4 AWG Copper Wire — Ampacity, Diameter & Sizes Reference
4 AWG copper wire is rated 85 A at 75 °C in NEC 310.16, has a solid diameter of 5.19 mm, and a DC resistance of 0.815 Ω/km. This page covers ampacity tables, the full copper sizes chart, stranded versus solid copper wire trade-offs, scrap pricing for #2 copper, and the relative permeability used in field calculations. Reviewed by a licensed PE.
What 4 AWG copper wire is
Copper is a chemical element (Cu, atomic number 29) with the highest electrical conductivity of any non-precious metal — 5.96 × 10⁷ S/m at 20 °C, second only to silver. Drawn into round conductors and sized by the American Wire Gauge (AWG) system, copper is the dominant material for building wiring, motor windings, transformer coils, and busbars worldwide.
The AWG number runs inversely to diameter: a higher AWG is thinner, a lower AWG is thicker. 4 AWG sits in the middle of the NEC range that an electrician actually pulls by hand — large enough to feed a 60–80 A subpanel or an electric range, small enough to terminate without a hydraulic lug. Above 4 AWG (3, 2, 1, 1/0…) you start needing crimping tools and a second person on the pull.
Resistance and ampacity formulas
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- R_dc = DC resistance, Ω
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- ρ = Resistivity of copper at 20 °C = 1.724 × 10⁻⁸, Ω·m
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- L = Conductor length, m
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- A = Cross-sectional area, m²
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- R_T = Resistance at operating temperature, Ω
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- R_20 = Resistance at 20 °C, Ω
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- α = Copper temperature coefficient = 0.00393, 1/°C
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- T = Operating temperature, °C
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- I_adj = Adjusted ampacity, A
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- I_table = Table 310.16 base ampacity, A
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- k_T = Ambient temperature factor (Table 310.15(B)(1))
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- k_fill = Conduit-fill factor for >3 CCCs (Table 310.15(C)(1))
Standards that govern copper wire sizing
Three families of standards — North American, IEC, and Australian/New Zealand — set the rules for selecting copper conductors. The numbers diverge slightly because each region picks different reference temperatures and grouping factors, so always cite the exact code section in your design package.
| Document | Scope | Key clause for 4 AWG |
|---|---|---|
| NFPA 70 (NEC) 310.16 | U.S. building ampacity | 85 A @ 75 °C, 95 A @ 90 °C |
| NEC 310.17 | Single conductors in free air | 140 A @ 75 °C |
| IEC 60228 | Conductor classes and sizes | Class 1 (solid), Class 2 (stranded) |
| IEC 60287 | Continuous current rating method | Heat-balance calculation basis |
| AS/NZS 3008.1.1 | AU/NZ cable selection (4 mm² ≈ 11 AWG) | Use 25 mm² for 4 AWG equivalent (85 A) |
| ASTM B3 / B8 | Bare/stranded copper wire material spec | Soft-drawn 99.95 % Cu min. |
| UL 83 | THHN/THWN insulation | Used to qualify the 90 °C dry / 75 °C wet rating |
Copper sizes chart — AWG, mm², ampacity
The full NEC-listed copper range spans 18 AWG (lighting) to 2000 kcmil (utility service). Below is the working segment that most electricians and designers use — 14 AWG through 4/0. All ampacity values are NEC 310.16, 75 °C column, three current-carrying conductors in raceway.
| AWG | Diameter (mm) | Area (mm²) | Rdc (Ω/km, 20 °C) | Ampacity 75 °C (A) |
|---|---|---|---|---|
| 14 | 1.628 | 2.08 | 8.286 | 20 |
| 12 | 2.053 | 3.31 | 5.211 | 25 |
| 10 | 2.588 | 5.26 | 3.277 | 35 |
| 8 | 3.264 | 8.37 | 2.061 | 50 |
| 6 | 4.115 | 13.30 | 1.296 | 65 |
| 4 | 5.189 | 21.15 | 0.815 | 85 |
| 3 | 5.827 | 26.67 | 0.647 | 100 |
| 2 | 6.544 | 33.62 | 0.513 | 115 |
| 1 | 7.348 | 42.41 | 0.407 | 130 |
| 1/0 | 8.252 | 53.49 | 0.323 | 150 |
| 2/0 | 9.266 | 67.43 | 0.256 | 175 |
| 3/0 | 10.40 | 85.01 | 0.203 | 200 |
| 4/0 | 11.68 | 107.2 | 0.161 | 230 |
- Identify the AWG size Read the cable jacket — every NEC-listed conductor is printed with its AWG (e.g. "4 AWG THHN CU"). For 4 AWG copper the bare diameter is 5.189 mm (0.2043 in).
- Look up base ampacity from NEC 310.16 For copper at 75 °C insulation in raceway, 4 AWG carries 85 A. At 90 °C (THHN, XHHW-2) it carries 95 A but is still limited to the 75 °C terminal column — 85 A.
- Apply temperature correction If the ambient is above 30 °C, multiply by the NEC 310.15(B)(1) factor. Example: 41–45 °C ambient → 0.82, so 4 AWG at 75 °C drops to 85 × 0.82 = 69.7 A.
- Apply conduit fill correction For 4–6 current-carrying conductors in the same raceway, multiply by 0.80 (NEC 310.15(C)(1)). 4 AWG with five other CCCs: 85 × 0.80 = 68 A.
- Verify voltage drop Use the resistance value (0.815 Ω/km for solid 4 AWG copper at 20 °C) to confirm the run is within 3 % branch / 5 % combined drop. A 30 m one-way 240 V circuit at 60 A drops 240 × 2 × 30 × 0.000815 / 60 ≈ 5.9 V — about 2.5 %.
