The Direct Answer: What Size Wire for 100 Amps?

100amp wire refers to the specific gauge and insulation rating of electrical conductors required to safely carry a 100-ampere load without exceeding thermal limits defined by the National Electrical Code (NEC). For a standard 100-amp breaker with 75°C rated terminals, the direct answer is 3 AWG copper or 1 AWG aluminum wire.

While that sounds straightforward, simply matching the breaker size to the wire ampacity is where many DIY installations fail inspection or create fire hazards. What you choose changes the physical footprint of your installation: 3 AWG copper is incredibly stiff and requires larger conduit bends, while 1 AWG aluminum is thicker in diameter but lighter, cheaper, and requires anti-oxidant paste at the terminations. Furthermore, the most common confusion around 100A circuits is looking at the 90°C ampacity column on wire charts and assuming a smaller wire (like 4 AWG copper) is legal. It is not, due to terminal temperature limitations.

The Continuous Load Trap: If your 100A load will run continuously for 3 hours or more (like a large commercial heater or a high-draw EV charger), NEC Article 210.20 requires you to size the breaker and wire at 125% of the continuous load. That means a 100A continuous load requires a 125A breaker and wire sized for 125A (1/0 AWG Copper or 2/0 AWG Aluminum). The guide below assumes a standard 100A peak/non-continuous breaker rating.

Ampacity and Sizing Table for 100A Circuits

To understand why 3 AWG Copper and 1 AWG Aluminum are the minimums, you have to look at how the NEC derates wire based on the weakest link in the circuit: the breaker lugs. According to NEC 110.14(C), unless the breaker is explicitly marked for 90°C terminations (which almost no residential 100A breakers are), you must use the 75°C column to size your wire, even if you are using 90°C rated THHN insulation.

Material AWG Size 60°C Ampacity 75°C Ampacity 90°C Ampacity (THHN) Legal for 100A Breaker?
Copper 4 AWG 70A 85A 95A No (Fails 75°C term. rule)
Copper 3 AWG 85A 100A 115A Yes (Minimum Copper)
Aluminum 2 AWG 75A 90A 100A No (Fails 75°C term. rule)
Aluminum 1 AWG 85A 100A 115A Yes (Minimum Aluminum)
Aluminum 1/0 AWG 100A 120A 135A Yes (Use for long runs)

Think of the 90°C wire insulation like a wide pipe, but the 75°C breaker lug is a narrow valve. The system can only flow as much heat as the narrowest point allows. You buy THHN (90°C) wire for its thin profile and conduit fill advantages, but you must calculate your ampacity based on the 75°C bottleneck at the termination points.

Worked Example: Sizing a 100A Subpanel Feeder

Ampacity tables only tell half the story. If your run is long, voltage drop will force you to upsize your 100amp wire. The NEC recommends keeping voltage drop under 3% for branch circuits and feeders combined. Let us run the math on a real-world scenario: feeding a detached garage subpanel.

  • Load: 100A (Peak)
  • Voltage: 240V Single Phase
  • Distance: 150 feet (one way)
  • Proposed Wire: 3 AWG Copper (CM = 52,620 circular mils)

Using the standard single-phase voltage drop formula: VD = (2 × K × I × L) / CM

Where K is the resistivity constant for copper (12.9 ohms per mil-foot), I is current (100A), L is length (150 ft), and CM is the circular mil area of 3 AWG wire.

VD = (2 × 12.9 × 100 × 150) / 52,620 = 7.35 Volts

To find the percentage: (7.35V / 240V) × 100 = 3.06%

Result: At exactly 150 feet, 3 AWG copper pushes just over the 3% recommended limit. If this garage is 200 feet away, the drop hits 4.08% (9.8V). To maintain optimal performance for sensitive electronics and power tools in the garage, you must upsize to 1 AWG Copper or 1/0 AWG Aluminum for any 100A run exceeding 140 feet. Always verify with a dedicated voltage drop calculator before pulling wire.

Where You Meet 100Amp Wire in Practice

You will typically encounter the requirement for 100A-rated conductors in three specific residential and light-commercial scenarios:

1. Subpanel Feeders

The most common application is feeding a 100A subpanel in a detached garage, a large workshop, or a basement finish. Because these runs often exceed 50 feet, aluminum (specifically 1-1-1-3 MH feeder cable) is the industry standard here. It is roughly 40% cheaper than copper and perfectly safe when terminated correctly with Noalox anti-oxidant paste and torqued to the manufacturer's exact specifications.

2. Future-Proofed EV Charger Circuits

While most current Level 2 EV chargers draw 32A to 48A (requiring 40A or 60A breakers), high-end home charging setups and future commercial-grade residential chargers can pull 80A continuous. Installing a 100A circuit using 1/0 AWG aluminum or 3 AWG copper to a junction box allows you to upgrade the EVSE later without re-pulling wire through finished walls.

3. Older Residential Service Entrances

Homes built in the 1960s and 70s often feature 100A main service panels. The service entrance conductors (the wires coming from the utility meter to the main breaker) are typically 1 AWG aluminum or 3 AWG copper. If you are upgrading a 100A service to 200A, you cannot reuse these existing wires; the physical lugs on a 200A main breaker will not even accept the smaller 100A wire, and the ampacity is insufficient for the new service.

Common Mistakes and Code Caveats

When working with wire this thick, the physical installation is just as critical as the math on the page. Avoid these common jobsite errors:

  • Ignoring Torque Specifications: Since the 2017 NEC update (110.14(D)), you are legally required to use a calibrated torque screwdriver or wrench when terminating breakers and lugs. A 100A lug typically requires between 40 and 50 inch-pounds of torque. Under-torquing causes arcing and fires; over-torquing snaps the lug screw or crushes the aluminum strands, creating a high-resistance hot spot.
  • Stripping Too Much Insulation: 3 AWG and 1 AWG wires are thick. If you strip back 1.5 inches of insulation but the breaker lug only requires 0.75 inches, you leave bare, energized copper exposed inside the panel. Use the stripping gauge printed on the breaker label.
  • Skipping Anti-Oxidant Paste on Aluminum: Aluminum oxidizes rapidly when exposed to air, creating a resistive layer that generates heat. You must apply a UL-listed anti-oxidant compound (like Noalox) to the stripped aluminum strands before inserting them into the breaker lug. Do not use it on copper-to-copper connections.
  • Conduit Fill Violations: If you are pulling individual THHN conductors for a 100A circuit (two hots, one neutral, one ground), you are dealing with very thick wires. According to standard conduit fill charts, three current-carrying 3 AWG copper wires will max out a 1-inch PVC Schedule 80 conduit. If you add a neutral for a multi-wire setup, you must step up to 1.25-inch conduit to prevent jamming and insulation damage during the pull.

Getting 100amp wire sizing right means respecting both the thermal limits of the breaker terminals and the physical realities of voltage drop over distance. Stick to the 75°C column for your baseline ampacity, run the voltage drop math for any distance over 50 feet, and always torque your lugs to the manufacturer's printed specifications.