1/0 AWG THHN Copper Ampacity & Derating
Calculation Walkthrough
Per NEC 2023 Table 310.16, 1/0 AWG copper THHN (90°C column) = 170A base. Assumes ≤3 CCC at 30°C ambient.
At 30.0°C (86°F), factor = 1.0. No reduction. At 50°C attic: factor drops to ~0.82 for 90°C insulation.
≤3 CCC = factor 1.0. EGC + neutral (unbalanced) not counted per 310.15(E). Typical 240V circuit: 2 hots + N + G = 2 CCC.
Equipment termination governs the final ampacity. Circuits >100A: 75°C column applies per 110.14(C)(1)(b). Modern panels and breakers are listed for 75°C terminations, so the 75°C column value of 150A becomes the ceiling. The 90°C rating is only used as the starting point for temperature correction and bundling derating. After all adjustments, the result cannot exceed the termination column value. This is the single most commonly failed inspection item.
NEC recommends limiting voltage drop to 3% on branch circuits and 5% total (branch + feeder combined). For 1/0 AWG copper carrying 150A at 240V single-phase over a 100-foot one-way run: Vd = (2 × L × I × R) / 1000. The resistance per 1000 ft for 1/0 AWG copper at 75°C is used from NEC Chapter 9 Table 8. At 150A and 100 ft, the drop is 3.7V (1.5%). For longer runs or higher loads, use the voltage drop calculator with your specific distance and load.
Common Applications
1/0 AWG THHN copper at 150A covers 125-150A feeders and 150A service entrances for small to medium-sized homes. Paired with a 150A breaker. This is the first gauge in the "ought" series where aluminum becomes the more practical choice for most installations. 1/0 AWG aluminum at 150A matches or exceeds the capacity of 2 AWG copper at significantly lower cost. For 200A dwelling services, NEC Table 310.15(B)(7) allows 2/0 AWG copper or 4/0 AWG aluminum. All terminations at this size require anti-oxidant compound for aluminum and proper torque per the connector listing. Verify conduit fill when pulling multiple conductors at this size.
NEC 2020 vs NEC 2023
Ampacity values for 1/0 AWG copper are identical across NEC 2017, 2020, and 2023 editions. The only change affecting this conductor is the table renumbering from Table 310.15(B)(16) to Table 310.16 in the 2023 edition. This renumbering was part of a broader reorganization of Article 310 that also moved temperature correction factors from Table 310.15(B)(2)(a) to Table 310.15(B)(1) and bundling adjustment factors from Table 310.15(B)(3)(a) to Table 310.15(C)(1). When citing NEC references on permits or inspection reports, use the table numbers that match your state's adopted code year. States on NEC 2020 or earlier should cite Table 310.15(B)(16); states that have adopted NEC 2023 should cite Table 310.16.
Quick Reference
| Conductor | Temp | Amps | Use |
|---|---|---|---|
| 1/0 AWG Copper TW | 60°C | 125A | Wet / underground |
| 1/0 AWG Copper THW | 75°C | 150A | Standard circuits |
| 1/0 AWG Copper THHN | 90°C | 170A | Derating start only |
| 1/0 AWG Copper free air | 75/90°C | 230A / 260A | Table 310.17 |
| 1/0 AWG Aluminum THHN | 75°C | 120A | Aluminum equivalent |
Frequently asked questions
What is the ampacity of 1/0 AWG Copper wire?
1/0 AWG THHN Copper has a base ampacity of 170A at 90°C per NEC 2023 Table 310.16. After applying the 110.14(C) termination temperature limit, the usable ampacity is 150A for most installations. This is the value you use for circuit sizing and breaker selection.
What size breaker do I need for 1/0 AWG copper wire?
Pair 1/0 AWG copper with a 150A breaker. The wire's usable ampacity of 150A must equal or exceed the breaker rating. For continuous loads (running 3+ hours), the load must not exceed 80% of the breaker rating (120A on a 150A breaker). See the full ampacity table for all wire sizes.
Can I use 1/0 AWG wire in conduit?
Yes. 1/0 AWG THHN is rated for conduit installations including EMT, PVC, and rigid metal conduit. The 170A base ampacity applies in conduit with no more than 3 current-carrying conductors. If bundling more than 3 conductors, apply the bundling adjustment factors from NEC Table 310.15(C)(1).
How far can I run 1/0 AWG copper wire?
The maximum distance depends on load and voltage. For 1/0 AWG copper at 150A on a 240V circuit, use the voltage drop calculator with your specific distance. As a rule of thumb, keep voltage drop under 3% for branch circuits. Longer runs may require upsizing the conductor.
Can I use aluminum instead of copper for 1/0 AWG?
Yes, but check the ampacity difference. 1/0 AWG aluminum THHN is rated at 120A versus 150A for copper. For equivalent capacity, you'll typically need to go up one or two gauge sizes in aluminum. See the 1/0 AWG copper vs aluminum comparison for cost and installation details.
Common products for 1/0 AWG Copper circuits
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NEC 2023 references verified April 2026