Worked example — 4 AWG copper feeding a 60 A subpanel
Take a detached-garage subpanel: 60 A, 240 V single-phase, 30 m one-way run from the main, four current-carrying conductors in PVC conduit (two hots, neutral, equipment ground does not count as CCC), 35 °C ambient inside the trench-pulled conduit. Verify that 4 AWG copper THWN-2 is the correct choice.
- Base ampacity (75 °C, NEC 310.16): 85 A — well above the 60 A breaker.
- Temperature correction (35 °C ambient): 0.91 → 85 × 0.91 = 77.4 A.
- Conduit-fill correction (3 CCCs): No derate needed for 1–3 CCCs. Adjusted ampacity remains 77.4 A → 60 A breaker still inside the rating.
- Voltage drop: 2 × 30 m × 60 A × 0.000815 Ω/m = 2.93 V → 2.93/240 = 1.22 %, well under the 3 % branch limit.
Verdict: 4 AWG copper THWN-2 in PVC conduit is correctly sized.
Stranded versus solid copper wire
Solid and stranded conductors with the same AWG carry essentially the same DC current, but they behave differently at termination, in vibration, and in long pulls. The table below shows where each type wins.
| Criterion | Solid copper | Stranded copper |
|---|---|---|
| Cost (per foot) | Lower (~10–15 % cheaper) | Higher |
| Outer diameter (4 AWG) | 5.19 mm | ~5.5 mm (Class B 7-strand) |
| AC resistance | Baseline | ~5 % higher (skin distribution) |
| Bending radius | Larger (≥ 6× OD) | Tighter (≥ 4× OD) |
| Vibration / flex life | Poor — work-hardens, breaks | Excellent |
| Best for | Building wiring 14–10 AWG, fixed runs | 8 AWG and larger, motor leads, robotics, machine wiring |
| Termination | Direct under screw lug | Use compression lug or ferrule |
Related copper wire questions
Number 4 copper wire — same conductor, different name
"Number 4 copper wire" and "4 AWG copper wire" describe the same conductor. The phrase "number 4" is more common in scrap-yard and contractor talk; "4 AWG" is the engineering and code language. Both point to the 21.15 mm² conductor with 5.19 mm bare diameter.
4 AWG copper in millimeters (mm copper)
The closest IEC metric size to 4 AWG is 25 mm² — the next size up. 4 AWG is 21.15 mm² (between 16 mm² and 25 mm²), so for AS/NZS 3008 or IEC 60364 work, designers usually round up to 25 mm² to match a stocked metric cable. The 25 mm² IEC ampacity in air is approximately 100 A, comfortably above the 85 A NEC value for 4 AWG.
Relative permeability of copper
Copper is diamagnetic, with relative permeability μᵣ = 0.999994. In every practical electromagnetic calculation — transformer flux paths, busbar proximity effects, induction-heating coil design — copper is treated as μᵣ = 1.0 with negligible error. The minute diamagnetism only becomes observable in superconducting magnetometry experiments.
- How much is number 2 copper?
- Number 2 copper is the scrap industry grade for unalloyed bare copper wire and tubing with light tarnish, paint or solder (≥ 96 % Cu purity). U.S. yard prices typically run $2.40–$3.20 per lb depending on region and London Metal Exchange (LME) settlement. As of late 2025 the LME copper cash benchmark sat around $4.10/lb, so #2 grade — which carries a recovery discount — usually clears at 65–75 % of the LME price. Always call two yards for a same-day quote; spreads of $0.30/lb between scrap dealers are common.
- How many amps can 4 AWG copper wire carry?
- 4 AWG copper carries 85 A in the NEC 310.16 75 °C column (THWN-2, XHHW-2 in raceway), 95 A in the 90 °C column, and 70 A in the 60 °C column. The 75 °C value is what most installations actually use because almost all breakers and lugs are 75 °C-rated terminals. For free-air installations (Table 310.17), 4 AWG copper is good for 140 A at 75 °C.
- What is the diameter of 4 AWG copper wire?
- A solid 4 AWG copper conductor is 5.189 mm (0.2043 in). Stranded 4 AWG nominal cross-section is 21.15 mm² (0.0413 in²) — the stranded outer diameter is slightly larger (~5.5 mm) because of the bundling factor. With THHN insulation, the overall jacketed diameter is about 7.6 mm (0.299 in).
- Should I use stranded or solid copper wire?
- Use solid for fixed building wiring up to 10 AWG (cheaper, easier to terminate under screw lugs). Use stranded for 8 AWG and larger, anything that flexes (motor leads, machine wiring, robotics), and for vibration-prone runs. Stranded conductors have ~5 % higher AC resistance and slightly larger overall diameter, but the trade-off is much better fatigue life and easier pulling through long conduit runs.
- What is the relative permeability of copper?
- The relative permeability of copper is μᵣ ≈ 0.999994 — copper is diamagnetic, so its permeability is fractionally less than free space. For all engineering field calculations (transformers, busbars, induction problems), you can treat it as 1.0 with negligible error. The diamagnetic offset only matters for very-high-precision magnetometry.
Copper conductors are preferred for the majority of indoor and equipment wiring because their high conductivity allows smaller cross-sections, their work-hardening behavior produces more reliable terminations than aluminum, and their mechanical strength permits long unsupported pulls without sag.
Sources
- NFPA 70 (NEC) — Articles 310.15, 310.16, 310.17 (2023 edition).
- IEEE Std 141 — Red Book, Chapter 9 (cable selection).
- IEC 60228 — Conductors of insulated cables (Classes 1, 2, 5, 6).
- IEC 60287-1-1 — Continuous current rating, AC resistance method.
- ASTM B3 / B8 — Soft / stranded copper wire material specifications.
- AS/NZS 3008.1.1 — Australian/New Zealand cable selection tables.