Ampacity of 3/0 Copper Wire: NEC Table 310.16 Chart & Sizing Guide

The Quick Answer: Ampacity of 3/0 Copper Wire

The allowable ampacity of 3/0 copper wire depends entirely on the temperature rating of the wire insulation and the equipment terminations it connects to. According to NFPA 70 (National Electrical Code) Table 310.16, the base ampacities for 3/0 AWG copper are:

• 165 Amps in the 60°C (140°F) column
• 200 Amps in the 75°C (167°F) column
• 225 Amps in the 90°C (194°F) column

How to read this data: Wire insulation (like THHN or XHHW-2) can handle high heat, but the circuit breakers, lugs, and busbars it connects to often cannot. The NEC requires you to use the lowest temperature rating in the entire circuit loop to determine your final allowable ampacity. The 90°C column is rarely used for final overcurrent protection sizing; it is primarily used as the starting baseline for calculating derating adjustments when wires are bundled in conduit or exposed to high ambient temperatures.

NEC Table 310.16: Copper Ampacity Chart

When sizing feeders or service entrance conductors, you rarely look at just one wire size in isolation. Below is a data-dense excerpt from NEC Table 310.16 for copper conductors, focusing on the heavy-gauge sizes most commonly queried for 150A to 250A installations. Bookmark this row for quick jobsite reference.

Table 310.16: Allowable Ampacities of Insulated Copper Conductors (Rated 0-2000 Volts, 60°C through 90°C, Not More Than Three Current-Carrying Conductors in Raceway, 30°C Ambient)

AWG Size	60°C (140°F) Column	75°C (167°F) Column	90°C (194°F) Column	Common Insulation Types
1/0 AWG	125 A	150 A	170 A	TW, UF (60°C); THW, THWN (75°C)
2/0 AWG	145 A	175 A	195 A	XHHW, USE (75°C)
3/0 AWG	165 A	200 A	225 A	THHN, THWN-2, XHHW-2 (90°C)
4/0 AWG	195 A	230 A	260 A	Most modern building wire

Which Temperature Column Applies to Your Installation?

The most common mistake DIYers and junior apprentices make is looking at the 90°C column because modern wire (like THHN/THWN-2) is stamped with a 90°C rating on the jacket. You cannot use the 90°C ampacity (225A) to size your breaker unless every single termination in the circuit is also rated for 90°C. In the real world, they almost never are.

The 75°C Default Rule (NEC 110.14(C))

For circuits rated over 100 amps, NEC 110.14(C) dictates that you must use the 75°C column unless the equipment is explicitly marked otherwise. Almost all modern 200A main breaker panels, subpanel lugs, and heavy-duty disconnects are rated for 75°C. Therefore, the 75°C column (200 Amps) is the governing value for a standard 3/0 copper wire installation. You can safely protect this wire with a 200A breaker.

When the 60°C Column Applies

The 60°C column (165 Amps) applies if you are terminating the 3/0 wire onto older equipment, specific types of HVAC disconnects, or NM-B (Romex) cable transitions that are strictly limited to 60°C. If your panel lugs are only rated for 60°C, a 3/0 copper wire is legally limited to 165 amps, meaning you must drop your breaker size to 150A (the next standard standard breaker size down, per NEC 240.4(B)).

When the 90°C Column is Actually Used

The 90°C column (225 Amps) is your starting point for derating. If your installation requires ampacity adjustments due to conduit fill or high ambient heat, you calculate the penalty against the 90°C value, then verify the final result doesn't exceed the 75°C termination limit.

Derating and What the Base Table Cannot Tell You

NEC Table 310.16 assumes ideal conditions: an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors in a single raceway. When real-world conditions deviate, the base ampacity of 3/0 copper wire must be modified.

Conduit Bundling Derating

If you pull two separate 240V circuits (four current-carrying hot wires) through the same conduit, NEC Table 310.15(C)(1) requires an 80% derating factor. Here is how the math works on the bench:

1. Start with the 90°C ampacity for 3/0 copper: 225A.
2. Apply the 80% adjustment factor: 225A × 0.80 = 180A.
3. Check the termination limit (75°C column): 200A.
4. Because 180A is lower than 200A, the derated value governs. Your 3/0 wire is now legally limited to 180 amps and must be protected by a 175A breaker.

Ambient Temperature Corrections

If you are running 3/0 THHN across the roof of a commercial building in Arizona where the ambient temperature hits 113°F (45°C), you must apply a temperature correction factor of 0.87 to the 90°C column (225A × 0.87 = 195.75A). Refer to the bottom of Table 310.16 or manufacturer spec sheets from Southwire for the exact correction matrices based on your specific insulation type.

What the Ampacity Table Cannot Tell You

Table 310.16 only tells you what the wire can handle thermally before the insulation degrades. It does not account for three critical jobsite realities:

• Voltage Drop: 3/0 copper has a DC resistance of roughly 0.0782 ohms per 1,000 feet at 75°C (per NEC Chapter 9, Table 8). If you are running a 200A feeder to a detached garage 150 feet away, you will experience roughly a 4.7V drop (nearly 4% on a 120V leg). While the NEC doesn't strictly mandate a hard voltage drop limit for feeders in all jurisdictions, exceeding 3% is considered poor design and can cause motor burnout or flickering lights. For a 150-foot 200A run, you should step up to 4/0 or 250 kcmil copper.
• Physical Lug Fitment: 3/0 AWG wire is thick and stiff. Bending it into the tight confines of a residential 200A load center can be a physical struggle. Always use a proper wire bending tool and verify the panel manufacturer's bending radius specifications before terminating.
• Torque Requirements: A 200A termination requires exact torque. Under-torquing a 3/0 lug causes high resistance and thermal runaway; over-torquing strips the lug threads. Always use a calibrated inch-pound torque screwdriver or torque wrench set to the exact value printed on the breaker or panel label (typically between 40 and 60 in-lbs for these lugs, but verify the manufacturer spec).