

• Amperage Load (e.g., 60A, 100A, 125A)
• Distance from the main panel to the sub panel (in feet)
• Voltage Level (typically 120V, 240V, or 208V)
• Allowable Voltage Drop (commonly 3% or less)
• Wire Type (copper or aluminum)

It simplifies a complex calculation and provides a safe, NEC-aligned recommendation in seconds.

📋 How to Use the Sub Panel Wire Size Calculator

Step-by-Step Instructions:

1. Enter the Sub Panel Load in Amps
   Example: 60A or 100A depending on your sub panel rating.
2. Enter the Distance from the Main Panel
   This is the one-way distance (in feet) the wire will run.
3. Select Voltage
   Usually 240V for residential panels.
4. Select Wire Material
   Choose between copper or aluminum (copper conducts better, but costs more).
5. Choose Allowable Voltage Drop
   Typically 3% for most installations.
6. Click Calculate
   The tool returns the recommended minimum wire gauge (AWG) needed.

🔧 Wire Sizing Formula (Plain Text)

While the calculator does the work for you, here’s the general formula used to calculate wire size based on voltage drop:

Voltage Drop (VD) = (2 × Length × Amps × Resistance) / 1000

Then:

Wire Size = Based on allowable voltage drop limit (typically ≤ 3%)

Where:

• Length = one-way distance in feet
• Amps = load current
• Resistance = ohms per 1000 ft for the wire type and size
• Use NEC tables for copper and aluminum resistance values

You compare the voltage drop to the allowable threshold and select the smallest wire gauge that keeps the drop below the limit.

📊 Example Calculations

Example 1: 100A Sub Panel, 75 Feet, 240V, Copper Wire

• Load: 100A
• Distance: 75 feet
• Voltage: 240V
• Material: Copper
• Allowable drop: 3%

✅ Result: Minimum wire size = #3 AWG Copper or #1 AWG Aluminum
(But many electricians prefer #1 AWG Copper for voltage stability.)

Example 2: 60A Sub Panel, 150 Feet, 240V, Aluminum Wire

• Load: 60A
• Distance: 150 feet
• Material: Aluminum
• Voltage: 240V

✅ Result: Recommended wire = #2 AWG Aluminum

⚙️ Common Wire Sizes for Sub Panels

Note: Distances over 50–75 feet usually require upsizing by one or two gauges.

🔒 Safety & Code Considerations

• Follow NEC (National Electrical Code) requirements.
• Use THHN/THWN wires in conduit or SER cable for sub panels.
• Sub panels must have separate neutral and ground bars.
• Always check local code and consult a licensed electrician.
• Oversize wire when in doubt, especially for long runs.

💡 Pro Tips for Accurate Wire Sizing

• Measure the actual one-way run, not just the distance between panels.
• Choose copper for better conductivity if budget allows.
• Account for future upgrades—oversize the wire slightly.
• Avoid exceeding a 3% voltage drop, especially for sensitive electronics.
• Use conduit rated for the wire size to ensure proper installation.

👷 Who Should Use This Calculator?

• 🏠 Homeowners installing sub panels in garages, basements, or additions
• 🛠️ Electricians and contractors planning electrical upgrades
• 🔌 DIYers seeking a safe, code-compliant sub panel setup
• 🏢 Property managers assessing infrastructure loads
• 🧰 Appliance installers needing sub panels for spas, EV chargers, or HVAC units

🤔 20 Frequently Asked Questions (FAQs)

1. What wire size do I need for a 100 amp sub panel?

Typically #3 AWG Copper or #1 AWG Aluminum, depending on distance.

2. Does wire size depend on distance?

Yes. Longer runs require thicker wires to prevent voltage drop.

3. Is copper better than aluminum?

Yes, copper has better conductivity, but it’s more expensive.

4. What’s the max voltage drop allowed?

Generally 3% is acceptable for sub panels.

5. Can I run 100A on 6 gauge wire?

No. That’s too small and would be unsafe.

6. What’s the difference between THHN and SER wire?

THHN is individual conductor wire; SER is bundled and often used for feeders.

7. How deep should I bury sub panel wire?

Typically 18–24 inches, but local code applies.

8. Do I need a ground rod for a sub panel?

Yes, especially for detached structures.

9. Can I use the same neutral and ground in a sub panel?

No. Neutrals and grounds must be isolated in sub panels.

10. Is 2 AWG aluminum okay for 100A panel?

Yes, if the distance is short. Otherwise, consider upsizing.

11. Does ambient temperature affect wire sizing?

Yes. Heat reduces ampacity—adjustments may be necessary.

12. Can I use the calculator for 208V setups?

Yes, just select 208V under the voltage input.

13. Is upsizing wire a good idea?

Yes, especially if you anticipate increased loads later.

14. Can I use this calculator for 3-phase power?

No, this tool is for single-phase residential panels.

15. Do I need a permit to install a sub panel?

Yes, in most jurisdictions.

16. Is wire size affected by conduit fill?

Not directly, but code limits how many wires can go in a conduit of a certain size.

17. What’s the maximum distance for 6 AWG wire at 60A?

Usually around 50–60 feet for copper. Beyond that, go to 4 AWG.

18. Is voltage drop dangerous?

Yes, it can cause inefficiency or damage equipment.

19. Can I use the calculator offline?

No, it requires an internet connection for online access.

20. Is this calculator free?

Yes, it’s 100% free to use anytime online.

✅ Final Thoughts

Choosing the correct wire gauge for a sub panel isn’t just about performance—it’s about safety. The Sub Panel Wire Size Calculator helps you make fast, informed, and code-compliant decisions for your electrical installations.