The maximum allowable ampacity for 1/0 AWG (frequently searched by DIYers and apprentices as 1 0 awg ampacity) copper wire is 150 Amps in the 60°C column, 170 Amps in the 75°C column, and 195 Amps in the 90°C column. For 1/0 AWG aluminum wire, the ratings are 120A, 135A, and 155A respectively. However, your actual usable ampacity is almost always limited by the 75°C termination rating of your breakers and lugs, making 170A (copper) and 135A (aluminum) the practical maximums for standard residential and commercial installations.

Wire in the 1/0 AWG size (read as "one-aught") is the workhorse for 200-amp residential service entrances, large subpanel feeders, and heavy commercial machinery. Because a single sizing error at this amperage can result in melted lugs or a structure fire, relying on memory or rule-of-thumb is unacceptable. Below is the definitive reference data you need to size, terminate, and derate 1/0 AWG conductors safely.

The NEC 310.16 Ampacity Table for 1/0 AWG Wire

Before pulling wire, you must know how to read the National Electrical Code (NEC) ampacity tables. The table is split by conductor material (Copper vs. Aluminum/Copper-Clad Aluminum) and insulation temperature rating (60°C, 75°C, 90°C). To use it, locate your wire gauge on the left, move right to your material, and stop at the column matching your installation's lowest temperature rating. Note that 1/0 AWG building wire is almost exclusively stranded; solid wire in this size is virtually non-existent in modern branch and feeder circuits.

The following data is extracted directly from NFPA 70 National Electrical Code (NEC) Table 310.16 (formerly 310.15(B)(16) in older code cycles). This table assumes an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors in a raceway.

NEC Table 310.16 Allowable Ampacities (Excerpt)
AWG Size Copper 60°C Copper 75°C Copper 90°C Aluminum 60°C Aluminum 75°C Aluminum 90°C
1 AWG 130A 150A 165A 100A 120A 130A
1/0 AWG 150A 170A 195A 120A 135A 155A
2/0 AWG 175A 195A 225A 135A 150A 170A
3/0 AWG 200A 225A 260A 155A 175A 200A
Bookmark Tip: If you are sizing a standard 200A residential service, 1/0 AWG copper is insufficient for the full 200A load (max 170A at 75°C). You must step up to 2/0 AWG copper (195A) or 4/0 AWG aluminum (180A) to meet the 200A breaker termination requirement.

Which Temperature Column Applies to Your Installation?

The most common mistake made when looking up 1 0 awg ampacity is blindly using the 90°C column because modern wire insulation (like THHN or XHHW-2) is rated for 90°C. This violates NEC 110.14(C), which dictates that the ampacity of a circuit is limited by the lowest temperature rating of any connected component, including breakers, lugs, and splices.

The 75°C Rule for Terminations

Virtually all modern circuit breakers (Square D, Eaton, Siemens) and panelboard lugs are tested and rated for 75°C terminations. Therefore, even if you pull 90°C THHN copper wire, your final allowable ampacity at the termination point is capped at the 75°C column value: 170 Amps. You cannot put 195 Amps through a 1/0 AWG wire into a standard 75°C rated breaker lug without risking thermal failure of the breaker's internal bimetallic trip strip.

When to Use the 60°C Column

You must use the 60°C column (150A for copper) if your wiring method is inherently limited to 60°C. The most common example is NM-B (Romex) cable. While you rarely see 1/0 AWG NM-B in the wild, if you are using it for a large feeder, or if you are connecting to older equipment explicitly marked with a 60°C rating, your ampacity drops to 150A.

When to Use the 90°C Column

The 90°C column (195A for copper) is only used as a starting baseline for derating calculations (discussed below) or for equipment specifically listed and marked for 90°C terminations, which is exceedingly rare in residential and light commercial work.

Derating Factors and What the Base Table Cannot Tell You

The base ampacity table assumes ideal conditions: an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors bundled together. When real-world conditions deviate, you must apply derating factors. Furthermore, the NEC ampacity table only solves for thermal limits; it completely ignores voltage drop.

Applying Derating Factors

Derating is calculated using the 90°C column, and the resulting derated value is then compared against the 75°C termination limit. You use the lower of the two numbers.

Scenario: You are running two separate 240V circuits in a single conduit, meaning you have four current-carrying conductors (four "hot" wires). According to NEC Table 310.15(C)(1), 4 to 6 conductors require an 80% derating factor.

  • Step 1: Start with the 90°C copper ampacity for 1/0 AWG: 195A.
  • Step 2: Apply the 80% derating factor: 195A × 0.80 = 156A.
  • Step 3: Compare to the 75°C termination limit (170A).
  • Result: 156A is lower than 170A. Your final allowable ampacity for this 1/0 AWG wire in this conduit is 156A. You must size your breaker accordingly (e.g., a 150A breaker).

If your ambient temperature exceeds 30°C (such as running conduit across a hot roof or through a high-temperature attic), you must also apply the ambient temperature correction factors from the bottom of NEC Table 310.16. For example, at 40°C (104°F), the 90°C column requires a 0.91 multiplier.

The Blind Spot: Voltage Drop

What the ampacity table cannot tell you is whether your wire will deliver adequate voltage to the load. NEC 310.16 ensures the wire won't melt or start a fire, but it does not guarantee performance. NEC Informational Note 210.19(A)(1) recommends a maximum 3% voltage drop for feeders.

Let's look at the math for a 1/0 AWG copper feeder carrying 150A over a 200-foot run on a 240V circuit:

  • Formula: VD = (2 × K × I × D) / Circular Mils
  • K (Copper constant) = 12.9
  • I (Current) = 150A
  • D (Distance) = 200 ft
  • Circular Mils (for 1/0 AWG) = 105,600
  • Calculation: (2 × 12.9 × 150 × 200) / 105,600 = 7.32 Volts dropped.

A 7.32V drop on a 240V system is a 3.05% voltage drop. While thermally safe according to the ampacity table, it slightly exceeds the 3% recommended feeder limit. If this feeder is supplying sensitive electronics or a long branch circuit will be added downstream, you would need to upsize to 2/0 AWG copper to maintain optimal performance, despite 1/0 AWG being thermally sufficient.

Pro-Tip on Termination Torque: When terminating 1/0 AWG wire, hand-tightening is a fire hazard. The large surface area requires specific compression to prevent high-resistance arcing. Always use a calibrated torque wrench or torque screwdriver. For most 200A panel main lugs and large breaker lugs accepting 1/0 AWG, the manufacturer's specified torque is typically between 35 and 50 ft-lbs (420 to 600 in-lbs). Always verify the exact value printed on the breaker or panel label, as mandated by NEC 110.14(D).

For further verification of specific insulation types or complex bundling scenarios, cross-reference your project plans with the Cerrowire Ampacity Tables or utilize the Southwire Ampacity Chart tool to ensure your local environmental factors are properly accounted for.