For a 100-amp circuit, the minimum code-compliant size is 3 AWG copper or 1 AWG aluminum with a 100A breaker. However, most electricians pull 2 AWG copper or 1/0 AWG aluminum to mitigate voltage drop. This assumes THHN/THWN-2 insulation, 75°C terminations, and 30°C ambient temperature.

Base Assumptions for This Guide:
  • Material: Copper (THHN/THWN-2) or Aluminum (XHHW-2)
  • Temperature Column: 75°C (per NEC 110.14(C) for standard residential breakers/lugs)
  • Ambient Temperature: 30°C (86°F) or lower
  • Installation Method: Raceway (conduit) or cable assembly, maximum 3 current-carrying conductors

The 100-Amp Ampacity Table: Copper vs. Aluminum

When sizing feeders for a 100-amp subpanel, an EV charger, or a large workshop, you must consult NEC Table 310.16. The most common mistake DIYers make is looking at the 90°C column because THHN wire is rated for 90°C. However, unless your breaker lugs and panel bus bars are explicitly stamped with a 90°C rating (which is exceptionally rare in residential gear), you are legally bound to use the 75°C column for your final ampacity and breaker sizing.

Material AWG Size Insulation Type 75°C Ampacity 90°C Ampacity 100A Breaker Match?
Copper 4 AWG THHN/THWN-2 85A 95A No (Undersized)
Copper 3 AWG THHN/THWN-2 100A 115A Yes (Absolute Minimum)
Copper 2 AWG THHN/THWN-2 115A 130A Yes (Recommended)
Aluminum 2 AWG XHHW-2 75A 90A No (Undersized)
Aluminum 1 AWG XHHW-2 100A 115A Yes (Absolute Minimum)
Aluminum 1/0 AWG XHHW-2 120A 135A Yes (Recommended)

Why This Size (and Why 4 AWG Fails)

You might look at a spool of 4 AWG copper THHN, see that it is rated for 95 amps in the 90°C column, and assume it is close enough to 100 amps to use the 'next size up' breaker rule (NEC 240.4(B)). This is a critical error.

The 'next size up' rule only applies when your exact calculated load does not correspond to a standard breaker size. Because 100A is a standard breaker size, your wire's ampacity must meet or exceed 100A in the applicable temperature column. Since 4 AWG copper maxes out at 85A in the 75°C column, placing it on a 100A breaker means the breaker will not trip before the wire's terminations overheat.

Furthermore, aluminum and copper are not interchangeable in the same AWG size. Aluminum has roughly 61% of the conductivity of copper by volume. Therefore, to carry the same 100A current safely, you must step up two AWG sizes when switching from copper to aluminum (e.g., from 3 AWG Cu to 1 AWG Al). Never use copper-rated torque specs on aluminum lugs; always apply an anti-oxidant compound like Noalox and torque to the manufacturer's exact inch-pound specification to prevent thermal creep and arcing.

What Changes the Answer: Length, Bundling, and Heat

The ampacity table above assumes a perfect, short run in a cool environment. In the real world, physics and the National Electrical Code (NEC) introduce variables that force you to upsize your wire.

1. Voltage Drop Over Distance

While the NEC doesn't strictly mandate a maximum voltage drop for standard feeders (it's an Informational Note in 310.15(B) recommending a max 3% drop for feeders), dropping below 3% is the hallmark of a quality installation. If you are feeding a 100A subpanel that will run continuous loads (like baseboard heaters or EV chargers), you calculate using 80A (100A x 0.80 continuous load factor).

Run Length (1-Way) Wire Size (Copper) Voltage Drop (at 80A / 240V) Percentage Drop Verdict
50 feet 3 AWG 1.96 V 0.8% Excellent
100 feet 3 AWG 3.92 V 1.6% Good
150 feet 3 AWG 5.88 V 2.4% Acceptable
200 feet 3 AWG 7.84 V 3.2% Fail (Upsize to 1 AWG)

Note: Calculations based on the Southwire Voltage Drop formula using K=12.9 for copper and Circular Mils for 3 AWG (52,620 CM).

2. Conduit Bundling (Derating)

If you pull more than three current-carrying conductors (CCCs) in a single raceway, the heat generated by the wires cannot dissipate properly. Per NEC Table 310.15(C)(1), you must apply a derating factor. If you have 4 to 6 CCCs, you multiply the 90°C ampacity by 80%. If you pull two separate 120/240V circuits in one PVC conduit (4 hots, 2 neutrals = 4 to 6 CCCs depending on neutral balancing), your 3 AWG copper (115A at 90°C) derates to 92A. Suddenly, 3 AWG is no longer legally allowed on a 100A breaker, and you must step up to 2 AWG.

3. High Ambient Temperatures

If your conduit runs through an unventilated attic in the summer where temperatures hit 50°C (122°F), you must apply an ambient temperature correction factor. At 50°C, the correction factor for 90°C THHN is 0.82. Again, this eats into your safety margin and often forces an upsize to 2 AWG copper or 1/0 aluminum.

Decision Tree: When an Engineer or AHJ Must Confirm

While sizing a standard 100A feeder to a detached garage is well within the scope of NEC-style guidance and a competent DIYer (where local code permits), certain scenarios cross the line from standard wiring into engineered systems. Use this decision tree to know when to stop and call a professional.

Scenario Why It's Complex Required Action
Run exceeds 300 feet Voltage drop requires massive wire (e.g., 2/0 Cu), which may not physically fit into the 100A breaker lugs. Consult an electrician to use a pull box and pigtails, or step up to a larger panel with larger lugs.
Paralleling conductors NEC 310.10(H) strictly forbids paralleling conductors smaller than 1/0 AWG. You cannot parallel two 4 AWG wires to make a 100A feeder. Redesign the run using a single set of properly sized 1/0 or larger conductors.
High Available Fault Current If the utility transformer is very close, the short-circuit current may exceed the 10kA Interrupting Capacity (AIC) of a standard 100A breaker. Utility or Electrical Engineer must calculate fault current; you may need 22kA or 65kA rated breakers.
Service Entrance (Meter to Main) This is utility-owned or utility-regulated infrastructure, not a standard branch circuit or feeder. Local AHJ (Inspector) and Utility Company must approve sizing and sealing.
Safety & Code Caveat: This guide provides NEC-style technical guidance. Your local Authority Having Jurisdiction (AHJ) always has the final say on code compliance. Working inside a live main panel or on service entrance conductors carries a lethal arc-flash and shock risk. Always de-energize, lock out/tag out, and verify dead with a Category III or IV multimeter before touching any bus bars or lugs. If you are unsure about local permitting requirements for subpanel feeders, hire a licensed electrical contractor.