1/0 AWG copper wire has a base ampacity of 150 amps in the 75°C column and 170 amps in the 90°C column. 1/0 AWG aluminum (or copper-clad aluminum) is rated for 120 amps at 75°C and 135 amps at 90°C. For 95% of residential and commercial feeders terminating on standard breakers, the National Electrical Code (NEC) requires you to use the 75°C column. Therefore, your maximum continuous overcurrent protection is 150A for copper and 120A for aluminum.
1/0 AWG Ampacity: The Quick Reference Table (NEC Table 310.16)
How to read this table: First, identify your conductor material (Copper vs. Aluminum). Next, check the insulation printed on the wire jacket (e.g., THHN, XHHW-2) to find its maximum temperature rating. Finally, apply the ambient temperature assumption: these base values assume an ambient temperature of 30°C (86°F). If your attic or conduit run exceeds 86°F, you must apply correction factors (covered below).
| AWG Size | Copper (60°C) | Copper (75°C) | Copper (90°C) | Aluminum (60°C) | Aluminum (75°C) | Aluminum (90°C) |
|---|---|---|---|---|---|---|
| 1 AWG | 110A | 130A | 145A | 85A | 100A | 115A |
| 1/0 AWG | 125A | 150A | 170A | 100A | 120A | 135A |
| 2/0 AWG | 145A | 175A | 195A | 115A | 135A | 150A |
| 3/0 AWG | 165A | 200A | 225A | 130A | 155A | 170A |
Bookmark the highlighted 1/0 AWG row for quick jobsite lookups. For the official NFPA 70 NEC standards, always consult the latest adopted edition in your municipality.
Which Temperature Column Actually Applies to Your Install?
The most common mistake DIYers and junior apprentices make is looking at a spool of 90°C THHN wire, seeing "170A" for 1/0 copper, and slapping it on a 175A breaker. This violates NEC 110.14(C) — the termination temperature rule.
The ampacity of a circuit is limited by the lowest temperature rating of any connected component, including the breaker lugs, busbars, and splices. Almost all modern residential and commercial breakers up to 600A are rated for 75°C terminations. Even if your wire insulation is rated for 90°C, the heat dissipation at the breaker lug is only validated up to 75°C.
Derating 1/0 Wire: When the 90°C Column Saves the Day
If the 75°C column dictates the breaker size, why does the 90°C column exist? It exists for derating. When you bundle multiple current-carrying conductors in a single conduit or run wire through a hot attic, the wire cannot dissipate heat as efficiently. The NEC requires you to reduce (derate) the wire's ampacity.
How derating modifies the base value: You always start your derating math using the 90°C column base value, apply the correction factor, and then verify that the final derated number is still higher than your load and your 75°C termination limit.
Worked Example: You are pulling four current-carrying conductors (two hots, one neutral, one ground doesn't count) through a conduit in an attic with an ambient temperature of 110°F (43°C).
- Base 90°C Ampacity (1/0 Cu): 170A
- Ambient Temp Correction (41-45°C): 0.87 multiplier
- Bundling Adjustment (4 conductors): 0.80 multiplier
- Math: 170A × 0.87 × 0.80 = 118.3A
In this scenario, your 1/0 copper wire is only good for 118.3A. You cannot use it for a 125A or 150A load. You would need to step up to 2/0 AWG or 3/0 AWG to compensate for the heat and bundling.
Decision Tree: Sizing Your 1/0 Breaker and Lugs
Use this decision path to lock in your exact hardware requirements without second-guessing the code book.
| Installation Condition | Required Action / Hardware Pick |
|---|---|
| Standard Subpanel Feeder (Copper, normal ambient, 3 wires in conduit) | Use 1/0 Cu THHN. Terminate on a 150A 75°C breaker. Torque lugs to manufacturer spec (usually 40-50 in-lbs). |
| Standard Subpanel Feeder (Aluminum, normal ambient) | Use 1/0 Al XHHW. Terminate on a 120A or 125A breaker. Apply antioxidant paste (e.g., Noalox) to aluminum strands before torquing. |
| High Ambient / Bundled (Derated ampacity drops below 150A) | Step up to 2/0 AWG Copper. Do not use 1/0 if the final derated math falls below your OCPD rating. |
| Long Distance Run (Over 150 feet at 240V) | Calculate voltage drop. You will likely need to upsize to 2/0 or 3/0 AWG to maintain ≤3% drop. |
What the Ampacity Table Cannot Tell You (Voltage Drop & Faults)
NEC Table 310.16 only tells you the thermal limit of the wire's insulation. It completely ignores two critical real-world physics problems: voltage drop and short-circuit let-through current.
1. Voltage Drop over Distance
Ampacity assumes the wire can handle the heat of 150A, but it doesn't guarantee the voltage will reach the other end. Using the standard voltage drop formula ($VD = \frac{2 \times K \times I \times D}{CM}$), we can map the limits of 1/0 copper at 150A on a 240V system:
- At 100 feet: ~3.6V drop (1.5%) — Excellent.
- At 150 feet: ~5.5V drop (2.3%) — Acceptable.
- At 200 feet: ~7.3V drop (3.0%) — Maximum recommended limit.
If your 150A feeder run exceeds 200 feet, 1/0 AWG is functionally useless for that load, even though it won't melt. You must upsize to 2/0 or 3/0 to keep motors and appliances from browning out. You can verify these calculations using the Southwire voltage drop calculators or similar engineering tools.
2. Short-Circuit Withstand (Let-Through Current)
If a dead short occurs, thousands of amps will surge through the wire before the breaker's magnetic trip clears the fault in a few milliseconds. 1/0 AWG copper has a specific cross-sectional area (105,600 circular mils) that dictates how much thermal energy it can absorb during a fault without the copper melting or the insulation vaporizing. If you are installing 1/0 wire on a service with exceptionally high available fault current (e.g., right next to a utility transformer), you must verify that your breaker's let-through energy ($I^2t$) does not exceed the wire's short-circuit withstand rating. For standard residential services, the 150A breaker clears fast enough to protect 1/0 wire, but in industrial settings, this requires an engineer's calculation.
