For a standard 100-amp residential service or subpanel feeder, use 1 AWG copper or 1/0 AWG aluminum wire, protected by a 100A breaker. This assumes 75°C rated terminals, THHN/THWN-2 insulation in conduit, and a maximum ambient temperature of 30°C (86°F).
- Material: Copper (Cu) and Aluminum (Al) evaluated separately.
- Temperature Column: 75°C (Standard for modern residential breakers and lugs).
- Insulation: THHN / THWN-2 (Rated for 90°C, but terminated at 75°C limits).
- Ambient Temperature: 30°C (86°F) or lower. No sunlight exposure on conduit.
- Installation Method: Single circuit in a standard raceway (conduit) or NM-B cable.
The Core Sizing Table: Copper vs. Aluminum for 100A
Before pulling any wire, you need to know exactly what the National Electrical Code (NEC) allows. The table below extracts the critical rows from NEC Table 310.16 that apply to a 100-amp load. Notice that we look at the 75°C column for final ampacity, even though THHN wire is technically rated for 90°C.
| Wire Size (AWG/kcmil) | Material | 60°C Column (Amps) | 75°C Column (Amps) | 90°C Column (Amps) | Verdict for 100A |
|---|---|---|---|---|---|
| 2 AWG | Copper | 75A | 95A | 115A | FAIL |
| 1 AWG | Copper | 85A | 110A | 130A | PASS |
| 1/0 AWG | Aluminum | 75A | 100A | 120A | PASS (Minimum) |
| 2/0 AWG | Aluminum | 90A | 115A | 135A | PASS (Preferred) |
Source: NFPA 70 (NEC) Table 310.16. Always defer to your local Authority Having Jurisdiction (AHJ) for final approval.
Why 1 AWG Copper (And Why 2 AWG Fails the NEC)
A common mistake on the jobsite is assuming 2 AWG copper is sufficient because it is rated for 95A at 75°C, and the next standard breaker size up is 100A. Under NEC 240.4(B), you are sometimes allowed to round up to the next standard overcurrent protective device (OCPD) size. However, this rounding rule applies to overcurrent protection, not to the minimum ampacity required for a calculated service.
Under NEC Article 310.12, service conductors must have an ampacity not less than the calculated load. If your calculated load is 100 amps, your wire must be rated for at least 100 amps. Since 2 AWG copper maxes out at 95A in the 75°C column, it is legally undersized for a full 100A service. You must step up to 1 AWG copper (110A at 75°C).
The Aluminum Alternative: If you are running aluminum (which is standard for utility service drops and increasingly common for long subpanel feeders due to cost), 1/0 AWG aluminum hits exactly 100A in the 75°C column. While legal, many electricians prefer 2/0 AWG aluminum for 100A feeders. The 1/0 size leaves zero headroom for voltage drop or minor ambient temperature spikes, whereas 2/0 provides a 115A buffer and is physically easier to terminate without overheating the lug.
Voltage Drop: When Distance Forces a Larger Wire Gauge
Ampacity tables assume the wire can handle the heat, but they do not account for voltage drop over distance. The NEC recommends (in Informational Note 210.19(A)(1)) that branch circuit and feeder voltage drop be limited to 3%, with a total combined drop of no more than 5%.
Let’s run the math for a 240V, 100A load using 1 AWG copper. According to the Southwire Voltage Drop Calculator and standard impedance tables:
- At 50 feet: 1.54V drop (0.64%) — 1 AWG is perfect.
- At 100 feet: 3.08V drop (1.28%) — 1 AWG is still well within limits.
- At 200 feet: 6.16V drop (2.56%) — 1 AWG is acceptable, but nearing the 3% threshold.
- At 250 feet: 7.70V drop (3.20%) — 1 AWG FAILS the 3% recommendation.
If your conduit run from the main panel to a detached garage subpanel exceeds 200 feet, you must upsize the wire to mitigate voltage drop, even if the ampacity is technically sufficient.
| One-Way Distance | Required Copper Size | Required Aluminum Size | Reasoning |
|---|---|---|---|
| 0 – 100 ft | 1 AWG | 1/0 AWG (or 2/0) | Ampacity dictates size; VD is negligible. |
| 101 – 200 ft | 1 AWG | 2/0 AWG | Aluminum needs upsizing to keep VD under 3%. |
| 201 – 250 ft | 1/0 AWG | 3/0 AWG | Both materials must upsize strictly for VD. |
| 251 – 350 ft | 2/0 AWG | 4/0 AWG | Long runs require heavy upsizing; consider 48V DC or step-up transformers if over 400ft. |
Derating, Bundling, and When to Call the AHJ
The baseline assumptions at the top of this article assume you are pulling a single feeder circuit through a conduit. The moment you change the physical environment, the NEC requires you to adjust your wire size.
The Bundling Derating Factor
If you pull two separate 100A subpanel feeders through the same piece of PVC conduit, you now have four current-carrying conductors (two hots per feeder; neutrals carrying only unbalanced linear load do not count per NEC 310.15(C)(1)). Four conductors trigger an 80% derating factor.
Here is where the 90°C column of THHN wire saves you. You are allowed to use the 90°C column for derating calculations, provided the final derated ampacity does not exceed the 75°C termination limit.
- 1 AWG Copper (90°C column): 130A × 0.80 = 104A. This is greater than 100A, and 104A is greater than the 75°C limit of 110A. 1 AWG still passes.
- 2 AWG Copper (90°C column): 115A × 0.80 = 92A. This fails the 100A requirement. 2 AWG fails.
Ambient Temperature Corrections
If your conduit runs across an unventilated attic in a southern climate where ambient temperatures regularly exceed 30°C (86°F), you must apply temperature correction factors from NEC Table 310.15(B)(1). At 40°C (104°F), the correction factor for 90°C insulation is 0.91. At 50°C (122°F), it drops to 0.82. If you are running wire through a hot attic, 1 AWG copper will derate to 106A (at 40°C ambient), which still passes, but leaves very little margin for error.
When an Engineer or the AHJ Must Confirm
While the math above covers 95% of residential 100A subpanel and service upgrades, you must pull a permit and defer to your local Authority Having Jurisdiction (AHJ) or a licensed electrical engineer in the following scenarios:
- Service Entrance Upgrades: If this 100A wire is the main service drop from the utility transformer to your meter/main panel, utility companies often have specific wire length, type (e.g., triplex/quadruplex aluminum), and termination requirements that supersede standard NEC feeder rules.
- Parallel Conductors: If you attempt to use two sets of smaller wires in parallel to achieve 100A (which is generally not permitted for 100A anyway, as NEC 310.10(H) requires parallel runs to be 1/0 AWG or larger), an engineer must design the load balancing.
- Continuous Loads: If the 100A load is considered "continuous" (running at maximum current for 3 hours or more, like a massive EV charging station or commercial HVAC), NEC 215.2 requires the conductors to be sized at 125% of the continuous load. A 100A continuous load requires wire rated for 125A, forcing you up to 1 AWG copper (if using 90C derating) or 1/0 copper at 75C.
