4 AWG NM-B Copper Ampacity & Derating
Calculation Walkthrough
Per NEC 2023 Table 310.16, 4 AWG copper NM-B (90°C column) = 95A 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 70A 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 4 AWG copper carrying 70A at 240V single-phase over a 100-foot one-way run: Vd = (2 × L × I × R) / 1000. The resistance per 1000 ft for 4 AWG copper at 75°C is used from NEC Chapter 9 Table 8. At 70A and 100 ft, the drop is 4.3V (1.8%). For longer runs or higher loads, use the voltage drop calculator with your specific distance and load.
Common Applications
4 AWG NM-B copper at 70A covers 70-85A feeders including large subpanel feeds, heavy workshop equipment, and commercial HVAC installations. Paired with a 70A or 80A breaker. At 70A capacity, 4 AWG Copper is the common choice for 100A subpanel feeders where the load calculation shows actual demand well below the panel rating. For a detached garage with a workshop, lighting, and a future EV charger provision, 4 AWG on a 100A subpanel provides adequate capacity with room for expansion. This gauge also serves as the minimum feeder size for many Level 2 commercial EV charger installations rated at 80A (requiring a 100A breaker per the 125% continuous load rule). For runs exceeding 100 feet, calculate voltage drop carefully. Aluminum 4 AWG at reduced cost is worth considering for longer feeder runs.
NEC 2020 vs NEC 2023
Ampacity values for 4 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 |
|---|---|---|---|
| 4 AWG Copper TW | 60°C | 70A | Wet / underground |
| 4 AWG Copper THW | 75°C | 85A | Standard circuits |
| 4 AWG Copper THHN | 90°C | 95A | Derating start only |
| 4 AWG Copper free air | 75/90°C | 125A / 140A | Table 310.17 |
| 4 AWG Aluminum THHN | 75°C | 65A | Aluminum equivalent |
Frequently asked questions
What is the ampacity of 4 AWG Copper wire?
4 AWG NM-B Copper has a base ampacity of 95A at 90°C per NEC 2023 Table 310.16. After applying the 110.14(C) termination temperature limit, the usable ampacity is 70A for most installations. This is the value you use for circuit sizing and breaker selection.
What size breaker do I need for 4 AWG copper wire?
Pair 4 AWG copper with a 70A breaker. The wire's usable ampacity of 70A must equal or exceed the breaker rating. For continuous loads (running 3+ hours), the load must not exceed 80% of the breaker rating (56A on a 70A breaker). See the full ampacity table for all wire sizes.
Can I use 4 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 4 AWG THHN for conduit ampacity.
How far can I run 4 AWG copper wire?
The maximum distance depends on load and voltage. For 4 AWG copper at 70A 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 4 AWG?
Yes, but check the ampacity difference. 4 AWG aluminum NM-B is rated at 55A versus 70A for copper. For equivalent capacity, you'll typically need to go up one or two gauge sizes in aluminum. See the 4 AWG copper vs aluminum comparison for cost and installation details.
Common products for 4 AWG Copper circuits
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NEC 2023 references verified April 2026