For a standard 100-amp circuit, use 3 AWG copper wire or 1 AWG aluminum wire paired with a 100-amp breaker. This assumes 75°C rated terminals, THHN/THWN-2 insulation, and an ambient temperature of 30°C (86°F). If your run exceeds 100 feet, you must upsize to mitigate voltage drop.
Sizing a 100-amp feeder for a subpanel, EV charger, or heavy workshop machinery is one of the most common mid-tier electrical tasks. However, simply looking at a basic ampacity chart without understanding the National Electrical Code (NEC) termination rules and derating factors is a fast track to a failed inspection or a melted lug. Below is the exact engineering and code-compliant framework for sizing 100-amp conductors.
The 100-Amp Wire Sizing Baseline
Before pulling any wire through conduit, you must establish the baseline assumptions for your installation. The NEC does not allow you to use the 90°C column of NFPA 70 (NEC) Table 310.16 for sizing your breaker unless every single component in the circuit—including the breaker, lugs, and splices—is rated for 90°C. Since standard residential and commercial breakers and panel lugs are rated for 75°C, the 75°C column is your legal limit for ampacity.
- Conductor Material: Copper (primary), Aluminum (secondary)
- Insulation Type: THHN/THWN-2 (dual-rated, standard for conduit)
- Termination Temperature: 75°C (standard for modern breakers/lugs)
- Ambient Temperature: 30°C (86°F) or lower
- Installation Method: Raceway (conduit) with no more than 3 current-carrying conductors
- Load Type: Non-continuous (operates for less than 3 hours at a time)
Here is the exact data from NEC Table 310.16 that dictates your wire size. Notice how the 90°C column is strictly reserved for derating calculations, not baseline sizing.
| AWG Size | Material | 60°C Column (Older Gear) | 75°C Column (The Sizing Rule) | 90°C Column (Derating Only) |
|---|---|---|---|---|
| 4 AWG | Copper | 70 Amps | 85 Amps | 95 Amps |
| 3 AWG | Copper | 85 Amps | 100 Amps | 115 Amps |
| 2 AWG | Aluminum | 75 Amps | 90 Amps | 100 Amps |
| 1 AWG | Aluminum | 85 Amps | 100 Amps | 115 Amps |
Why 3 AWG Copper (And Why Not 4 AWG?)
A frequent question on the workbench is why we cannot use 4 AWG copper wire and simply rely on the NEC 240.4(B) "next size up" rule. The 240.4(B) rule allows you to round up to the next standard breaker size if your wire's ampacity doesn't perfectly match a standard breaker, provided the load isn't continuous and the breaker is under 800 amps.
Here is why that fails for a 100-amp circuit: 4 AWG copper in the 75°C column is rated for 85 amps. The next standard breaker size up from 85 amps is 90 amps. You cannot legally jump from an 85-amp wire rating to a 100-amp breaker. The gap is too wide, and the wire will overheat before the breaker trips. To safely and legally land on a 100-amp breaker, your wire must have a baseline ampacity of at least 100 amps in the 75°C column. That strictly requires 3 AWG copper or 1 AWG aluminum.
Variables That Force You to Upsize
The baseline answer assumes perfect conditions. In the real world, physics and installation environments frequently force you to buy thicker wire. Here are the three variables that will invalidate 3 AWG copper and force an upsize.
1. Voltage Drop Over Distance
The NEC recommends a maximum 3% voltage drop on branch circuits and feeders. For a 240V, 100-amp circuit, a 3% drop is 7.2 volts. Using the standard voltage drop formula ($VD = \frac{2 \times K \times I \times L}{CM}$), 3 AWG copper (Circular Mills = 52,620) will hit that 7.2V limit at roughly 146 feet.
However, accounting for real-world AC reactance, startup surges, and slightly higher K-factors in warmer conduit, the practical threshold is shorter. If your one-way wire run exceeds 100 feet, upsize to 2 AWG copper to keep voltage drop well under 3% under heavy load.
