8 AWG THHN Aluminum Ampacity & Derating
NEC Quick Answer
8 AWG aluminum THHN has a base ampacity of 45A at 90C per NEC Table 310.16. Usable ampacity is 40A at the 75C termination limit, or 30A if equipment terminals are rated 60C only. Always verify terminal temperature ratings per NEC 110.14(C).
NEC 2023: Table 310.16, Section 110.14(C)
Calculation Walkthrough
Per NEC 2023 Table 310.16, 8 AWG aluminum THHN (90°C column) = 45A 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 40A 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 8 AWG aluminum carrying 40A at 240V single-phase over a 100-foot one-way run: Vd = (2 × L × I × R) / 1000. The resistance per 1000 ft for 8 AWG aluminum at 75°C is used from NEC Chapter 9 Table 8. At 40A and 100 ft, the drop is 10.2V (4.3%). For longer runs or higher loads, use the voltage drop calculator with your specific distance and load.
Common Applications
8 AWG THHN aluminum at 40A covers 40-50A circuits including electric ranges, cooktops, large central air conditioning units (3-5 ton), and Level 2 EV charger installations rated at 40A. Paired with a 40A or 50A breaker depending on the connected load. For electric ranges, the NEC permits a demand factor calculation under 220.55 that often allows 8 AWG for ranges rated up to 12 kW. Hot tub installations typically require 8 AWG minimum with mandatory GFCI protection per 680.44. EV chargers drawing 40A continuous require a 50A breaker (125% continuous load rule per 210.20) and 8 AWG meets this requirement. For loads above 50A or long runs with voltage drop concerns, upsize to 6 AWG.
NEC 2020 vs NEC 2023
Ampacity values for 8 AWG aluminum 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 |
|---|---|---|---|
| 8 AWG Aluminum TW | 60°C | 35A | Wet / underground |
| 8 AWG Aluminum THW | 75°C | 40A | Standard circuits |
| 8 AWG Aluminum THHN | 90°C | 45A | Derating start only |
| 8 AWG Aluminum free air | 75/90°C | 55A / 60A | Table 310.17 |
| 8 AWG Copper THHN | 75°C | 50A | Copper equivalent |
Frequently asked questions
What is the ampacity of 8 AWG Aluminum wire?
8 AWG THHN Aluminum has a base ampacity of 45A at 90°C per NEC 2023 Table 310.16. After applying the 110.14(C) termination temperature limit, the usable ampacity is 40A for most installations. This is the value you use for circuit sizing and breaker selection.
What size breaker do I need for 8 AWG aluminum wire?
Pair 8 AWG aluminum with a 40A breaker. The wire's usable ampacity of 40A must equal or exceed the breaker rating. For continuous loads (running 3+ hours), the load must not exceed 80% of the breaker rating (32A on a 40A breaker). See the full ampacity table for all wire sizes.
Can I use 8 AWG wire in conduit?
Yes. 8 AWG THHN is rated for conduit installations including EMT, PVC, and rigid metal conduit. The 45A 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 8 AWG aluminum wire?
The maximum distance depends on load and voltage. For 8 AWG aluminum at 40A 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.
Common products for 8 AWG Aluminum circuits
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NEC 2023 references verified April 2026