12 AWG NM-B Copper Ampacity & Derating
NEC Quick Answer
12 AWG NM-B copper has a usable ampacity of 20A. Like all NM-B cable, NEC 334.80 limits it to the 60C column of Table 310.16 despite having 90C-rated conductors. Maximum overcurrent protection is 20A per NEC 240.4(D).
NEC 2023: Table 310.16 (60C column), Section 334.80, Section 240.4(D)
Calculation Walkthrough
Per NEC 2023 Table 310.16, 12 AWG copper NM-B (90°C column) = 30A 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 60°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 60°C terminations, so the 60°C column value of 20A 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 12 AWG copper carrying 20A at 240V single-phase over a 100-foot one-way run: Vd = (2 × L × I × R) / 1000. The resistance per 1000 ft for 12 AWG copper at 75°C is used from NEC Chapter 9 Table 8. At 20A and 100 ft, the drop is 7.9V (3.3%). For longer runs or higher loads, use the voltage drop calculator with your specific distance and load.
Common Applications
12 AWG NM-B copper at 20A covers 20A general-purpose and small-appliance circuits, making it the most commonly installed wire size in residential construction. Required for kitchen countertop circuits, bathroom receptacles, garage circuits, and laundry areas per NEC 210.11(C). Paired with a 20A breaker. In a typical home, 12 AWG Copper makes up more than half of all branch circuit wiring. NM-B is limited to the 60°C column per NEC 334.80 regardless of installation method. For conduit installations, use individual THHN or XHHW-2 conductors. For dedicated 30A appliance circuits like dryers and water heaters, upsize to 10 AWG.
NEC 2020 vs NEC 2023
Ampacity values for 12 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 |
|---|---|---|---|
| 12 AWG Copper TW | 60°C | 20A | Wet / underground |
| 12 AWG Copper THW | 75°C | 25A | Standard circuits |
| 12 AWG Copper THHN | 90°C | 30A | Derating start only |
| 12 AWG Copper free air | 75/90°C | 35A / 40A | Table 310.17 |
| 12 AWG Aluminum THHN | 75°C | 20A | Aluminum equivalent |
Frequently asked questions
What is the ampacity of 12 AWG Copper wire?
12 AWG NM-B Copper has a base ampacity of 30A at 90°C per NEC 2023 Table 310.16. After applying the 110.14(C) termination temperature limit, the usable ampacity is 20A for most installations. This is the value you use for circuit sizing and breaker selection.
What size breaker do I need for 12 AWG copper wire?
Pair 12 AWG copper with a 20A breaker. The wire's usable ampacity of 20A must equal or exceed the breaker rating. For continuous loads (running 3+ hours), the load must not exceed 80% of the breaker rating (16A on a 20A breaker). See the full ampacity table for all wire sizes.
Can I use 12 AWG wire in conduit?
NM-B cable can be sleeved through short sections of conduit for physical protection per NEC 334.15(B), but it is not a standard conduit wiring method. For conduit installations, use individual THHN or XHHW-2 conductors instead. See Romex vs conduit and 12 AWG THHN for conduit ampacity.
How far can I run 12 AWG copper wire?
The maximum distance depends on load and voltage. For 12 AWG copper at 20A 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 12 AWG?
Yes, but check the ampacity difference. 12 AWG aluminum NM-B is rated at 15A versus 20A for copper. For equivalent capacity, you'll typically need to go up one or two gauge sizes in aluminum. See the 12 AWG copper vs aluminum comparison for cost and installation details.
Common products for 12 AWG Copper circuits
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NEC 2023 references verified April 2026