1/0 AWG aluminum wire is rated for 120 amps under the 75°C column (standard for THHN/XHHW-2 insulation in conduit) and 100 amps under the 60°C column (required for NM-B cable or older equipment terminations). If you are pulling a feeder to a modern 100A residential subpanel, 1/0 aluminum is the exact, code-compliant pick. Wire ampacity is a thermal limit, not a power conversion, but translating that 120A limit into usable kilowatts requires factoring in your system voltage and power factor.
The Core Ampacity Numbers and the Power Formula
Wire doesn't "convert" to amps; it has a maximum thermal threshold before the insulation degrades. According to standard NEC Table 310.16 ampacity charts, 1/0 aluminum hits its 120A limit at 75°C. However, to understand what that 120A limit actually powers, we use the single-phase power formula:
Substituted Values (240V Resistive Load): P = 240V × 120A × 1.0 (PF) = 28,800 Watts (28.8 kW)
This means a 1/0 aluminum feeder on a 240V split-phase system can safely deliver 28.8 kW of continuous real power to a purely resistive load (like a bank of baseboard heaters) before the conductor exceeds its thermal rating.
Neighboring Wire Sizes (±20% Ampacity Range)
When sizing feeders, it is critical to know the exact step-up and step-down values to avoid over-purchasing copper or under-sizing aluminum. Here are the NEC 310.16 limits for aluminum conductors in a standard 30°C ambient environment with no more than three current-carrying conductors in a raceway.
| AWG Size | 60°C Column (NM-B / Old Terminals) | 75°C Column (THHN / XHHW-2) | 90°C Column (Derating Only) |
|---|---|---|---|
| 1 AWG AL | 85 Amps | 100 Amps | 115 Amps |
| 1/0 AWG AL | 100 Amps | 120 Amps | 135 Amps |
| 2/0 AWG AL | 115 Amps | 135 Amps | 150 Amps |
| 3/0 AWG AL | 130 Amps | 155 Amps | 170 Amps |
Note: The 90°C column is strictly used for ambient temperature derating calculations per NEC 310.15(B). The final derated ampacity must still be compared against the 75°C or 60°C termination limits.
How Voltage, Phase, and Power Factor Shift the Load
A common point of confusion is assuming the wire's ampacity changes based on the system voltage. The thermal ampacity (120A) is fixed regardless of voltage. What shifts drastically is the usable power delivery (kilowatts) based on your regional grid and phase configuration.
- 120V (US Single-Phase): 120A × 120V × 1.0 PF = 14.4 kW
- 230V (EU/UK Single-Phase): 120A × 230V × 1.0 PF = 27.6 kW
- 240V (US Split-Phase): 120A × 240V × 1.0 PF = 28.8 kW
- 400V (EU 3-Phase): 120A × 400V × 1.732 × 0.8 PF = 66.5 kW
If you are trying to size this wire based on a motor's kilowatt rating and the Power Factor (PF) is unknown, the conversion is meaningless. A 25 kW resistive heater (PF=1.0) draws 60A at 240V. A 25 kW industrial compressor motor (PF=0.6) draws 100A at 240V. Without knowing the PF, you cannot safely convert a kW load into amps to verify if 1/0 aluminum is sufficient.
Decision Tree: Picking the Right Aluminum Feeder
Stop guessing which temperature column applies to your project. Use this decision path to terminate on a concrete purchase.
| If your installation scenario is... | Then apply this NEC rule... | Concrete Pick / Action |
|---|---|---|
| Feeding a modern 100A main-breaker subpanel using individual wires in PVC conduit. | NEC 110.14(C) allows 75°C column for modern breakers. | Buy 1/0 AWG XHHW-2 Aluminum. (Rated 120A, safely protected by 100A breaker). |
| Running a direct-burial underground feeder to a detached garage. | Underground service entrance cable (USE-2) defaults to 75°C wet location ratings. | Buy 1/0 AWG USE-2 Aluminum. (Rated 120A). |
| Wiring an older home with original 1960s breakers or using NM-B (Romex) cable. | NEC 110.14(C) mandates 60°C column for older equipment and NM-B insulation. | Buy 1/0 AWG Aluminum. (Strictly limited to 100A. Do not exceed 100A breaker). |
| Connecting to a 125A main breaker panel. | NEC 240.4(B) "Next Size Up" rule applies if ampacity doesn't match a standard breaker. | Buy 1/0 AWG XHHW-2 Aluminum. (120A wire can be protected by the next standard size up: 125A). |
Critical Assumptions: When 120 Amps Becomes Unsafe
The 120A rating for 1/0 aluminum is not an absolute universal constant. It is fixed by three specific assumptions outlined in the National Electrical Code (NFPA 70). If your jobsite violates these assumptions, your wire will overheat.
- Ambient Temperature (30°C / 86°F): The 120A rating assumes the air or earth around the conduit does not exceed 30°C. If you are pulling this wire through a hot attic in Arizona where ambient temps hit 50°C (122°F), you must apply a temperature correction factor. At 50°C, the 75°C column requires a 0.67 multiplier. 120A × 0.67 = 80.4 Amps. In a hot attic, 1/0 aluminum is only good for 80 amps.
- Conductor Bundling (Max 3 Current-Carrying): The baseline rating assumes 3 or fewer current-carrying conductors in a raceway. If you pull four 1/0 aluminum wires (e.g., two 240V circuits sharing a neutral) through the same PVC conduit, you must apply an 80% derating factor. 120A × 0.80 = 96 Amps.
- Termination Torque: Aluminum expands and contracts more than copper under thermal load. You must use a calibrated torque screwdriver or wrench to tighten the breaker lugs to the manufacturer's exact spec (usually printed on the breaker label, often between 25-45 in-lbs for smaller lugs, or higher for main lugs). Hand-tightening aluminum feeders is the leading cause of melted lugs and subpanel fires.
Default Recommendation: For 95% of modern residential subpanel feeder runs in conduit, purchase 1/0 AWG XHHW-2 aluminum. It provides a robust 120A capacity, handles wet/dry locations, and easily slides into standard 1.5-inch PVC conduit without excessive pulling tension.