2. Conductor Bundling (Derating)
This is where DIYers and even some apprentices fail inspections. If you pull more than three current-carrying conductors through a single conduit, the wires heat each other up. NEC 310.15(C)(1) requires you to derate the wire's ampacity. This is the one time you are allowed to use the 90°C column.
| Current-Carrying Conductors in Conduit | Derating Factor | 3 AWG Cu (90°C = 115A) | Resulting Ampacity | Passes 100A? |
|---|---|---|---|---|
| 1 to 3 | 100% | 115A x 1.0 | 115 Amps | Yes |
| 4 to 6 | 80% | 115A x 0.8 | 92 Amps | No (Must upsize to 2 AWG) |
| 7 to 9 | 70% | 115A x 0.7 | 80.5 Amps | No (Must upsize to 1 AWG) |
If you are running two separate 100-amp circuits (4 hot wires total) in the same PVC conduit, 3 AWG copper derates to 92 amps. It is no longer legal for a 100-amp breaker. You must pull 2 AWG copper.
3. High Ambient Temperatures
If your conduit runs through an unventilated attic in a southern climate where ambient temperatures routinely exceed 30°C (86°F), you must apply ambient temperature correction factors from NEC Table 310.15(B)(1). At 40°C (104°F), the correction factor for 90°C wire is 0.91. 115A x 0.91 = 104.6A. It barely passes, but if the attic hits 45°C (113°F), the factor drops to 0.87, yielding 100.05A—dangerously close to the limit. Upsize to 2 AWG for any run through hot attics.
Copper vs. Aluminum for 100-Amp Feeders
While 3 AWG copper is the gold standard for branch circuits, 1 AWG aluminum is incredibly common for 100-amp subpanel feeders due to cost. According to major wire manufacturers like CerroWire, aluminum building wire has improved significantly, but it requires specific handling.
| Criteria | 3 AWG Copper (THHN) | 1 AWG Aluminum (XHHW-2) |
|---|---|---|
| Material Cost (Approx. per foot) | $3.50 - $5.00 | $1.20 - $1.80 |
| Wire Diameter & Bend Radius | Thinner, easier to pull through 1-inch conduit and bend into tight panel gutters. | Thicker, stiffer. Requires larger sweep bends and more physical effort to terminate. |
| Termination Prep | Strip and insert. Standard torque applies. | Must be wire-brushed and coated with Noalox (anti-oxidant paste) to prevent galvanic corrosion and high-resistance heating. |
| Lug Compatibility | Compatible with all standard CU/AL rated lugs. | Must ONLY be terminated on lugs explicitly marked 'AL' or 'CU/AL'. Never use on copper-only lugs. |
The Verdict: Choose 3 AWG copper for runs under 50 feet, tight conduit bends, or when terminating into compact breaker enclosures. Choose 1 AWG aluminum (specifically XHHW-2 insulation, which is thinner than THHN) for long subpanel feeders where the 60% cost savings easily justifies the extra physical effort and anti-oxidant prep.
When to Call an Engineer or the AHJ
While the rules above cover 95% of residential and light-commercial 100-amp installations, certain scenarios require a stamped engineering drawing or a direct consultation with your local Authority Having Jurisdiction (AHJ):
- Service Entrance Conductors: If this 100-amp wire is feeding the main service disconnect from the utility meter (rather than a subpanel from an existing main breaker), utility company specs and local service entrance rules override standard NEC feeder rules. Many utilities require 1/0 AWG aluminum minimum for service drops regardless of load.
- Parallel Runs: If you are trying to parallel two smaller wires to achieve 100 amps (e.g., two 6 AWG wires), stop. NEC 310.10(G) strictly prohibits paralleling conductors smaller than 1/0 AWG. You cannot parallel wires for a 100-amp circuit.
- Harmonic Loads: If the 100-amp load consists of heavy variable frequency drives (VFDs) or massive LED lighting arrays, the neutral conductor may carry significant harmonic current. An engineer must calculate the neutral sizing, as it may need to be oversized to 200% of the phase conductors.
Always verify your final wire and breaker selection against the specific terminal torque specifications printed on the breaker label. A 100-amp breaker with 3 AWG copper typically requires between 35 and 45 in-lbs of torque. Use a calibrated torque screwdriver; guessing the tightness is the leading cause of thermal failures at the lug.
