The baseline ampacity of 1/0 AWG copper wire is 150 amps in the 75°C column and 170 amps in the 90°C column. For 1/0 AWG aluminum (or copper-clad aluminum), the ampacity is 120 amps (75°C) and 135 amps (90°C). These values are sourced directly from NEC Table 310.16. However, simply matching a 170A breaker to 1/0 copper THHN is a code violation in almost all residential and commercial scenarios. To size your overcurrent protection correctly, you must understand how termination temperature limits and derating factors modify these baseline numbers.
The 1/0 AWG Ampacity Reference Table (NEC Table 310.16)
Before pulling wire, you need to know how to read the ampacity charts. The National Electrical Code (NEC) organizes conductor ampacity by material (Copper vs. Aluminum) and by the temperature rating of the wire's insulation (60°C, 75°C, and 90°C). The values below assume an ambient air temperature of 30°C (86°F) and not more than three current-carrying conductors in a raceway or cable. If your installation deviates from these baseline conditions, you must apply correction factors, which we will cover below.
| AWG Size | Material | 60°C (140°F) NM-B, UF-B |
75°C (167°F) THHW, THWN, XHHW |
90°C (194°F) THHN, XHHW-2 |
|---|---|---|---|---|
| 1 AWG | Copper | 110 A | 130 A | 145 A |
| 1 AWG | Aluminum | 85 A | 100 A | 115 A |
| 1/0 AWG | Copper | 125 A | 150 A | 170 A |
| 1/0 AWG | Aluminum | 100 A | 120 A | 135 A |
| 2/0 AWG | Copper | 145 A | 175 A | 195 A |
| 2/0 AWG | Aluminum | 115 A | 135 A | 150 A |
Note: Ampacity values are for standard installation conditions. Always verify with the latest adopted edition of the NEC in your jurisdiction, as local AHJs may have specific amendments. For comprehensive code context, refer to the National Fire Protection Association (NFPA) NEC portal.
Which Temperature Column Applies to Your Installation?
The most common mistake DIYers and junior electricians make is looking at the 90°C column because they bought THHN or XHHW-2 wire, and then sizing the breaker to that higher number. This violates NEC 110.14(C), which governs termination temperature limits.
The Weakest Link Rule: The ampacity of your circuit is limited by the lowest temperature rating of any connected component, termination, or conductor in the circuit. Almost all standard residential and commercial circuit breakers, panelboard lugs, and disconnect switches are tested and rated for 75°C terminations.
Code Caveat: Even if your 1/0 AWG THHN wire has a 90°C insulation rating (170A), the breaker lug it lands on is likely only rated for 75°C. Therefore, you must use the 75°C column (150A) to determine your maximum overcurrent protection size. The 90°C column is generally only used as a starting point for calculating derating factors.
When would you use the 60°C column? You are forced down to the 60°C column (125A for 1/0 copper) if you are using older NM-B (Romex) cable, UF-B underground feeder cable, or if the equipment is explicitly marked for 60°C conductors, which is common in older panels or specific HVAC disconnects.
How Derating Modifies the Base 1/0 Ampacity
The values in Table 310.16 assume your wire is in a raceway with no more than three current-carrying conductors (e.g., two hots and a neutral in a single-phase 120/240V system) and an ambient temperature not exceeding 86°F (30°C). When you bundle wires together or run them through hot spaces, the heat cannot dissipate. The insulation will melt or degrade before the breaker trips. To prevent this, you must apply derating (adjustment and correction) factors.
Worked Numeric Example:
Imagine you are running a 1/0 AWG copper THHN feeder through an attic in Texas where the ambient temperature reaches 115°F (46°C), and you have pulled 4 current-carrying conductors in the same conduit.
- Start with the 90°C base ampacity: 170A (from the table above).
- Apply the bundling adjustment factor: NEC Table 310.15(C)(1) states that 4 to 6 current-carrying conductors require an 80% adjustment factor.
170A × 0.80 = 136A. - Apply the ambient temperature correction factor: NEC Table 310.15(B)(1) dictates that for 90°C insulation at 113-122°F, the correction factor is 87% (0.87).
136A × 0.87 = 118.32A.
Your final derated ampacity for this specific 1/0 AWG copper run is 118 amps. Because standard breakers do not come in 118A, and NEC 240.4(B) allows rounding up to the next standard size only if the calculated load doesn't exceed the conductor's derated ampacity, you would be limited to a 110A breaker for this specific installation, despite using massive 1/0 wire. If you need a 150A feeder in this hot attic, you must upsize to 2/0 or 3/0 AWG to compensate for the thermal derating.
What the Ampacity Table Cannot Tell You
While NEC Table 310.16 is the bible for thermal limits, it completely ignores the physics of voltage drop and physical installation constraints. Relying solely on the ampacity chart can result in a system that is legally compliant but functionally useless.
- Voltage Drop Over Distance: Ampacity measures heat, not voltage loss. If you run a 1/0 AWG aluminum feeder 200 feet to a detached garage subpanel, the wire won't overheat at 100A, but the voltage at the far end will sag well below 114V under load, causing motors to overheat and lights to flicker. Industry best practice (and a requirement in some local codes) limits voltage drop to 3% for branch circuits and 5% total from service to appliance. For long runs, you must calculate voltage drop using the specific resistance (ohms per 1000 ft) of your wire, often requiring you to upsize beyond what the ampacity table demands.
- Conduit Fill Limits: Chapter 9 of the NEC limits how much physical space wires can take up inside a conduit (typically 40% for three or more wires). 1/0 AWG wire is exceptionally thick. Pulling four 1/0 THHN conductors requires a minimum of 1.5-inch Schedule 80 PVC or 1.25-inch EMT, which drastically increases material costs and pulling tension.
- Bending Radius and Pulling Tension: 1/0 cable is stiff. The NEC mandates specific bending radii (typically 4 to 8 times the cable diameter depending on the jacket). If you are pulling this into a tight junction box, you may physically not be able to make the bend without kinking the copper, which creates a high-resistance hot spot that the ampacity table cannot predict.
Always treat the ampacity table as the minimum thermal baseline. For long feeders, subpanel supplies, or high-ambient environments, run a voltage drop calculation and consult industry resources on termination temperature ratings to ensure your final wire size covers both thermal safety and functional performance.
