Wire Size Calculator for Subpanels and Feeders – NEC Guidelines Included

Understanding Subpanel Wire Size and Its Importance

A subpanel is a secondary breaker panel. It’s typically installed in locations like garages, workshops, or outbuildings. It helps distribute power safely across different areas of a building or property.

Choosing the wrong subpanel wire size can cause voltage drop, overheating, or even fire. On the other hand, oversized wires cost more and are harder to install. So, it’s essential to strike the right balance between safety, efficiency, and cost.

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Wire Size Calculator for Subpanels and Feeders – NEC Guidelines Included
Wire Size Calculator for Subpanels and Feeders – NEC Guidelines Included

The NEC (National Electrical Code) provides guidance on how to size wires correctly for feeders and subpanels. These rules change slightly from one edition to the next. This guide focuses on NEC 2023 standards.

Key Takeaways

  • Choosing the correct subpanel wire size ensures safety, prevents overheating, and keeps your installation compliant with the National Electrical Code (NEC).
  • Proper feeder cable size depends on load, distance, and voltage drop considerations.
  • Use an accurate NEC feeder wire calculator to simplify sizing for subpanels in detached garages, workshops, or outbuildings.
  • Copper or aluminum conductor choice also influences wire size.
  • Learn how to size feeder wires for both 100A, 125A, 150A, and 200A subpanels based on NEC 2023 guidelines.

Use our free online tool Amps to Wire Size Calculator – Choose the Right Cable for 10A, 20A, 40A Loads

What is a Feeder Wire and Why Its Size Matters?

A feeder wire connects the main service panel to a subpanel. It’s different from branch circuit wires, which connect outlets and fixtures. Feeder cables must carry more current over longer distances. Hence, choosing the correct feeder cable size is critical.

The size depends on three major factors:

  • Ampacity (how much current the wire carries)
  • Distance from the main panel (longer distances mean more voltage drop)
  • Type of wire (copper or aluminum)

How to Use a NEC Feeder Wire Calculator

A NEC feeder wire calculator simplifies the complex formulas used for load calculation, voltage drop, and wire size selection. Here’s how it usually works:

  1. Input Load (in Amps): This is the maximum load your subpanel will handle.
  2. Voltage: Typically 120/240V for residential applications.
  3. Distance: From main panel to subpanel.
  4. Conductor Material: Copper or Aluminum.

The calculator then provides the minimum wire size based on NEC ampacity tables and voltage drop limits.

Now let’s explore what NEC actually says about these parameters.

Use our free online tool Wire Size Calculator for Solar Panels – Avoid Power Loss in Off-Grid and Hybrid Systems

NEC Guidelines for Subpanel and Feeder Cable Sizing

According to NEC 2023:

  • Feeder conductors must be sized according to Article 310.16 (formerly 310.15(B)(16))
  • Voltage drop should not exceed 3% for feeders
  • Ampacity adjustments must consider ambient temperature and number of conductors

NEC Feeder Wire Size Table (Copper Conductors)

Subpanel AmperageMinimum Copper Wire SizeDistance ≤ 50 ftDistance 50–100 ft (Voltage Drop Adjusted)Breaker Size
60 Amps#6 AWGYesUse #4 AWG60A
100 Amps#4 AWGYesUse #2 AWG100A
125 Amps#2 AWGYesUse #1/0 AWG125A
150 Amps#1 AWGYesUse #2/0 AWG150A
200 Amps2/0 AWGYesUse 3/0 AWG200A

NEC Feeder Wire Size Table (Aluminum Conductors)

Subpanel AmperageMinimum Aluminum Wire SizeDistance ≤ 50 ftDistance 50–100 ft (Voltage Drop Adjusted)Breaker Size
60 Amps#4 AWGYesUse #2 AWG60A
100 Amps#2 AWGYesUse #1/0 AWG100A
125 Amps#1/0 AWGYesUse 2/0 AWG125A
150 Amps2/0 AWGYesUse 3/0 AWG150A
200 Amps3/0 AWGYesUse 4/0 AWG200A

These values already include adjustments based on common distances and residential voltage drop standards.

