3/0 Ampacity Chart: Copper vs Aluminum Wire Sizing Guide

The base ampacity for 3/0 AWG copper wire is 200 amps in the 75°C column and 225 amps in the 90°C column. For 3/0 AWG aluminum wire, the base ampacity is 155 amps (75°C) and 175 amps (90°C). These values are sourced directly from NEC Table 310.16 (2023/2026 editions). However, the number you can actually use to size your breaker depends entirely on your termination equipment, ambient temperature, and conduit bundling.

3/0 AWG Ampacity Chart (NEC Table 310.16)

Before pulling wire, you must understand how to read the ampacity table. The National Electrical Code (NEC) organizes ampacity by conductor material (Copper vs. Aluminum/Copper-Clad) and insulation temperature rating.

• 60°C Column: Used for older insulation types (TW, UF) and circuits rated 100A or less where the equipment termination rating is unknown.
• 75°C Column: The standard for modern feeder wire (THW, THWN, XHHW) and the default termination limit for most breakers and lugs rated over 100A.
• 90°C Column: Applies to modern thermoplastic like THHN and THWN-2. You almost never use this column for final breaker sizing. It exists solely to provide a higher baseline for derating calculations before you hit the 75°C termination bottleneck.

The following data is extracted from Southwire's official ampacity charts and aligned with NEC Table 310.16 for 60°C to 90°C rated conductors, not more than three current-carrying conductors in a raceway, at an ambient temperature of 30°C (86°F).

Applying Derating Factors to 3/0 Feeders

The base values in the table above assume ideal conditions: an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors (CCCs) in a single conduit. When you deviate from these conditions, you must apply derating factors to the 90°C column, even if your final termination limit is 75°C.

Example Scenario: You are pulling four current-carrying conductors (e.g., two hots, a neutral carrying unbalanced nonlinear load, and a grounded phase for a multi-wire branch circuit or specific 3-phase setup) through a conduit in an attic where the ambient temperature reaches 35°C (95°F).

1. Start with the 90°C base: 3/0 Copper THHN = 225A.
2. Apply bundling derating: NEC Table 310.15(C)(1) dictates an 80% adjustment factor for 4-6 CCCs. (225A × 0.80 = 180A).
3. Apply ambient temperature correction: NEC Table 310.15(B)(1) dictates a 0.94 correction factor for 90°C wire at 31-35°C. (180A × 0.94 = 169.2A).

Your final derated ampacity for this 3/0 copper wire is 169.2 amps. Because this is below the 75°C termination limit of 200A, you must size your overcurrent protective device (breaker) at or below 169.2A. The next standard breaker size down is 150A. If you need a full 200A capacity in this hot, bundled environment, 3/0 wire is insufficient; you must step up to 2/0 or 4/0 depending on the exact math.

What the Ampacity Table Cannot Tell You

Looking up 3/0 ampacity is only the first step in feeder design. The NEC table does not account for three critical real-world constraints:

1. Termination Temperature Limits (NEC 110.14(C))

As detailed in Mike Holt's NEC 110.14(C) breakdown, you cannot use the 90°C ampacity to size a breaker unless every single termination point in the circuit (lugs, busbars, breaker terminals) is explicitly rated for 90°C. Standard residential and commercial breakers are rated 75°C. Therefore, a 3/0 copper wire is capped at 200A by the breaker lugs, regardless of the wire's 225A 90°C rating.

2. Voltage Drop Over Distance

Ampacity dictates thermal safety; it does not guarantee performance. If you run a 3/0 copper feeder at a full 200A load over a distance of 250 feet on a 240V system, the voltage drop will be approximately 7.68V (3.2%). This exceeds the NEC recommended 3% maximum for feeders. For long runs, you must upsize to 4/0 or 250 kcmil to maintain voltage stability, even if 3/0 is thermally adequate.

3. Conduit Fill and Physical Bending Radius

3/0 AWG wire is exceptionally thick and stiff. If you are pulling three 3/0 THHN conductors, the total cross-sectional area is roughly 0.801 square inches. According to NEC Chapter 9, Table 5, a 1.5-inch Schedule 40 PVC conduit only allows 0.794 square inches at 40% fill. You will physically jam the wires if you use 1.5-inch PVC. You must step up to a 2-inch conduit (1.15 sq in capacity) to meet code and actually get the wires pulled without damaging the insulation.