The base ampacity for 1/0 AWG copper wire is 150 amps in the 75°C column and 170 amps in the 90°C column. For 1/0 AWG aluminum wire, the base ampacity is 120 amps (75°C) and 135 amps (90°C). These values are derived directly from NEC Table 310.16, assuming an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors in a raceway. However, the number you can actually use for your breaker sizing depends entirely on your termination equipment and conduit fill.
The 1/0 AWG Ampacity Reference Table (NEC 310.16)
Before sizing your feeder or service entrance, you need to know how to read the ampacity tables. The rows represent the wire gauge (AWG or kcmil), while the columns represent the temperature rating of the wire's insulation (60°C, 75°C, and 90°C). The values below assume standard installation conditions: an ambient air temperature of 30°C and a maximum of three current-carrying conductors bundled together. If your installation exceeds these parameters, you must apply derating factors, which we will cover below.
Bookmark this section for quick reference. The 1/0 AWG rows are highlighted for fast lookup.
| Conductor Size (AWG/kcmil) | Copper 60°C (140°F) | Copper 75°C (167°F) | Copper 90°C (194°F) | Aluminum 60°C (140°F) | Aluminum 75°C (167°F) | Aluminum 90°C (194°F) |
|---|---|---|---|---|---|---|
| 2 AWG | 95A | 115A | 130A | 75A | 90A | 100A |
| 1 AWG | 110A | 130A | 145A | 85A | 100A | 115A |
| 1/0 AWG (Quick Jump) | 125A | 150A | 170A | 100A | 120A | 135A |
| 2/0 AWG | 145A | 175A | 195A | 115A | 135A | 150A |
| 3/0 AWG | 165A | 200A | 225A | 130A | 155A | 170A |
| 4/0 AWG | 195A | 230A | 260A | 150A | 180A | 205A |
THHN and XHHW-2 are rated for the 90°C column. NM-B (Romex) is strictly limited to the 60°C column, regardless of the individual wire insulation inside the sheath. UF-B cable is also limited to 60°C. Always verify the jacket printing before selecting your column.
Which Temperature Column Applies to Your Installation?
The most common mistake DIYers and junior electricians make is looking at a spool of 90°C-rated THHN wire, seeing 170 amps in the table, and installing it on a 170-amp breaker. This is a direct violation of NEC 110.14(C), which governs termination provisions.
Here is the rule of thumb for selecting the correct column:
- The 60°C Column: Use this for NM-B (Romex), UF-B, or when connecting to equipment rated 100 amps or less that does not explicitly state a higher temperature rating on its label.
- The 75°C Column: This is the default for almost all modern residential and commercial breakers, panelboard lugs, and disconnect switches rated over 100 amps. If you are pulling 1/0 THHN through conduit to a 150-amp main breaker, your maximum allowable ampacity for termination is 150 amps (Copper) or 120 amps (Aluminum).
- The 90°C Column: You can almost never use this column for final breaker sizing because the breaker lugs themselves are rarely rated for 90°C. However, you are permitted to use the 90°C column as your starting baseline for derating calculations (explained in the next section).
If you are using 1/0 aluminum wire (like SER cable) for a 120-amp subpanel feeder, the 75°C column gives you exactly 120 amps, making it a perfect, code-compliant match. If you are using 1/0 copper for a 150-amp service, the 75°C column yields 150 amps, matching the service disconnect perfectly.
Derating 1/0 Wire: When Base Ampacity Drops
The values in NEC Table 310.16 assume ideal conditions. In the real world, heat is the enemy of electrical conductors. When you bundle multiple wires together in a single conduit, or when you run wire through a hot attic, the wire's ability to dissipate heat drops. To prevent the insulation from melting or degrading, the NEC requires you to 'derate' (reduce) the ampacity.
This is where the 90°C column becomes your best friend. According to NEC guidelines, you apply derating factors to the 90°C ampacity of the wire, provided the final derated number does not exceed the termination limit of the 75°C column.
Derating Example: Bundled Conductors
Imagine you are pulling four current-carrying conductors (two hots, one neutral, one ground does not count) through a single PVC conduit to a subpanel. NEC Table 310.15(C)(1) requires an 80% adjustment factor for 4 to 6 current-carrying conductors.
- Start with the 90°C base: 1/0 Copper THHN = 170A.
- Apply the 80% factor: 170A × 0.80 = 136A.
- Check the termination limit: The 75°C column limit is 150A.
- Result: Because 136A is less than 150A, your new maximum allowable ampacity for this specific conduit run is 136 amps. You cannot protect this wire with a 150-amp breaker; you must drop down to a 125-amp breaker (the next standard size down per NEC 240.6) or upsize your wire to 2/0 AWG.
If your conduit runs across an attic space where temperatures regularly exceed 86°F (30°C), you must apply an additional temperature correction factor from NEC Table 310.15(B)(1). For example, in a 110°F attic, you must multiply your base ampacity by 0.87 before applying any bundling derating. Failure to do this is a leading cause of melted wire insulation and electrical fires in residential retrofits.
What the Ampacity Table Cannot Tell You
While NEC Table 310.16 is the ultimate authority on thermal limits (ampacity), it completely ignores voltage drop and physical limitations. Relying solely on the ampacity chart can lead to a system that is legally compliant but functionally inadequate.
Voltage Drop Calculations
Ampacity measures how much current a wire can carry before it gets too hot. It does not measure how much voltage is lost over distance. The Copper Development Association (CDA) and NEC Informational Notes recommend keeping voltage drop under 3% for branch circuits and 5% total for feeder and branch circuits combined.
Let us run the math on a 1/0 AWG copper feeder carrying a full 150-amp load at 240V over a distance of 100 feet:
- Formula: VD = (2 × K × I × D) / Circular Mils
- K (Copper constant): 12.9
- I (Current): 150A
- D (Distance): 100 ft
- Circular Mils for 1/0 AWG: 105,600
- Calculation: (2 × 12.9 × 150 × 100) / 105,600 = 3.66 Volts
A 3.66V drop on a 240V system is a 1.52% drop. This is well within the acceptable 3% limit, meaning 1/0 copper is electrically sound for a 100-foot, 150-amp run. However, if that same run was 250 feet long, the drop would be 9.15V (3.8%), exceeding the recommended limit. In that scenario, you would need to upsize to 2/0 or 3/0 AWG, even though 1/0 AWG technically has the thermal ampacity to handle the 150 amps without catching fire.
Physical Termination Limits
1/0 AWG wire is physically thick (approximately 0.37 inches in diameter for bare copper). Many standard 100-amp or even 125-amp breakers and lugs are simply not physically large enough to accept a 1/0 conductor. Before purchasing your wire, check the manufacturer's datasheet for your specific breaker or lug kit to verify the 'Wire Range' (e.g., '#4 - 1/0'). If the lug maxes out at #1 AWG, you will need to use a mechanical splice reducer or upsize your terminal hardware.
Disclaimer: This guide provides NEC-style guidance for educational purposes. Your local Authority Having Jurisdiction (AHJ) or electrical inspector always has the final say on code compliance and permitted installations. Always de-energize panels and verify dead with a tested meter before working on mains connections.
