3 AWG THHN Aluminum Ampacity & Derating
Calculation Walkthrough
Per NEC 2023 Table 310.16, 3 AWG aluminum THHN (90°C column) = 85A 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 75A 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 3 AWG aluminum carrying 75A at 240V single-phase over a 100-foot one-way run: Vd = (2 × L × I × R) / 1000. The resistance per 1000 ft for 3 AWG aluminum at 75°C is used from NEC Chapter 9 Table 8. At 75A and 100 ft, the drop is 6.0V (2.5%). For longer runs or higher loads, use the voltage drop calculator with your specific distance and load.
Common Applications
3 AWG THHN aluminum at 75A covers 85-100A feeders and commercial equipment circuits. Paired with a 90A or 100A breaker. This gauge fills the gap between the more common 4 AWG and 2 AWG sizes, appearing primarily in commercial and industrial installations where load calculations land precisely in the 85-100A range. In residential work, 3 AWG is less commonly stocked at supply houses, so electricians frequently round up to 2 AWG for availability and future capacity. Where conduit fill is tight or the run is short, 3 AWG saves conduit size compared to 2 AWG. The 75A rating at 75°C provides clean breaker pairing with standard 100A overcurrent devices.
NEC 2020 vs NEC 2023
Ampacity values for 3 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 |
|---|---|---|---|
| 3 AWG Aluminum TW | 60°C | 65A | Wet / underground |
| 3 AWG Aluminum THW | 75°C | 75A | Standard circuits |
| 3 AWG Aluminum THHN | 90°C | 85A | Derating start only |
| 3 AWG Aluminum free air | 75/90°C | 115A / 130A | Table 310.17 |
| 3 AWG Copper THHN | 75°C | 100A | Copper equivalent |
Frequently asked questions
What is the ampacity of 3 AWG Aluminum wire?
3 AWG THHN Aluminum has a base ampacity of 85A at 90°C per NEC 2023 Table 310.16. After applying the 110.14(C) termination temperature limit, the usable ampacity is 75A for most installations. This is the value you use for circuit sizing and breaker selection.
What size breaker do I need for 3 AWG aluminum wire?
Pair 3 AWG aluminum with a 80A breaker. The wire's usable ampacity of 75A must equal or exceed the breaker rating. For continuous loads (running 3+ hours), the load must not exceed 80% of the breaker rating (64A on a 80A breaker). See the full ampacity table for all wire sizes.
Can I use 3 AWG wire in conduit?
Yes. 3 AWG THHN is rated for conduit installations including EMT, PVC, and rigid metal conduit. The 85A 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 3 AWG aluminum wire?
The maximum distance depends on load and voltage. For 3 AWG aluminum at 75A 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 3 AWG Aluminum circuits
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NEC 2023 references verified April 2026