3/0 THHN Ampacity Chart: NEC Table 310.16 Reference & Derating

The direct answer for 3/0 AWG copper THHN wire ampacity depends on the temperature column you are referencing in NEC Table 310.16. In the 90°C column, 3/0 copper THHN is rated for 225 amps. In the 75°C column, it is rated for 200 amps, and in the 60°C column, it is rated for 165 amps. For aluminum 3/0 THHN, the ratings are 180A (90°C), 155A (75°C), and 130A (60°C). However, because most standard residential and commercial breakers and lugs are rated for 75°C, your final usable ampacity for termination purposes is almost always capped at 200A for copper and 155A for aluminum, regardless of the wire's 90°C insulation rating.

The 3/0 THHN Ampacity Reference Table (NEC 310.16)

How to read this table: This data is extracted directly from NEC Table 310.16 (formerly 310.15(B)(16)). The columns represent the temperature rating of the wire's insulation, not the temperature of your breaker or lugs. THHN wire is typically dual-rated as THWN-2, meaning it carries a 90°C rating in dry locations and a 90°C rating in wet locations. To use this table, find your conductor material (Copper or Aluminum) and trace across to the temperature column that matches either your termination equipment (usually 75°C) or your derating starting point (90°C). Bookmark the specific rows below for quick jobsite reference.

Which Temperature Column Applies to Your Installation?

A common mistake among DIYers and junior apprentices is looking at the 90°C column (225A for 3/0 copper) and assuming they can put that wire on a 225-amp breaker. You cannot. NEC Article 110.14(C) governs termination temperature limitations. This rule states that the ampacity of a conductor must be selected based on the lowest temperature rating of any connected termination, conductor, or device.

In 99% of residential and commercial panels, the lugs on the breakers and the bus bars are tested and rated for 75°C. Therefore, even though your 3/0 THHN wire has 90°C insulation that can physically handle 225 amps without melting, the breaker lug it connects to will overheat if pushed past its 75°C limit. Consequently, the 75°C column (200A for copper, 155A for aluminum) is your maximum allowable ampacity for standard terminations.

There is one major exception where the 60°C column applies: if you are connecting to equipment explicitly marked for 60°C, or if you are wiring circuits rated 100 amps or less using conductors sized 14 AWG through 1 AWG. Since 3/0 AWG is larger than 1 AWG, this 60°C restriction generally does not apply to 3/0 wire unless you are dealing with very old, specific legacy equipment.

How Derating Rows Modify the Base 3/0 Value

This is where the 90°C column finally gets to shine. While you cannot use the 90°C column for your final termination ampacity, NEC 310.14(B) allows you to use the 90°C column as your starting base when calculating derating factors for ambient temperature and conductor bundling. After you apply the derating math, you compare the result to the 75°C termination limit, and the lower of the two numbers becomes your final legal ampacity.

Let's run a real-world jobsite calculation. Imagine you are pulling four current-carrying 3/0 copper THHN conductors through a single conduit on a rooftop where the ambient temperature reaches 110°F (43°C).

1. Establish the Base: Start with the 90°C column value for 3/0 Copper: 225A.
2. Ambient Temperature Correction: Per NEC Table 310.15(B)(1), the correction factor for 90°C insulation at 105-113°F (41-45°C) is 0.82.
   Math: 225A × 0.82 = 184.5A.
3. Bundling Adjustment: Per NEC Table 310.15(C)(1), having 4 to 6 current-carrying conductors in a single raceway requires an adjustment factor of 0.80.
   Math: 184.5A × 0.80 = 147.6A.
4. Final Comparison: Your derated ampacity is 147.6A. Compare this to your 75°C termination limit (200A). Since 147.6A is lower than 200A, 147.6A is your final allowable ampacity.

In this scenario, you cannot use a 150A breaker (as standard breaker sizes don't allow rounding up past the derated limit unless specific next-size-up rules apply, and 147.6A doesn't qualify for the next standard 150A size under standard continuous load rules without specific overcurrent protection adjustments). You would need to step down to a 125A breaker, or pull a larger wire like 4/0 AWG to compensate for the heat and bundling.

What the Ampacity Table Cannot Tell You

While manufacturer spec sheets and NEC Table 310.16 are the ultimate authority on thermal limits, ampacity tables are only half the battle when sizing 3/0 wire for a heavy feeder or service entrance. You must also account for the following physical and electrical realities:

• Voltage Drop: Ampacity tells you if the wire will melt; it does not tell you if your equipment will run properly. For a 200A feeder running 150 feet, 3/0 copper will experience roughly a 2.5% voltage drop at full load, which is acceptable (under the 3% NEC recommendation for feeders). However, if that same 200A load is 300 feet away, the voltage drop jumps to 5%, which will cause motors to overheat and electronics to brown out. You would need to upsize to 4/0 or 250 kcmil purely for voltage drop, even though 3/0 is thermally sufficient.
• Pulling Tension and Bend Radius: 3/0 THHN is thick, stiff, and heavy. The maximum pulling tension for copper conductors is generally calculated at 0.008 pounds per circular mil. Exceeding this stretches the copper, altering its resistance and potentially damaging the insulation. Furthermore, NEC Chapter 9 dictates minimum bend radiuses; you cannot jam 3/0 wire into a tight LB conduit body without violating the bend radius and risking insulation tearing.
• Torque Specifications: A 3/0 wire improperly torqued into a 200A main breaker lug will create a high-resistance connection. This generates localized heat that will trip the breaker thermally long before the wire itself reaches its 200A ampacity. Always use a calibrated torque screwdriver or wrench and follow the exact inch-pound or foot-pound value printed on the breaker's wiring diagram label.
• Local AHJ Amendments: The NEC is a model code. Your local Authority Having Jurisdiction (AHJ) or city inspector may have local amendments that restrict aluminum wire for certain interior feeder applications or require specific colored insulation for neutrals and grounds. Always verify local codes before purchasing 300 feet of 3/0 wire.