How Many Amps Is 1/0 Aluminum Good For? NEC Ampacity & Load Guide

1/0 AWG aluminum wire is good for 120 amps in standard residential and commercial applications. This baseline assumes a 75°C temperature rating at the terminations (like standard breakers and lugs) and an ambient temperature of 30°C (86°F). If your equipment is strictly rated for 60°C, the limit drops to 100 amps. While the wire's 90°C insulation (like THHN or XHHW-2) can physically handle 135 amps without melting, NEC 110.14(C) termination rules cap your usable ampacity at the lowest-rated component in the circuit. Since almost all standard breakers and panel lugs are rated for 75°C, your 1/0 aluminum feeder is legally and safely limited to 120A.

The Core Ampacity Assumptions (What Fixes the Number)

Wire doesn't have a single universal ampacity; it has a thermal limit based on its insulation and its environment. The numbers above are fixed by three critical assumptions derived from NFPA 70 (NEC) Table 310.16:

• Insulation Type: We assume modern THHN, THWN-2, or XHHW-2. If you are using older USE-2 or UF-B, the limits drop significantly.
• Ambient Temperature: The base table assumes 30°C (86°F). If your wire runs through a hot attic or outdoors in a desert climate, you must derate.
• Termination Limits: As outlined by EC&M's guide on NEC 110.14(C), you must use the 60°C column for circuits 100A or less (unless marked otherwise), and the 75°C column for circuits over 100A. Since 1/0 Al is typically used for 100A-120A feeders, the 75°C column (120A) is your governing limit for loads over 100A.

Neighboring Wire Sizes (±20% Ampacity Range)

When designing a subpanel feeder or service entrance, you rarely land exactly on the wire's maximum limit. Here is how 1/0 AWG aluminum compares to its immediate neighbors in the 75°C column, which is the standard for modern terminations.

How Voltage and Phase Shift the Power (Not the Amps)

A common point of confusion on the workbench is asking how the ampacity shifts when moving from 120V to 240V or 3-phase. The ampacity does not shift. A 1/0 aluminum wire will overheat and fail at 121 amps whether you push 12 volts or 600 volts through it (assuming 600V-rated insulation). Voltage dictates the power (Watts) the wire can deliver, not the current limit.

Here is how the total power delivery shifts across common system voltages at the full 120A limit:

• 120V Single-Phase: $120A \times 120V = 14,400W$ (14.4 kW)
• 240V Single-Phase: $120A \times 240V = 28,800W$ (28.8 kW) — Standard residential subpanel.
• 208V 3-Phase: $120A \times 208V \times \sqrt{3} = 43,265W$ (43.2 kW) — Light commercial.
• 480V 3-Phase: $120A \times 480V \times \sqrt{3} = 99,763W$ (99.7 kW) — Industrial motor feeds.

When this conversion is meaningless: Calculating the actual real power (Watts) this wire can deliver to an AC motor or inductive load is meaningless if the Power Factor (PF) is unknown. The true AC power formula is $P = V \times I \times PF$. Without knowing the PF, you only calculate Apparent Power (VA). If you size a 120A breaker for a 3-phase motor with a terrible 0.6 PF, the wire will carry 120A of current but only deliver a fraction of the expected mechanical work, potentially causing voltage drop issues while the breaker happily stays closed.

When the Base Ampacity Fails: The Derating Formula

The 120A limit assumes perfect conditions. In the real world, you bundle wires in conduit or run them through hot attics. When conditions change, you must use the NEC ambient temperature and bundling derating formula:

$I_{adjusted} = I_{base} \times C_{temp} \times C_{bundling}$

Worked Example: You are running three current-carrying 1/0 Al THHN conductors through a conduit in an attic that reaches 45°C (113°F).

1. Base ($I_{base}$): We use the 90°C column for derating math: 135A.
2. Temp Correction ($C_{temp}$): NEC Table 310.15(B)(1) gives a factor of 0.82 for 90°C wire at 45°C.
3. Bundling ($C_{bundling}$): 3 conductors = 1.0 (no adjustment).
4. Substitution: $135A \times 0.82 \times 1.0 = 110.7A$.

Now, compare the adjusted 90°C ampacity (110.7A) to the 75°C termination limit (120A). You must use the lower of the two. Your 1/0 aluminum wire in this hot attic is now only good for 110 amps. You cannot put it on a 120A breaker; you must downsize the breaker to 110A (or use the next standard size down, 100A) or upsize the wire to 2/0 AWG.

Decision Tree: Is 1/0 Aluminum Right for Your Feeder?

Use this decision path to lock in your exact wire and breaker size without guessing.