100 Amp Sub Panel Wiring Size: Exact AWG, Breaker, and Voltage Drop Guide

The correct 100 amp sub panel wiring size is 3 AWG copper or 1 AWG aluminum THHN/THWN-2, protected by a 100A double-pole breaker. This baseline assumes a 4-wire feeder, 75°C terminations, 30°C ambient temperature, and a single conduit run.

The Baseline Assumptions

Before pulling any wire, we must lock in the physical and environmental variables. Electrical sizing is not a guessing game; it is a math equation based on fixed parameters. If your installation deviates from the assumptions below, your wire size will change.

Ampacity and the 75°C Column Rule

A common mistake on the jobsite is looking at the 90°C column on NEC Table 310.16 because the wire insulation (THHN) is rated for 90°C. However, the terminations inside your main panel and subpanel lugs are almost universally rated for 75°C. Per NEC 110.14(C), you must size the wire based on the lowest temperature rating of any connected component.

As the table shows, 3 AWG copper and 1 AWG aluminum both hit exactly 100A in the 75°C column, making them the minimum legal sizes for a 100A breaker.

Voltage Drop: When to Upsize Your Feeder

The NEC recommends (but does not strictly mandate for feeders) a maximum voltage drop of 3% for branch circuits and feeders to ensure equipment operates efficiently. For a 240V system, a 3% drop equals 7.2 volts.

Let us run a voltage drop check at a stated distance of 150 feet using 3 AWG copper. Assuming a realistic continuous load of 80A (80% of the 100A breaker rating):

• Formula: VD = (2 x K x I x D) / CM
• K (Copper): 12.9 ohms
• I (Current): 80A
• D (Distance): 150 ft
• CM (Circular Mils for 3 AWG): 52,620

Calculation: (2 x 12.9 x 80 x 150) / 52,620 = 5.88 Volts.
5.88V / 240V = 2.45% drop.

At 150 feet, 3 AWG copper passes the 3% rule comfortably. However, if your run extends to 200 feet, the drop becomes 7.84V (3.26%). At that distance, you must upsize to 2 AWG copper or 1/0 AWG aluminum to maintain power quality, especially if the subpanel will run heavy inductive loads like a well pump or air compressor.

Decision Tree: Choosing Your Exact Feeder

Use this decision path to lock in your materials list. Do not mix and match; follow the logic to its terminal recommendation.

Default Concrete Pick: For 90% of standard residential garage or workshop subpanels located within 100 feet of the main service, buy four individual THHN/THWN-2 conductors: two 3 AWG Black (Hots), one 3 AWG White (Neutral), and one 8 AWG Green (Ground).

Why Not 4 AWG Copper? (The 'One Size Smaller' Trap)

Many DIYers look at 4 AWG copper and assume it is close enough, or they rely on the NEC 240.4(B) 'next size up' rule, which allows you to protect a wire with the next standard breaker size if the wire's ampacity does not perfectly match a standard breaker.

Here is why that fails for a 100A panel: 4 AWG copper in the 75°C column is rated for 85 amps. The standard breaker sizes (per NEC 240.6) are 80A, 90A, and 100A. If your wire is rated 85A, the 'next size up' breaker is 90A, not 100A. Therefore, you cannot legally protect 4 AWG copper with a 100A breaker. You must step up to 3 AWG, which hits exactly 100A, perfectly matching the breaker without invoking the next-size-up exception.

Variables That Change the Answer

Your wire size must increase if any of the baseline assumptions are violated. Watch for these three jobsite realities:

• Conductor Bundling: If you pull two separate 240V circuits in the same conduit, you now have 4 current-carrying conductors. Per NEC Table 310.15(C)(1), you must derate the ampacity to 80%. A 3 AWG wire (100A) derates to 80A, meaning it can no longer be used on a 100A breaker. You would need to upsize to 1 AWG copper.
• High Ambient Temperatures: If your conduit runs through an unventilated attic in a southern climate where ambient temperatures exceed 113°F (45°C), you must apply a temperature correction factor (0.82 for 75°C wire). 100A x 0.82 = 82A. Again, you must upsize the wire.
• Aluminum Terminations: If you choose aluminum to save money, you must use an anti-oxidant compound (like Noalox) on the lugs and torque them to the manufacturer's exact inch-pound specification. Aluminum creeps under pressure and oxidizes, which can cause high-resistance arcing if not installed perfectly.

When an Engineer or the AHJ Must Confirm

While the National Electrical Code (NEC) provides the framework, local Authorities Having Jurisdiction (AHJ) have the final say. You must pull a permit and have an inspector or licensed engineer review your plans if:

For standard residential outbuildings, stick to 3 AWG copper or 1 AWG aluminum, torque your lugs with a calibrated inch-pound screwdriver, and keep your neutral and ground bars isolated in the subpanel.