For a standard 100-amp service, use 3 AWG copper or 1 AWG aluminum wire, protected by a 100A breaker. This assumes 75°C terminations, 30°C ambient temperature, and no more than three current-carrying conductors in a raceway. Always verify voltage drop if the run exceeds 140 feet.
- Insulation: THHN/THWN-2 in conduit or XHHW-2 in SER cable (rated 90°C, but terminated at 75°C).
- Termination Rating: 75°C (standard for modern breakers and panel lugs).
- Ambient Temperature: 30°C (86°F) or lower.
- Raceway: Single conduit or cable with a maximum of 3 current-carrying conductors.
- System Voltage: 240V split-phase residential.
The Core Sizing Table: Copper vs. Aluminum Ampacity
Wire sizing is not a guessing game; it is dictated by the 75°C column of NEC Table 310.16. While modern wire insulation like THHN is rated for 90°C, the lugs inside your breaker and panel are almost universally rated for 75°C. Therefore, the 75°C column governs your final ampacity limit.
| AWG Size | Material | 75°C Ampacity (Governing) | 90°C Ampacity (Derating Base) | Max Standard Breaker |
|---|---|---|---|---|
| 4 AWG | Copper | 85A | 95A | 90A |
| 3 AWG | Copper | 100A | 115A | 100A |
| 2 AWG | Aluminum | 90A | 100A | 90A |
| 1 AWG | Aluminum | 100A | 115A | 100A |
| 1/0 AWG | Aluminum | 120A | 135A | 125A |
Note on Aluminum: While 1 AWG aluminum meets the 100A requirement, many electricians and inspectors prefer 1/0 AWG aluminum for service feeders. The marginal cost increase is minimal, but the thicker strand provides superior mechanical strength at the lugs and inherently reduces voltage drop on longer runs.
Why 3 AWG Copper and Not One Size Smaller?
A frequent point of confusion on the jobsite is why we cannot use 4 AWG copper (rated 85A at 75°C) on a 100A breaker. This stems from a misinterpretation of the NEC 240.4(B) 'next size up' rule.
NEC 240.4(B) allows you to use the next standard breaker size only if the calculated load does not exceed the wire's ampacity, and the next standard breaker does not exceed 800A. For example, if your calculated load is 82A, you can use 4 AWG copper (85A) protected by a 90A breaker.
However, when sizing a feeder for a 100-amp subpanel or a 100-amp service disconnect, the panel's main breaker rating is the load capacity you must supply. You are not sizing the wire to a specific calculated load of 82A; you are sizing it to deliver the full 100A capacity of the panel. Because 4 AWG copper is only rated for 85A, it cannot be protected by a 100A breaker for a 100A panel feed. The wire must have an ampacity equal to or greater than the breaker rating. Therefore, you must step up to 3 AWG copper, which hits exactly 100A in the 75°C column.
Variables That Force a Wire Size Increase
The 3 AWG Copper / 1 AWG Aluminum baseline assumes perfect conditions. Real-world installations rarely match the textbook. Use the decision tree below to determine if your specific installation requires upsizing the wire.
| Variable | Threshold / Condition | Impact on 3 AWG Copper | Required Action |
|---|---|---|---|
| Voltage Drop | Run exceeds 140 feet at a full 100A load. | Drop exceeds 3% (7.2V on a 240V system). | Upsize to 2 AWG Copper or 1/0 AWG Aluminum. |
| Conductor Bundling | 4 to 6 current-carrying conductors in one raceway. | Ampacity derates to 80% (115A x 0.80 = 92A). | Upsize to 2 AWG Copper (130A x 0.80 = 104A). |
| Ambient Heat | Attic or roof space reaches 40°C (104°F). | 90°C column derates to 88% (115A x 0.88 = 101A). | Borderline. Upsize to 2 AWG Copper for safety margin. |
| Continuous Load | The 100A load runs continuously for 3+ hours. | NEC requires 125% sizing (100A x 1.25 = 125A). | Upsize to 1 AWG Copper (130A at 75°C). |
The Voltage Drop Calculation in Practice
Voltage drop is not just a theoretical inefficiency; it causes motors to overheat, lights to flicker, and electronics to brown out. The Southwire voltage drop formula for single-phase systems is:
VD = (2 × K × I × L) / CM
Where K = 12.9 (Copper), I = 100A, L = Length in feet, CM = Circular Mills (52,620 for 3 AWG).
At 140 feet, the drop is 6.86V (2.85%), which is safely under the NEC recommended 3% limit for feeders. At 150 feet, it crosses 7.35V (3.06%), triggering the need for an upsized wire. Always use the Copper Development Association's guidelines or a verified calculator when runs exceed 100 feet.
When an Engineer or the AHJ Must Confirm
While the rules above cover 95% of residential subpanel and service disconnect upgrades, certain scenarios cross the line from standard DIY/trade practice into engineered territory. You must pull a permit and have the local AHJ (or a licensed professional engineer) review the design in the following cases:
- Utility Service Entrance Conductors: The wires running from the utility transformer or meter base to your main panel are governed by utility company specifications (the 'Green Book' or local equivalent), not just the NEC. Utilities often mandate specific aluminum triplex/quadruplex cables and have strict rules about meter socket ratings and grounding electrode conductors.
- High-Temperature Environments: If your conduit runs through a boiler room, above a kiln, or in an unventilated attic in a desert climate where ambient temperatures regularly exceed 50°C (122°F), standard derating tables fall apart. An engineer must calculate the exact thermal resistivity of the environment.
- Complex Grounding and Bonding: A 100-amp subpanel requires a strictly isolated neutral bar and a bonded ground bar, tied back to the main panel via a dedicated equipment grounding conductor (EGC). If you are upgrading an older 3-wire feeder (which lacked a dedicated ground) to a modern 4-wire system, the AHJ must inspect the grounding electrode system to ensure equipotential bonding is maintained.
Getting the service wire for a 100 amp panel right the first time saves you from failing a rough-in inspection or, worse, dealing with a melted terminal lug three years down the road. Stick to the 75°C column, respect the voltage drop limits, and always torque your lugs to the manufacturer's specified inch-pound rating using a calibrated torque screwdriver.
