For a 100-amp service, use 3 AWG copper wire (THHN/THWN-2 insulation) paired with a 100A breaker. This baseline assumes standard 75°C terminations, a 30°C (86°F) ambient temperature, and no more than three current-carrying conductors in the conduit per NEC Table 310.16.
The Core Sizing Rules for 100A Copper Feeders
Sizing wire is never just about matching a number to a breaker; it is about matching the wire to the termination temperature limits and the physical environment. Before pulling any wire through EMT or PVC conduit, you must lock in your baseline assumptions. If your installation deviates from the block below, the 3 AWG answer changes.
- Material: Copper (never interchange with aluminum sizing)
- Insulation: THHN/THWN-2 (rated 90°C in the wire, but terminated at 75°C)
- Temperature Column: 75°C (per NEC 110.14(C) for equipment rated 100A or less)
- Ambient Temperature: 30°C (86°F) or lower
- Conduit Fill: Maximum 3 current-carrying conductors in a single raceway
According to the Cerrowire Ampacity Tables and NEC Table 310.16, 3 AWG copper wire in the 75°C column has an allowable ampacity of exactly 100A. This perfectly matches a standard 100A two-pole breaker.
Why 3 AWG and Not One Size Smaller?
A common mistake on the jobsite is attempting to use 4 AWG copper wire to save money. In the 75°C column, 4 AWG copper is only rated for 85A. While NEC 240.4(B) allows you to round up to the next standard breaker size (e.g., using a 90A breaker on 85A wire for an 80A calculated load), you cannot use 4 AWG wire on a 100A breaker. The overcurrent protective device must protect the wire at its rated ampacity. If the panel bus or service disconnect is rated for 100A, the wire feeding it must be capable of safely carrying 100A without the insulation degrading or the termination overheating.
When the Standard Answer Changes: Voltage Drop & Derating
The NEC provides minimum safety thresholds for fire prevention, but it does not guarantee your equipment will operate efficiently over long distances. If your 100A feeder runs to a detached garage, a workshop subpanel, or an EV charger, you must calculate voltage drop.
Voltage Drop Check at 100 Feet
The NFPA National Electrical Code recommends a maximum voltage drop of 3% for feeders. Let us run the math for a 240V, 100A load using 3 AWG copper at a distance of 100 feet.
Formula: VD = (2 × K × I × D) / CM
- K (Copper Resistivity): 12.9
- I (Current): 100A
- D (One-way Distance): 100 ft
- CM (Circular Mils for 3 AWG): 52,620
Calculation: (2 × 12.9 × 100 × 100) / 52,620 = 4.9V drop.
Percentage: 4.9V / 240V = 2.04%.
At 100 feet, 3 AWG copper passes the 3% rule with room to spare. However, if that same run extends to 150 feet, the drop increases to 3.06%, pushing you over the recommended limit. At that point, you must bump the wire size to 2 AWG copper.
| One-Way Distance | Voltage Drop % | Required Copper AWG | Reasoning |
|---|---|---|---|
| Up to 115 ft | < 2.4% | 3 AWG | Meets 100A ampacity and <3% drop |
| 116 ft to 150 ft | 2.4% - 3.1% | 2 AWG | Bumped to maintain <3% drop |
| 151 ft to 225 ft | 3.1% - 4.5% | 1 AWG | Required for long-run efficiency |
Bundling and Temperature Derating
If you pull multiple circuits through the same conduit, the wires heat each other up. NEC Table 310.15(C)(1) requires derating when you have 4 to 6 current-carrying conductors in a single raceway. You must multiply the wire's 90°C ampacity by 80%.
For 3 AWG copper, the 90°C ampacity is 115A. Derated by 80%, it drops to 92A. Because 92A is less than your 100A breaker, 3 AWG is no longer legal in a bundled scenario. You must step up to 2 AWG copper (130A at 90°C, derated to 104A) to safely carry the 100A load.
Copper vs. Aluminum: What Changes the Answer
Copper and aluminum are not interchangeable in sizing math. Aluminum has higher resistance and requires a larger physical cross-section to carry the same current safely. If your budget or utility feed dictates aluminum (such as XHHW-2 or SER cable), you cannot use 3 AWG.
In the 75°C column of NEC Table 310.16, 3 AWG aluminum is only rated for 75A. To achieve a 100A rating with aluminum, you must use 1 AWG aluminum wire, which is rated for exactly 100A at 75°C.
- Copper (THHN): 3 AWG | Ampacity: 100A | Approx Cost: $3.50/ft per conductor
- Aluminum (XHHW-2): 1 AWG | Ampacity: 100A | Approx Cost: $1.20/ft per conductor
Note: When terminating aluminum wire, you must use anti-oxidant compound (like Noalox) and torque the lugs to the manufacturer's exact inch-pound specification to prevent thermal creep and arcing over time.
When an Engineer or the AHJ Must Confirm
While the math above covers 95% of residential subpanel and service upgrades, specific edge cases require a stamped engineering drawing or a direct sign-off from your local Authority Having Jurisdiction (AHJ).
- Continuous Loads: If your 100A load is considered 'continuous' (operating for 3 hours or more, like a commercial HVAC unit or a continuous-duty EV charger), NEC 210.19(A)(1) requires the wire to be sized at 125% of the load. 100A × 1.25 = 125A. You would need 1 AWG copper (130A at 75°C) to meet this requirement.
- Service Entrance Conductors: If this 100A wire is the main service drop from the utility pole to your meter/main panel, NEC 310.12 might allow smaller wire based on the 83% rule for dwelling services. However, many utilities have strict minimums (often 2 AWG copper or 1/0 aluminum) regardless of NEC allowances. Always check with the utility provider.
- High Ambient Environments: If the conduit runs across a hot roof or through an attic that regularly exceeds 30°C (86°F), you must apply the temperature correction factors in Table 310.15(B)(1). An attic at 110°F requires a 0.88 correction factor, which will drop 3 AWG copper below the 100A threshold.
Frequently Asked Questions
Can I use 4 AWG copper wire for a 100 amp subpanel if my calculated load is only 80 amps?
Yes, but with a critical caveat regarding the breaker. If your actual calculated load is 80A, 4 AWG copper (rated 85A) is sufficient for the load. However, you must protect that 4 AWG wire with a 90A or 80A breaker, not a 100A breaker. You cannot install a 100A breaker on 4 AWG wire simply because the panel bus is rated for 100A. The breaker must protect the wire's ampacity. If you want to use the 100A breaker already installed in the main panel, you must pull 3 AWG copper.
What size ground wire do I need for a 100 amp copper 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). If you had to upsize your current-carrying conductors to 2 AWG or 1 AWG to compensate for voltage drop over a long distance, NEC 250.122(B) requires you to proportionally increase the size of the ground wire as well to maintain the same fault-current clearing capability.
Does the 100 amp wire size change if I use THHN in a 90°C rated lug?
In almost all residential and light commercial applications, no. While THHN wire insulation is rated for 90°C, NEC 110.14(C) dictates that you must size the wire based on the lowest temperature rating in the entire circuit path. Standard breakers, panel lugs, and disconnect switches are typically rated for 75°C. Even if you find a specialized 90°C lug, the breaker itself is the bottleneck. Therefore, you must always use the 75°C column for 100A terminations, keeping the requirement at 3 AWG copper.
