Wire size for a 100A circuit refers to the minimum American Wire Gauge (AWG) cross-sectional area required to safely carry 100 amps of current without exceeding the thermal limits of the wire insulation or equipment terminations. For a standard residential or commercial installation, the direct answer is 1 AWG copper or 1/0 AWG aluminum. Getting this right dictates how well your circuit dissipates heat under continuous load and how much voltage is lost over the physical length of the run. The most common mistake DIYers and junior apprentices make is confusing the wire's 90°C insulation rating with the 75°C temperature limit of standard breaker lugs, leading to undersized wires that overheat and fail at the termination point.

The Core Rule: NEC Ampacity and the Termination Bottleneck

When sizing conductors, we rely on NEC Table 310.16, which lists the allowable ampacities for insulated conductors based on their material (copper or aluminum) and temperature rating (60°C, 75°C, or 90°C). Modern wire like THHN/THWN-2 is manufactured with 90°C insulation. However, NEC Article 110.14(C) mandates that you must size your wire based on the lowest temperature rating of any connected component in the circuit.

Virtually all standard 100A breakers, panelboard lugs, and disconnect switches are rated for 75°C terminations. Therefore, you must use the 75°C column in Table 310.16 to determine your baseline wire size, regardless of the fact that your THHN wire can technically handle more heat.

The Toll Booth Analogy: Think of the 90°C wire insulation as a six-lane highway, but the breaker lug is a toll plaza only built to handle four lanes of traffic (75°C). No matter how wide the highway is, traffic must bottleneck to pass through the toll plaza. If you push 100 amps through a wire sized only for the 90°C column (like 3 AWG copper), the bottleneck at the 75°C lug will overheat, melt the terminal, and cause a fire.

Looking at the 75°C column for copper, 1 AWG is rated for 130A, which comfortably covers a 100A load. For aluminum, 1/0 AWG is rated for 120A. (Note: 2 AWG copper is only rated for 115A in the 90°C column, but drops to 100A exactly in the 75°C column. Because NEC 240.4(B) allows the next standard breaker size up only if the ampacity doesn't match a standard breaker, and 100A is a standard breaker size, you cannot use 2 AWG copper for a 100A breaker if the load is continuous or if you want a safety margin for voltage drop. 1 AWG is the correct baseline).

Worked Numeric Example: Sizing a 100A Feeder at 150 Feet

Ampacity tables assume a standard ambient temperature of 30°C (86°F) and a short run where voltage drop is negligible. In the real world, distance introduces resistance, which causes voltage drop. NEC recommends a maximum voltage drop of 3% for feeders to maintain efficiency and prevent equipment malfunctions.

Let's calculate the voltage drop for a 100A, 240V feeder running 150 feet from a main panel to a detached workshop subpanel using 1 AWG copper.

  • Formula: Voltage Drop (VD) = (2 × K × I × L) / Circular Mils (CM)
  • K (Copper constant): 12.9
  • I (Current): 100 Amps
  • L (One-way length): 150 Feet
  • CM (1 AWG Copper): 83,690

Calculation:
VD = (2 × 12.9 × 100 × 150) / 83,690
VD = 387,000 / 83,690 = 4.62 Volts

To find the percentage: (4.62V / 240V) × 100 = 1.92%. This is well under the 3% NEC recommendation, meaning 1 AWG copper is perfectly adequate for a 150-foot run.

What if the run is 250 feet?
VD = (2 × 12.9 × 100 × 250) / 83,690 = 7.70 Volts (3.2%).
Because 3.2% exceeds the 3% guideline, we must bump up to the next size: 1/0 AWG copper (CM = 105,600).
Recalculating: VD = (2 × 12.9 × 100 × 250) / 105,600 = 6.10 Volts (2.54%). This brings us back into compliance.

Where You Meet This in Practice

You will encounter the 100A wire sizing requirement in several specific residential and light commercial scenarios:

  • Detached Garage or Workshop Subpanels: A 100A subpanel is the standard baseline for a detached garage running power tools, lighting, and a mini-fridge. You will pull 4 wires (two hots, one neutral, one ground) through PVC conduit using individual THWN-2 conductors.
  • Hardwired EV Chargers: While many Level 2 chargers run on 40A or 50A circuits, high-speed residential chargers (like the Tesla Wall Connector configured for maximum output or commercial-grade units) can require up to 80A continuous draw. NEC 210.19(A)(1) requires conductors to be sized at 125% of continuous loads (80A × 1.25 = 100A), pushing you directly into 100A wire sizing territory.
  • Whole-House Electric Heating or Tankless Water Heaters: Large electric tankless water heaters often require multiple 40A breakers, but some centralized electric boiler systems or older 100A service drops utilize this exact wire gauge for their primary feed.

Copper vs. Aluminum: Cost and Termination Trade-offs

When pulling 100A feeders, the cost difference between copper and aluminum becomes massive. Here is how they compare in a real-world 2026 market context.

Criteria 1 AWG Copper (THHN/THWN-2) 1/0 AWG Aluminum (XHHW-2)
Baseline Ampacity (75°C) 130 Amps 120 Amps
Approx. Cost per Foot $3.50 - $5.00 $1.20 - $1.80
Termination Prep Strip and torque Requires wire brushing and anti-oxidant paste (e.g., Noalox)
Conduit Fill / Stiffness Smaller diameter, very stiff to bend Larger diameter, easier to bend in sweeps
Lug Compatibility Universal Must verify lugs are AL/CU rated
Safety Warning: If you choose aluminum to save money, you MUST use an anti-oxidant compound on the terminations and torque the lugs to the manufacturer's exact inch-pound specification using a calibrated torque screwdriver or wrench. Loose aluminum connections expand and contract under load, eventually arcing and causing fires. For detailed safety standards, refer to the NFPA National Electrical Code and OSHA electrical safety guidelines.

Frequently Asked Questions

Can I use 2 AWG wire for a 100 amp breaker?

In most standard installations, no. While 2 AWG copper has an ampacity of 115A in the 90°C column, its rating in the 75°C column (which governs breaker terminations) is exactly 100A. NEC 240.4(B) allows you to round up to the next standard breaker size if the wire ampacity doesn't match a standard breaker, but since 100A is already a standard breaker size, you cannot round up. Furthermore, if your load is continuous (running for 3 hours or more), you must derate the wire by 125%, meaning 2 AWG copper is severely undersized. Always use 1 AWG copper for a 100A breaker to ensure compliance and account for minor voltage drops.

What size ground wire do I need for a 100A feeder?

According to NEC Table 250.122, the minimum size equipment grounding conductor for a 100A overcurrent device is 8 AWG copper or 6 AWG aluminum. However, if you had to increase your ungrounded (hot) conductors to compensate for voltage drop over a long distance, NEC 250.122(B) requires you to proportionally increase the size of your ground wire as well. For example, if you bumped your hot wires from 1 AWG to 1/0 AWG for a long run, you should bump your copper ground wire from 8 AWG to 6 AWG to maintain the same ratio of fault-current capacity.

Does a 100A subpanel need a main breaker?

If the 100A subpanel is installed in the same building as the main service panel, it does not require a main breaker; the 100A breaker in the main panel serves as the disconnect. However, if the subpanel is in a detached structure (like a separate garage or barn), NEC Article 225.31 and 225.32 require a readily accessible disconnecting means. While this can technically be a standalone 100A disconnect switch, it is almost always more cost-effective and practical to simply buy a subpanel with a 100A main breaker built in to satisfy this code requirement.