The Direct Answer: 1/0 AWG Ampacity at a Glance

When sizing conductors for heavy feeders or service entrances, the 1/0 AWG wire amp rating (pronounced "one-aught") depends entirely on the conductor material and the temperature rating of your termination points.

Quick Reference Base Values (NEC Table 310.16):
  • 1/0 AWG Copper: 150 Amps (75°C column) | 170 Amps (90°C column)
  • 1/0 AWG Aluminum: 120 Amps (75°C column) | 135 Amps (90°C column)

Here is the most critical rule that catches DIYers and junior apprentices off guard: You must use the 75°C column for your final breaker sizing in 95% of residential and commercial installations. Even if you pull 90°C-rated THHN copper wire, the lugs on standard breakers and panelboards are only rated for 75°C. Per NEC 110.14(C), the lowest temperature rating in the circuit chain dictates the allowable ampacity. Therefore, your legal maximum overcurrent protection for 1/0 AWG copper is a 150A breaker, and for 1/0 AWG aluminum, it is a 125A breaker (the next standard size down from 120A, or exactly 120A if using a specific adjustable trip unit).

How to Read the NEC 310.16 Ampacity Table

The standard ampacity charts published by wire manufacturers are derived directly from NEC Table 310.16. To use this table correctly, you need to understand what the columns actually represent.

Table 1: Allowable Ampacities for Insulated Conductors (Excerpt from NEC Table 310.16, 30°C Ambient)
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

Column Breakdown:

  • 60°C Column: Used for older installations, NM-B (Romex) cable, and circuits rated 100A or less where the equipment temperature rating is unknown.
  • 75°C Column: The default column for modern THHN/THWN-2 or XHHW-2 wire terminating on standard breakers, lugs, and disconnects rated 100A and above.
  • 90°C Column: Used exclusively as the starting point for derating calculations (adjusting for heat and conduit fill). You cannot use this column to size your breaker unless every single termination point in the circuit is explicitly stamped "90°C" (which is exceedingly rare in standard gear).

Derating 1/0 AWG: When Base Values Drop

The base 1/0 AWG amp rating assumes two things: an ambient air temperature of exactly 30°C (86°F), and no more than three current-carrying conductors in a raceway. When real-world conditions violate these assumptions, you must apply correction and adjustment factors.

Safety Note: When derating wire, you always start your math using the 90°C column value, apply the multiplier, and then compare the result to the 75°C column terminal limit. The final ampacity is whichever number is lower.

Scenario: Four current-carrying conductors in a single conduit
Imagine you are pulling two hots, a neutral, and a ground for a multi-wire branch circuit or a 120/240V single-phase feeder where the neutral carries unbalanced current (counting as 4 current-carrying conductors). Per NEC 310.15(C)(1), you must apply an 80% adjustment factor.

  1. Start at 90°C: 1/0 AWG Copper = 170A.
  2. Apply Adjustment: 170A × 0.80 = 136A.
  3. Check Terminal Limit: The 75°C terminal limit is 150A.
  4. Final Ampacity: 136A is lower than 150A. Your 1/0 AWG copper wire is now legally rated for 136 Amps.

Because 136A is not a standard breaker size, NEC 240.4(B) allows you to round up to the next standard size for feeders, meaning you could protect this derated 1/0 AWG wire with a 150A breaker, provided the calculated continuous load does not exceed 136A.

Decision Path: Choosing Your 1/0 AWG Conductor

Use this decision tree to lock in the exact wire type and size for your project. Do not guess; follow the path that matches your panel and load requirements.

If Your Scenario Is... Then Your Concrete Pick Is...
150A residential subpanel feeder, under 100 feet, pulling through PVC conduit. 1/0 AWG Copper THHN/THWN-2 (3 hots/neutral + 1 ground). Protect with a 150A breaker.
150A feeder, but you want to save money using aluminum. 2/0 AWG Aluminum XHHW-2. (1/0 AWG Aluminum is only rated 120A; you must upsize to 2/0 for a 150A breaker).
120A maximum load (e.g., large workshop subpanel or EV charger circuit), under 75 feet. 1/0 AWG Aluminum XHHW-2. Protect with a 125A breaker. Use anti-oxidant paste (Noalox) on all aluminum terminations.
150A feeder running outdoors in direct burial (no conduit). 1/0 AWG Copper USE-2 or UF-B (Check local AHJ for UF-B at this size; USE-2 or direct burial MHF is preferred). Note: UF-B defaults to the 60°C column (125A), requiring upsizing.

Default Recommendation: For a standard, trouble-free 150A residential subpanel feeder under 100 feet, buy 1/0 AWG Copper THHN/THWN-2. It terminates cleanly, resists creep under lugs better than aluminum, and perfectly matches the 150A breaker terminal limits without requiring upsizing.

What the Ampacity Table Cannot Tell You

The NEC 310.16 table only solves for thermal limits (preventing the wire insulation from melting). It completely ignores three critical physical realities of your installation:

1. Voltage Drop Over Distance

Ampacity assumes the wire can handle the heat, but it doesn't guarantee the voltage at the other end will be usable. The NEC recommends a maximum 3% voltage drop for feeders.

  • At 100 feet: 1/0 AWG Copper carrying 150A at 240V yields a ~2.5% drop. (Acceptable).
  • At 150 feet: The drop increases to ~3.8%. (Unacceptable).

The Fix: If your run exceeds 115 feet at full 150A load, the ampacity table is irrelevant. You must upsize to 2/0 AWG Copper or 4/0 AWG Aluminum strictly to mitigate voltage drop.

2. Conduit Fill Capacity

1/0 AWG wire is physically thick. Per NEC Chapter 9, Table 1, you cannot exceed 40% conduit fill for three or more wires. Three 1/0 AWG THHN wires and one 1/0 AWG ground will not fit in 1-inch PVC; you must step up to 1.5-inch Schedule 80 PVC or 2-inch conduit to allow for proper heat dissipation and physical pulling clearance.

3. Physical Pulling Tension and Bend Radius

1/0 AWG copper is incredibly stiff. When pulling through sweeps and bends, the pulling tension can exceed the physical limits of the wire or the conduit joints. Always use wire pulling lubricant (like Polywater J) and ensure your conduit bends do not exceed 360 degrees total between pull boxes to prevent jamming and insulation tearing.