Use our free online tool Free Electrical Wire Size Chart & Calculator – Find the Right Cable Every Time

Subpanel Wire Size for Detached Garage or Workshop

Let’s take a practical example. Suppose you want to install a 100A subpanel in a detached garage 80 feet away.

  • Load: 100 Amps
  • Distance: 80 feet
  • Wire Type: Copper
  • Voltage: 240V

From the table above, #4 AWG copper is standard, but due to the longer distance, you must use #2 AWG copper to reduce voltage drop.

For aluminum, instead of #2 AWG, you would go with #1/0 AWG aluminum to stay on the safe side.

Always add 20% margin when sizing for workshops, especially if tools like welders or compressors are used.

Voltage Drop Consideration – Don’t Ignore It

Voltage drop happens when current travels through long wire runs. It causes lights to dim and motors to overheat.

According to NEC, the total voltage drop in feeder + branch circuit should not exceed 5%. For feeders alone, it’s limited to 3%.

Use the formula:

Voltage Drop (%) = (2 × Length × Amperes × Resistance per 1000 ft) ÷ (Voltage × 1000)

To keep voltage drop under control:

  • Increase wire size
  • Use copper instead of aluminum
  • Minimize bends and junctions

A NEC feeder wire calculator does this behind the scenes.

Feeder Cable Size for 100 Amp Subpanel – Detailed Insight

Copper:

  • For distances under 50 feet, #4 AWG copper is sufficient.
  • For 50–100 feet, upgrade to #2 AWG.
  • Beyond 100 feet, use #1/0 AWG to prevent voltage issues.

Use our free online tool Online Wire Gauge Calculator for 1 Phase and 3 Phase Load – Amp and Distance Based

Aluminum:

  • Aluminum has higher resistance.
  • Use #2 AWG aluminum up to 50 feet.
  • Use 1/0 AWG beyond 80 feet for consistent performance.

Subpanel Wire Size for 200 Amp Load

A 200A subpanel is common in large homes or separate buildings with HVAC units, heaters, or EV chargers.

  • Copper: Use 2/0 AWG minimum
  • Aluminum: Use 3/0 AWG minimum
  • For distances over 100 ft, bump up to 3/0 copper or 4/0 aluminum

Always ensure your main breaker supports this load, and your meter panel can handle the feed.

How to Calculate Feeder Load as per NEC

Before picking wire size, calculate total connected load.

Use this formula:

Feeder Load (Watts) = Lighting Load + Receptacle Load + Appliance Load + HVAC Load + Continuous Load × 1.25

Then:

Amps = Total Load (Watts) ÷ Voltage

Use the amp value in a NEC feeder wire calculator to get the exact size.

Copper vs Aluminum – Which Conductor to Choose?

Copper:

  • Higher conductivity
  • Less prone to oxidation
  • More expensive
  • More flexible and easier to pull

Aluminum:

  • Cheaper
  • Lightweight
  • Requires antioxidant paste at connections
  • Larger size needed for same current

For indoor short runs, copper is ideal. For long outdoor feeds, aluminum can be more cost-effective.

Use our free online tool What Gauge Wire Do I Need? Free Online Calculator for Electricians & Homeowners

Common Mistakes in Subpanel Wire Sizing

  • Ignoring voltage drop: Especially for long distances
  • Using wire rated for less than 75°C or 90°C: May result in de-rating
  • Overloading feeder wire: Always future-proof your installation
  • Skipping permit and inspection: Subpanel wiring must meet NEC code

Final Thoughts on Subpanel Wire Sizing

Properly sizing subpanel wire and feeder cable protects your property and appliances. It ensures your installation meets NEC standards and runs efficiently.

Using a reliable NEC feeder wire calculator helps take the guesswork out of the process. Always double-check your wire size based on distance, load, and conductor type. Voltage drop should never be ignored.

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