10 AWG THHN Copper Ampacity & Derating
NEC Quick Answer
10 AWG copper THHN has a base ampacity of 40A at 90C per NEC Table 310.16. Usable ampacity is 35A after applying the 75C termination limit. Maximum overcurrent protection for 10 AWG copper is 30A per NEC 240.4(D).
NEC 2023: Table 310.16, Section 240.4(D), Section 110.14(C)
Calculation Walkthrough
Per NEC 2023 Table 310.16, 10 AWG copper THHN (90°C column) = 40A 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: use 60°C column unless conductor + equipment both rated 75°C. Modern panels and breakers are listed for 75°C terminations, so the 75°C column value of 35A 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 10 AWG copper carrying 35A at 240V single-phase over a 100-foot one-way run: Vd = (2 × L × I × R) / 1000. The resistance per 1000 ft for 10 AWG copper at 75°C is used from NEC Chapter 9 Table 8. At 35A and 100 ft, the drop is 8.7V (3.6%). For longer runs or higher loads, use the voltage drop calculator with your specific distance and load.
Common Applications
10 AWG THHN copper at 35A covers 30A dedicated appliance circuits including electric dryers, water heaters, window air conditioning units, and small workshop equipment. Paired with a 30A breaker. The 10 AWG Copper conductor at 35A handles the continuous loads typical in these applications with margin to spare. For electric dryers, NEC 250.140 requires a 4-wire connection (2 hots, neutral, ground) in new construction. Water heater circuits are typically 240V with a 30A 2-pole breaker and no neutral. When installing in conduit runs exceeding 100 feet, verify voltage drop stays under 3%. For circuits above 30A such as electric ranges, upsize to 8 AWG.
NEC 2020 vs NEC 2023
Ampacity values for 10 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 |
|---|---|---|---|
| 10 AWG Copper TW | 60°C | 30A | Wet / underground |
| 10 AWG Copper THW | 75°C | 35A | Standard circuits |
| 10 AWG Copper THHN | 90°C | 40A | Derating start only |
| 10 AWG Copper free air | 75/90°C | 50A / 55A | Table 310.17 |
| 10 AWG Aluminum THHN | 75°C | 30A | Aluminum equivalent |
Frequently asked questions
What is the ampacity of 10 AWG Copper wire?
10 AWG THHN Copper has a base ampacity of 40A at 90°C per NEC 2023 Table 310.16. After applying the 110.14(C) termination temperature limit, the usable ampacity is 35A for most installations. This is the value you use for circuit sizing and breaker selection.
What size breaker do I need for 10 AWG copper wire?
Pair 10 AWG copper with a 30A breaker. The wire's usable ampacity of 35A must equal or exceed the breaker rating. For continuous loads (running 3+ hours), the load must not exceed 80% of the breaker rating (24A on a 30A breaker). See the full ampacity table for all wire sizes.
Can I use 10 AWG wire in conduit?
Yes. 10 AWG THHN is rated for conduit installations including EMT, PVC, and rigid metal conduit. The 40A 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 10 AWG copper wire?
The maximum distance depends on load and voltage. For 10 AWG copper at 35A 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 10 AWG?
Yes, but check the ampacity difference. 10 AWG aluminum THHN is rated at 30A versus 35A for copper. For equivalent capacity, you'll typically need to go up one or two gauge sizes in aluminum. See the 10 AWG copper vs aluminum comparison for cost and installation details.
Common products for 10 AWG Copper circuits
As an Amazon Associate we earn from qualifying purchases.
NEC 2023 references verified April 2026