100 Amp Wire Size Chart for 240V: AWG, Copper vs. Aluminum & NEC Rules

For a standard 100 amp 240V circuit, you need 3 AWG copper or 1 AWG aluminum wire. This assumes standard 75°C terminations, an ambient temperature of 30°C (86°F), and no more than three current-carrying conductors in a raceway. If your run exceeds 100 feet or involves bundling multiple circuits in one conduit, you will need to step up the wire size to compensate for voltage drop and thermal derating.

The 100 Amp Wire Size Chart (NEC Table 310.16)

The following data is extracted directly from NFPA 70 (National Electrical Code) Table 310.16. This table dictates the allowable ampacities for insulated conductors rated up to 2000 volts.

Bookmark Quick-Jump (The 100A Rows): As highlighted in the table above, the exact match for a 100-amp load in the 75°C column is 3 AWG Copper (100A) and 1 AWG Aluminum (100A). If you are pulling THHN/THWN-2 in conduit, you will buy 90°C wire, but you must still size it based on these 75°C termination limits unless your equipment is explicitly rated and marked for 90°C terminations (which is exceptionally rare for residential 100A breakers).

Which Column Applies and How Derating Modifies the Base Value

A common mistake on the jobsite is looking at the 90°C column for THHN wire, seeing that 4 AWG copper is rated for 95A, and assuming it is close enough, or misapplying the 90°C rating to the breaker lug. Here is how the NEC actually forces you to use the columns.

The Termination Rule (NEC 110.14(C)): The weakest link in the circuit dictates the temperature limit. Standard 100A breakers and panel lugs are tested and rated for 75°C. Therefore, your final circuit ampacity cannot exceed the value in the 75°C column. For 100A, that means 3 AWG Copper or 1 AWG Aluminum.

The Derating Exception (NEC 310.15(C)): You are allowed to use the 90°C column as your starting baseline when calculating thermal derating for bundling conductors. If you pull more than three current-carrying conductors through a single conduit, the wires heat each other up, and you must reduce their allowable ampacity.

What This Chart Cannot Tell You

While Table 310.16 is the bible for thermal ampacity, it operates in a vacuum. It assumes a 30°C ambient environment and short wire runs. It completely ignores the physics of resistance over distance.

Voltage Drop: The NEC does not strictly mandate voltage drop limits for branch circuits in the enforceable text, but Informational Note 210.19(A) strongly recommends keeping it under 3% for efficiency and equipment longevity. On a 240V circuit, a 3% drop is 7.2 volts.

If you are running a 100A 240V feeder to a detached garage 150 feet away using 3 AWG copper, the voltage drop formula (VD = 2 × K × I × D / CM) reveals a drop of roughly 9.6 volts (4%). This exceeds the 3% recommendation and can cause hard-starting motors (like air compressors or HVAC units) to overheat and trip their internal overloads. For a 150-foot run at a continuous 80A realistic load, you would need to step up to 1 AWG Copper or 2/0 AWG Aluminum to keep the voltage drop under 3%.

Conduit Fill Capacity: The chart tells you how much current the wire can carry, but Chapter 9 of the NEC dictates how many wires physically fit inside a pipe. Three 1 AWG aluminum wires plus an 8 AWG ground will not fit inside a 3/4-inch EMT conduit; you will need to step up to 1-inch conduit to avoid jamming and damaging the insulation during the pull.

Frequently Asked Questions

What size wire do I need for a 100 amp 240V subpanel?

For a 100A subpanel feeder, you need three current-carrying conductors (two hots, one neutral) and one equipment grounding conductor. Assuming standard distances and no bundling, use 3 AWG copper or 1 AWG aluminum for the hots and neutral. Modern NEC code (250.32) strictly requires a 4-wire feeder to a detached structure; you cannot use the neutral as a ground. The neutral and ground bars must remain isolated in the subpanel.

Can I use 2 AWG aluminum wire for a 100 amp 240V circuit?

No, 2 AWG aluminum is not sufficient for a full 100A load. According to the 75°C column in Table 310.16, 2 AWG aluminum is only rated for 90 amps. While you could technically protect it with a 90A breaker, if your main disconnect or subpanel main breaker is rated at 100A, you must use 1 AWG aluminum (rated exactly 100A at 75°C) to comply with NEC 240.4. Many electricians jump straight to 1/0 AWG aluminum (120A) for 100A feeders to provide a safety buffer and make the physical termination easier, as 1 AWG aluminum can be stiff and difficult to bend into tight panel gutters.

Does the 240V voltage change the wire size compared to 120V for 100 amps?

No. Wire ampacity is a measure of thermal dissipation based on current (amps) and resistance, not voltage. Whether you are pushing 100 amps at 120V or 100 amps at 240V, the wire generates the exact same amount of heat (I²R losses). Therefore, the AWG size remains 3 AWG copper or 1 AWG aluminum. However, the 240V system will experience a lower percentage of voltage drop over the same distance compared to a 120V system, which is why 240V is preferred for high-draw appliances and subpanels.

What size ground wire is required for a 100 amp 240V feeder?

According to NEC Table 250.122, the minimum equipment grounding conductor for a 100A overcurrent protective device is 8 AWG copper or 6 AWG aluminum. If you had to upsize your ungrounded (hot) conductors to compensate for voltage drop over a long distance, NEC 250.122(B) requires you to proportionally increase the size of your ground wire as well to maintain a low-impedance fault path.