For a 100-amp service, use 3 AWG copper or 1 AWG aluminum wire with a 100-amp breaker, assuming 75°C rated terminations. If your panel lugs are only rated for 60°C, or the run exceeds 50 feet, upsize to 1 AWG copper or 1/0 aluminum to maintain safe ampacity and limit voltage drop.
- Material: Copper or Aluminum THHN/THWN-2
- Ambient Temperature: 30°C (86°F)
- Installation: Single conduit, no more than 3 current-carrying conductors
- Termination Rating: 75°C (Standard for modern 100A+ breakers per NEC 110.14(C))
The Core Sizing Matrix and NEC Termination Rules
When pulling wire for a 100-amp subpanel feeder or a dedicated 100-amp service disconnect, the NFPA 70 National Electrical Code (NEC) dictates your minimum gauge based on the temperature rating of your equipment terminations, not just the insulation on the wire. Even though THHN wire is rated for 90°C, NEC 110.14(C) requires you to size the wire using the 75°C column if your breakers and lugs are rated for 75°C, or the 60°C column if they are older or unmarked.
| Wire Gauge | Material | 75°C Ampacity | 60°C Ampacity | Max Standard Breaker |
|---|---|---|---|---|
| 3 AWG | Copper | 100A | 85A | 100A |
| 2 AWG | Copper | 115A | 95A | 110A (or 100A) |
| 1 AWG | Copper | 130A | 110A | 125A (or 100A) |
| 1 AWG | Aluminum | 100A | 85A | 100A |
| 1/0 AWG | Aluminum | 120A | 100A | 125A (or 100A) |
Why 3 AWG Copper and Not 4 AWG?
A common mistake on the jobsite is trying to use 4 AWG copper to save money. In the 75°C column, 4 AWG copper is rated for exactly 85 amps. Under NEC 240.4(B), you are allowed to round up to the next standard breaker size if your wire ampacity doesn't match a standard breaker. However, NEC 240.6 lists standard breaker sizes as 80A, 90A, and 100A. The next size up from 85A is 90A. Therefore, 4 AWG copper cannot legally be protected by a 100-amp breaker. You must step up to 3 AWG, which hits exactly 100A in the 75°C column.
What Changes the Answer: Distance, Heat, and Continuous Loads
The table above assumes a perfect, short run in a climate-controlled garage. Real-world conditions force you to upsize your wire. Here are the three factors that will change your baseline gauge.
1. Voltage Drop Over Distance
The NEC recommends a maximum voltage drop of 3% for feeders. If you are running a 100-amp feeder 150 feet from your main panel to a detached garage subpanel, 3 AWG copper will fail this recommendation.
Let's run the math using the standard voltage drop formula: VD = (2 × K × I × D) / CM.
- K (Copper constant) = 12.9
- I (Current) = 100A
- D (Distance) = 150 ft
- CM (Circular Mils for 3 AWG) = 52,620
VD = (2 × 12.9 × 100 × 150) / 52,620 = 7.35 Volts.
On a 240V circuit, 7.35V is a 3.06% drop. This barely fails the 3% guideline. If you have heavy 120V branch loads on that subpanel, the drop on the 120V leg doubles to over 6%, which will cause lights to dim and motors to overheat. By upsizing to 1 AWG copper (CM = 83,690), the drop falls to 4.62V (1.9%), passing easily. You can verify your specific run using the Southwire Voltage Drop Calculator.
2. Ambient Temperature and Bundling Derating
If your conduit runs across a hot attic space where ambient temperatures regularly exceed 86°F (30°C), or if you pull more than three current-carrying conductors through a single pipe (like two multi-wire branch circuits), NEC 310.15(B)(1) requires you to apply a derating factor. Because derating is applied to the 90°C column of THHN wire, you start with a higher baseline, but the final derated ampacity must still meet or exceed your breaker size. In high-heat attics, 3 AWG will quickly derate below 100A, forcing an upgrade to 1 AWG or 1/0 aluminum.
3. The 125% Continuous Load Rule
Is your 100-amp load continuous? NEC Article 100 defines a continuous load as one where the maximum current is expected to continue for 3 hours or more (e.g., EV chargers, data center servers, or heavy commercial HVAC). If your calculated continuous load is exactly 100A, NEC 210.20(A) requires the breaker to be sized at 125% of the load. That means you need a 125-amp breaker and wire rated for 125A, which bumps your minimum copper size to 1 AWG (130A at 75°C).
Copper vs. Aluminum Feeders: Cost, Torque, and Oxidation
For a 100-amp service, aluminum is incredibly common and code-compliant, provided you follow the installation rules. Here is how the two materials stack up for this specific amperage.
| Criteria | 3 AWG Copper | 1/0 Aluminum (Recommended) |
|---|---|---|
| Material Cost (per 100ft) | ~$350 - $450 | ~$120 - $160 |
| Wire Stiffness | Moderate (Hard to bend in tight panels) | Low (Easy to route and terminate) |
| Termination Prep | Strip and insert | Must apply anti-oxidant paste (Noalox) |
| Lug Compatibility | Universal | Must verify lugs are rated AL/CU |
NEC 110.14(D) mandates that any termination marked with a torque specification must be tightened using a calibrated torque tool. Aluminum expands and contracts more than copper under thermal cycling. If you do not torque your aluminum lugs to the exact inch-pound specification printed on the breaker or panel label, the connection will loosen over time, arc, and potentially start a fire. Never use an uncalibrated cordless impact driver for panel terminations.
When the AHJ or an Engineer Must Step In
While the NEC provides the baseline for safe wiring, there are specific scenarios where your local Authority Having Jurisdiction (AHJ) or a licensed electrical engineer will override these general guidelines.
Service Entrance vs. Subpanel Feeder: If this 100-amp wire is acting as your main service entrance (from the utility meter to your main panel), the utility company's 'Green Book' or service requirements will dictate the rules. Many utilities strictly require a minimum of 2 AWG copper or 1/0 aluminum for any 100-amp service drop, regardless of the NEC minimums, to ensure mechanical strength against wind and ice loads on the mast.
High Fault Current Availability: If you are installing a 100-amp service in an industrial park or near a utility substation where the available fault current exceeds 10,000 amps, an engineer must verify that your chosen breaker's AIC (Ampere Interrupting Capacity) rating and the wire's bracing are sufficient. Standard residential 100A breakers are typically rated for 10kAIC; high-fault environments may require 22kAIC or 65kAIC breakers and specific conduit fill limits.
Always pull a permit and have your local electrical inspector verify your wire sizing and torque marks before energizing a 100-amp feeder. Local amendments to the NEC can and do supersede national baselines.
