The Direct Answer: 1/0 AWG Ampacity Quick Lookup
For standard residential and commercial installations, the ampacity of 1/0 AWG (one-aught) copper wire is 150 amps, and for 1/0 AWG aluminum wire, it is 120 amps. These values are based on the 75°C temperature column of the National Electrical Code (NEC), which governs the vast majority of modern breaker and lug terminations.
Below is the complete baseline data extracted directly from NFPA 70 (NEC) Table 310.16. Use the quick-jump anchors if you are verifying a specific material on the jobsite.
| Conductor Material | 60°C Column (140°F) | 75°C Column (167°F) | 90°C Column (194°F) |
|---|---|---|---|
| Copper (THHN, THWN-2, XHHW-2) | 125A | 150A (Standard Default) | 170A |
| Aluminum (XHHW-2, THWN-2) | 100A | 120A (Standard Default) | 135A |
How to Read NEC Table 310.16 for 1/0 Wire
The most common mistake DIYers and junior apprentices make is looking at the 90°C column because the wire insulation (like THHN) is rated for 90°C, and assuming they can run 170 amps through 1/0 copper. You cannot. The NEC requires you to match the lowest temperature rating in the entire circuit, which is almost always the termination point (the breaker lug or panel busbar).
The 75°C Column: This is your default. Modern breakers rated 100A and higher, as well as standard panel lugs, are rated for 75°C. Use this column to determine your maximum breaker size and base ampacity.
The 90°C Column: You only use this column as the starting point for derating calculations (ambient temperature corrections or bundling adjustments). The final derated number must then be compared against the 75°C termination limit.
The 60°C Column: Rarely used for 1/0 wire today. It applies if you are terminating into legacy equipment explicitly marked for 60°C, or if you are somehow using 1/0 NM-B cable (which is virtually non-existent in modern manufacturing).
Derating 1/0 AWG: When Base Ampacity Drops
Table 310.16 assumes an ambient air temperature of 30°C (86°F) and no more than three current-carrying conductors (CCCs) in a raceway. When your installation deviates from this, you must apply correction factors to the 90°C column, not the 75°C column.
Worked Example: 1/0 Copper THHN in a Hot Attic
Imagine you are pulling three 1/0 copper THHN conductors through an attic where the ambient temperature reaches 110°F (43°C) in the summer.
- Identify the base 90°C ampacity: 170A.
- Find the ambient correction factor: Per NEC Table 310.15(B)(1), the correction factor for 110°F (43°C) at the 90°C rating is 0.87.
- Calculate the derated ampacity: 170A × 0.87 = 147.9A.
- Apply the termination limit: Your breaker lugs are rated 75°C (150A limit). Since your derated wire ampacity (147.9A) is lower than the termination limit, the wire's true allowable ampacity is now 147.9A.
Because 147.9A is below the standard 150A breaker threshold, you cannot protect this wire with a 150A breaker if the continuous load exceeds the wire's derated capacity. You must either step down to a 125A breaker, move the wire out of the hot attic, or upsize to 2/0 AWG.
Decision Tree: Sizing Your 1/0 Breaker and Installation
Use this decision matrix to lock in your exact material and breaker size based on your specific feeder or service entrance scenario. This eliminates the 'it depends' guesswork.
| Installation Scenario | Maximum Continuous Load | Environment / Routing | Concrete Pick: Wire & Breaker |
|---|---|---|---|
| Scenario A: 150A Subpanel Feeder (Standard) | 120A - 150A | Standard basement/crawlspace, ≤ 3 CCCs, ≤ 86°F | Buy: 1/0 AWG Copper THHN/THWN-2 Breaker: 150A (75°C rated) |
| Scenario B: 120A Subpanel Feeder (Budget) | 90A - 120A | Standard routing, looking to save on copper costs | Buy: 1/0 AWG Aluminum XHHW-2 Breaker: 120A (75°C rated, use antioxidant paste) |
| Scenario C: 150A Feeder in Hot Attic (110°F+) | 140A - 150A | Ambient temp exceeds 86°F, requires derating | Buy: 2/0 AWG Copper THHN (Upsized to survive derating) Breaker: 150A |
| Scenario D: High-Load 4-Wire Bundled Conduit | 130A | 4 to 6 current-carrying conductors in one conduit (80% derating) | Buy: 2/0 AWG Copper THHN (170A × 0.80 = 136A) Breaker: 150A (Load must not exceed 136A) |
What the Ampacity Table Cannot Tell You
While NEC Table 310.16 dictates thermal limits and breaker sizing, it is blind to three critical physical realities of your installation. If you ignore these, your 1/0 AWG wire will fail or perform poorly regardless of its ampacity rating.
1. Voltage Drop Over Distance
Ampacity tables assume the wire can handle the heat, but they do not account for voltage drop. 1/0 AWG copper has a cross-sectional area of 105,600 circular mils. If you are running a 150A load more than 100 feet from the main panel, you will exceed the recommended 3% voltage drop limit for branch circuits/feeders. For a 150A load at 150 feet on a 240V system, 1/0 copper will drop roughly 4.5% of the voltage. You must upsize to 2/0 or 3/0 AWG purely for voltage drop mitigation, even though 1/0 is thermally rated for the breaker.
2. Physical Lug and Conduit Fit
1/0 AWG wire is thick and stiff, especially in copper. Standard 1-inch PVC conduit can comfortably hold three 1/0 THHN wires (fill capacity is roughly 40% for three wires), but pulling it around multiple 90-degree sweeps without damaging the insulation requires proper pulling compound and a heavy-duty fish tape. Furthermore, verify that your destination lugs (like on a small disconnect switch or a specific HVAC unit) are physically rated to accept 1/0 wire; some 100A lugs max out at #2 AWG.
3. Short-Circuit Withstand Rating
Ampacity protects against sustained overloads. It does not protect against the explosive magnetic and thermal forces of a dead short. The available fault current at your panel (often 10,000A to 22,000A in modern utility drops) requires your breaker to have an adequate AIC (Ampere Interrupting Capacity) rating. 1/0 copper can withstand high short-circuit currents for the fractions of a second it takes a breaker to trip, but only if the breaker itself is rated for the available fault current at the service entrance.
Final Recommendation: For a standard 150A residential subpanel feeder under 100 feet, purchase 1/0 AWG Copper THHN/THWN-2 and pair it with a 150A 75°C-rated breaker. If the run exceeds 100 feet, or if the wire passes through an environment exceeding 86°F, upsize to 2/0 AWG Copper to maintain both voltage stability and derated thermal headroom.
