For a 100-amp residential service entrance, you need 4 AWG copper or 2 AWG aluminum wire, protected by a 100-amp main breaker. If routing a 100-amp feeder to a subpanel rather than a main service, you must upsize to 3 AWG copper or 1 AWG aluminum.
- Material: Copper (Cu) and Aluminum (Al) evaluated separately.
- Temperature Column: 75°C (standard for residential breaker and lug terminations).
- Ambient Temperature: 30°C (86°F).
- Installation Method: Single raceway/conduit or NM-B cable, not buried directly in earth.
The 83% Rule: Service Entrance vs. Subpanel Feeder
The most common mistake DIYers make when sizing wire for a 100-amp load is ignoring the distinction between a service entrance and a feeder. The National Electrical Code (NEC) treats these two applications differently due to load diversity in whole-home calculations.
Under NEC Article 310.12, service-entrance conductors supplying a whole dwelling unit are permitted to be sized at 83% of the service rating. This means for a 100-amp service, your wire only needs an ampacity of 83 amps (100A × 0.83). Looking at the 75°C column, 4 AWG copper is rated for 85 amps, making it perfectly legal for a main service.
However, if you are running a 100-amp feeder to a detached garage, a workshop subpanel, or an EV charger subpanel, the 83% rule does not apply. You must size the wire to carry the full 100 amps. This requires stepping up to 3 AWG copper.
| Application | Conductor Material | Minimum AWG Size | 75°C Ampacity | 90°C Ampacity (THHN) |
|---|---|---|---|---|
| Main Service Entrance (NEC 310.12) | Copper | 4 AWG | 85A (Passes 83A req) | 95A |
| Main Service Entrance (NEC 310.12) | Aluminum | 2 AWG | 90A (Passes 83A req) | 100A |
| Subpanel Feeder (Standard) | Copper | 3 AWG | 100A | 115A |
| Subpanel Feeder (Standard) | Aluminum | 1 AWG | 100A | 115A |
Why This Size and Not One Smaller?
Looking at the table above, you might notice that 4 AWG THHN copper has a 90°C ampacity of 95A. If you are building a 100-amp subpanel feeder, why can't you use 4 AWG wire and rely on the 90°C rating? After all, 95A is very close to 100A, and the breaker will trip at 100A anyway.
The answer lies in NEC 110.14(C) - Temperature Limitations of Equipment. While the THHN insulation on the wire can physically withstand 90°C without melting, the brass or aluminum lugs inside your breaker panel and the breaker terminals themselves are typically only tested and rated for 75°C.
If you push 95 amps through a 4 AWG wire connected to a 75°C-rated lug, the wire will act as a heat sink, pulling heat away from the lug. But if the lug gets too hot, it can degrade the breaker's internal thermal-magnetic trip mechanism, causing nuisance tripping or, worse, failing to trip during a fault. Therefore, the NEC mandates that you must size your wire based on the lowest temperature rating in the circuit. Since the terminals are 75°C, you must use the 75°C column. In that column, 4 AWG copper is only good for 85 amps. You must use 3 AWG copper (rated 100A at 75°C) for a subpanel feeder.
What Changes the Answer: Distance, Bundling, and Aluminum
The AWG sizes listed above are your baseline. Real-world jobsite conditions frequently force you to upsize your wire. Here are the three variables that will change your material list.
1. Voltage Drop Over Distance
The NEC recommends a maximum voltage drop of 3% for feeders. On a 240V system, a 3% drop is 7.2 volts. Let's run the math for a 100-amp subpanel feeder located 100 feet from the main panel using 3 AWG copper.
Using the standard voltage drop formula: VD = (2 × K × I × D) / CM
- K (Copper resistance) = 12.9 ohms-cmil/ft
- I (Current) = 100A
- D (Distance) = 100 ft
- CM (Circular mils for 3 AWG) = 52,620
VD = (2 × 12.9 × 100 × 100) / 52,620 = 4.9 Volts.
4.9V is a 2.04% drop. 3 AWG is safe at 100 feet.
Now, what if that detached garage is 200 feet away? The voltage drop doubles to 9.8V (4.08%). This exceeds the 3% recommendation, causing dimming lights and potential motor burnout on heavy tools. At 200 feet, you must upsize to 1 AWG copper (CM = 83,690) to bring the drop down to an acceptable 6.1V (2.5%). Always run a voltage drop check using a trusted calculator like the Cerrowire technical resources before pulling wire for long runs.
2. Conduit Bundling and Derating
If you are pulling multiple circuits through the same conduit, the wires heat each other up. Under NEC 310.15(C)(1), if you have more than three current-carrying conductors in a single raceway, you must apply a derating factor. For example, if you pull two 240V circuits (4 current-carrying conductors) plus a neutral through one PVC conduit, you must derate the ampacity to 80%. Your 3 AWG copper (100A at 75°C) drops to 80A. You would be forced to upsize to 1 AWG copper just to maintain your 100-amp capacity.
3. Aluminum Conductors
Aluminum is significantly cheaper and lighter than copper, making it the standard for utility drops and large feeders. However, aluminum expands and contracts more than copper under thermal cycling, which can loosen terminal connections over time and cause arcing fires. If you choose 1 AWG or 2 AWG aluminum for your 100-amp run, you must:
- Use an anti-oxidant compound (like Noalox) on the stripped wire ends to prevent aluminum oxide buildup, which is highly resistive.
- Use a calibrated torque screwdriver or wrench to tighten the lugs to the exact inch-pound specification printed on the breaker or panel label. Hand-tightening is a fire hazard.
- Ensure the breaker and panel lugs are explicitly rated for aluminum (most modern 75°C lugs are, but always verify).
When an Engineer or AHJ Must Confirm
While the guidelines above cover 95% of residential 100-amp installations, certain edge cases require professional sign-off. You must consult a licensed Professional Engineer (PE) or your local Authority Having Jurisdiction (AHJ) in the following scenarios:
| Scenario | Why It Requires AHJ/Engineer Review |
|---|---|
| Continuous Loads | If the 100A load will run for 3 hours or more continuously (e.g., a commercial EV charging station or large server rack), NEC 210.20(A) requires the breaker and wire to be sized at 125% of the load (125A). A standard 100A setup will fail inspection. |
| High Ambient Temperatures | If the conduit runs through an unconditioned attic in a hot climate where ambient temperatures regularly exceed 30°C (86°F), you must apply temperature correction factors from NEC Table 310.15(B)(1). The wire ampacity drops, requiring a larger AWG. |
| Local Code Amendments | Some municipalities explicitly ban the use of aluminum wire for interior feeders, or they reject the NEC 310.12 83% rule entirely, requiring full-size wire for all services. Always check local amendments before buying materials. |
Sizing wire for a 100-amp service is not just about matching a number on a breaker handle. It requires understanding the physical limitations of your terminations, the mathematical reality of voltage drop over distance, and the specific legal definitions of a service versus a feeder. By anchoring your decisions to the 75°C ampacity column and verifying your run length, you will build a system that passes inspection and operates safely for decades.
