Wire Size for 100 Amps: NEC Ampacity & AWG Sizing Guide

The minimum standard wire size for 100 amps is 3 AWG copper or 1/0 AWG aluminum when using standard 75°C rated terminations, though voltage drop and ambient temperature can force you to upsize. Selecting the correct wire size for 100 amps dictates not just the physical diameter of the conductor, but the bending radius inside your panel, the torque required on the lugs, and the ultimate safety margin against insulation failure. When you change the wire gauge in a 100-amp circuit, you alter the thermal dissipation profile and the mechanical stress on the breaker terminals. The most common mistake DIYers and junior electricians make is confusing the wire's insulation rating with the termination rating, leading to dangerously undersized feeders.

The Core Rule: Ampacity, Temperature Columns, and Material

To understand wire sizing, you have to look at NEC Table 310.16, which lists the allowable ampacities for insulated conductors. The table is divided into temperature columns: 60°C, 75°C, and 90°C.

What people commonly confuse is the difference between the wire's insulation rating and the termination rating. You can buy THHN wire, which has a 90°C insulation rating. Looking at the 90°C column, you might think 4 AWG copper (rated 95A) or even a specialized 3 AWG setup is plenty. However, NEC 110.14(C) dictates that you must size the wire based on the lowest temperature rating of any connected component in the circuit. Since almost all standard residential breakers, lugs, and bus bars are rated for 75°C, you are legally and physically bound to the 75°C column for your baseline ampacity.

Think of wire ampacity like a multi-lane highway. The 90°C rating is the absolute maximum speed the pavement can handle before melting, but the 75°C rating is the legal speed limit posted by the traffic authority because the on-ramps (the breaker lugs) cannot safely handle the higher speed.

Note: 2 AWG Aluminum is only rated for 90A at 75°C, making it illegal for a 100A breaker unless specific continuous load derating exceptions apply, which is rare for general feeders.

Worked Example: Sizing a 100-Amp Subpanel Feeder

Let's run the math on a real-world installation. You are installing a 100-amp subpanel in a detached garage to run a welder, a compressor, and general lighting. The distance from the main panel to the subpanel is 80 feet. The system is 240V split-phase.

Step 1: Base Ampacity Sizing
We need a wire that can safely carry 100A without exceeding the 75°C rating of the breaker lugs. We select 3 AWG Copper (100A) or 1/0 AWG Aluminum (120A).

Step 2: Voltage Drop Calculation
The NEC recommends a maximum voltage drop of 3% for feeders. We use the standard voltage drop formula: VD = (2 × K × I × D) / CM

• K = 12.9 (Copper) or 21.2 (Aluminum)
• I = 100 Amps (Worst-case full load)
• D = 80 feet (One-way distance)
• CM = Circular Mils (52,620 for 3 AWG Cu; 105,600 for 1/0 AWG Al)

Scenario A: 3 AWG Copper
VD = (2 × 12.9 × 100 × 80) / 52,620 = 3.92 Volts
Percentage Drop = (3.92 / 240) × 100 = 1.63%. This is well under the 3% limit. 3 AWG Copper is perfectly acceptable.

Scenario B: 1/0 AWG Aluminum
VD = (2 × 21.2 × 100 × 80) / 105,600 = 3.21 Volts
Percentage Drop = (3.21 / 240) × 100 = 1.33%. Also excellent, and significantly cheaper to purchase.

Step 3: The Decision
If you are pulling individual wires through PVC conduit, 3 AWG Copper THHN is easier to pull due to its smaller diameter and stiffness. If you are trenching and direct-burying or running along a surface, 1/0 AWG Aluminum SER (Service Entrance) cable is vastly more economical, saving you roughly $150 to $200 on an 80-foot run based on 2026 material pricing. For the exact voltage drop parameters of your specific cable geometry, cross-reference with the Southwire Voltage Drop Calculator.

Where You Meet This in Practice

You will typically encounter the requirement for 100-amp wire sizing in three specific residential and light-commercial scenarios:

1. Detached Garage and Workshop Subpanels: Modern workshops often require 100A to simultaneously support a 50A plasma cutter, a 30A RV receptacle, and 20A general lighting circuits. 1/0 Aluminum SER is the undisputed king of these overhead or trench runs due to cost.
2. Accessory Dwelling Units (ADUs): As municipalities push for more ADUs, builders are pulling 100A feeders from the main house to the new structure to handle electric ranges, mini-split heat pumps, and electric water heaters without requiring a separate utility meter.
3. Heavy HVAC and Heat Pump Upgrades: While most central air units run on 30A to 60A breakers, large commercial-grade heat pumps or multi-zone VRF (Variable Refrigerant Flow) systems for large homes frequently require a dedicated 100A disconnect and matching feeder.