Sources and scope for this page: NFPA 70 (NEC) Chapter 9, Table 8 — Conductor properties (NEC 2023 edition), covering the area and DC resistance columns for uncoated copper at 20 °C; and IEC 60228:2004, covering the metric conductor cross-sectional areas in the mapping column (one more source is tabulated below). No independent source check is recorded for this page: the content rests on the sources listed and on the working shown on the page. Full sources, values and limits on this page.
Page last changed 2026-10-19 · based on NEC 2023 edition
Quick Answer
Conductor Size, Diameter and DC Resistance (Copper)
| Size[3] | Area mm²[3] | Area kcmil[3] | Conductor diameter mm[1] | DC resistance Ω / 1,000 ft[3] | DC resistance Ω / km[3] | Nearest IEC metric size[2] |
|---|---|---|---|---|---|---|
| 14 AWG # | 2.08 | 4.11 | 1.63 | 3.07 | 10.1 | 1.5 mm² |
| 12 AWG # | 3.31 | 6.53 | 2.05 | 1.93 | 6.34 | 2.5 mm² |
| 10 AWG # | 5.26 | 10.4 | 2.59 | 1.21 | 3.97 | 4 mm² |
| 8 AWG # | 8.37 | 16.5 | 3.26 | 0.764 | 2.51 | 6 mm² |
| 6 AWG # | 13.3 | 26.2 | 4.11 | 0.491 | 1.61 | 10 mm² |
| 4 AWG # | 21.2 | 41.7 | 5.19 | 0.308 | 1.01 | 16 mm² |
| 3 AWG # | 26.7 | 52.6 | 5.83 | 0.245 | 0.804 | 25 mm² |
| 2 AWG # | 33.6 | 66.4 | 6.54 | 0.194 | 0.637 | 25 mm² |
| 1 AWG # | 42.4 | 83.7 | 7.35 | 0.154 | 0.505 | 35 mm² |
| 1/0 AWG # | 53.5 | 106 | 8.25 | 0.122 | 0.400 | 50 mm² |
| 2/0 AWG # | 67.4 | 133 | 9.27 | 0.0967 | 0.317 | 70 mm² |
| 3/0 AWG # | 85.0 | 168 | 10.4 | 0.0766 | 0.251 | 70 mm² |
| 4/0 AWG # | 107 | 212 | 11.7 | 0.0608 | 0.200 | 95 mm² |
| 250 kcmil # | 127 | 250 | 12.7 | 0.0515 | 0.169 | 120 mm² |
| 300 kcmil # | 152 | 300 | 13.9 | 0.0429 | 0.141 | 150 mm² |
| 350 kcmil # | 177 | 350 | 15.0 | 0.0367 | 0.121 | 185 mm² |
| 400 kcmil # | 203 | 400 | 16.1 | 0.0321 | 0.105 | 185 mm² |
| 500 kcmil # | 253 | 500 | 18.0 | 0.0258 | 0.0847 | 240 mm² |
| 600 kcmil # | 304 | 600 | 19.7 | 0.0214 | 0.0703 | 300 mm² |
| 750 kcmil # | 380 | 750 | 22.0 | 0.0171 | 0.0562 | 400 mm² |
Areas, diameters and resistances describe the conductor, not the cable. A finished cable adds insulation, a jacket and often a filler, so its outside diameter is much larger and depends entirely on the insulation type and voltage rating. Resistance values are for uncoated copper at 20 °C; aluminium is about 61% less conductive for the same area, and resistance rises roughly 0.4% per °C above 20 °C. This table deliberately gives no ampacity — current-carrying capacity depends on insulation temperature rating, installation method and ambient temperature, and is covered on the ampacity chart.
AWG, kcmil and mm² — Three Ways to Say the Same Thing
The North American system is logarithmic in the small sizes and linear in the large ones, which is why it looks irregular. AWG sizes from 36 down to 0000 are defined so that each three-gauge step doubles the area: 12 AWG to 9 AWG doubles the copper, and 12 AWG to 6 AWG quadruples it. Above 4/0 the system switches to kcmil — thousands of circular mils — and simply counts area: 250 kcmil, 300 kcmil, 500 kcmil and so on.
A circular mil is not a metric unit. It is the area of a circle one thousandth of an inch in diameter, which makes 1 kcmil equal to 0.5067 mm². The unit survives because it makes the arithmetic of wire area trivial: the circular-mil area of a round wire is simply its diameter in mils squared, with no factor of π/4 anywhere.
The IEC system is straightforward metric cross-sectional area — 1.5, 2.5, 4, 6, 10 mm² and the R10 preferred series above that. Because the two series were designed independently, they do not line up: 12 AWG is 3.31 mm² and the nearest IEC size is 2.5 mm², which is 24% less copper. The next IEC size up, 4 mm², is 21% more. Substituting a metric cable for an AWG one always means choosing between an undersized and an oversized conductor, which is why the mapping column shows the nearest size rather than an equivalent.
Resistance Is the Number That Matters
Voltage drop, power loss and fault-loop impedance are all driven by conductor resistance, not by the size label. Two cables that look similar can differ by 20% in copper cross-section and therefore in resistance.
Three corrections apply to any resistance figure. Temperature: copper's resistivity rises about 0.393% per °C, so a conductor at 75 °C has roughly 22% more resistance than at 20 °C, which is the temperature this table is based on. Material: aluminium has 61% more resistivity than copper for the same cross-section, which is why aluminium cable is always two AWG sizes larger for the same job. Stranding and coating: tinned copper is slightly more resistive than bare, and fine-stranded flexible cable slightly more again, though for power work the difference is small.
For a voltage-drop calculation the whole path counts — out on the live conductor and back on the neutral, so a single-phase circuit has twice the resistance of one conductor length. Three-phase circuits are shorter by a factor of √3 because the return path is shared. That is why a voltage-drop calculator asks for the one-way length and applies the factor itself.
Frequently Asked Questions
- What is the difference between a cable's size and its diameter?
- Size is the cross-sectional area of the conducting metal — 2.5 mm², 12 AWG, 250 kcmil. Diameter is a linear dimension of the conductor or, more often, of the finished cable including insulation. The two are not interchangeable: a 12 AWG conductor is 2.05 mm in diameter, but the same conductor in a 600 V jacket is typically 3 to 4 mm outside. Cable trays, glands and conduit are sized on outside diameter; current capacity and voltage drop are sized on area.
- How do I convert AWG to mm²?
- Look up the area in the table — AWG is not a linear scale, so there is no simple formula for a mental conversion. As a rough anchor: 12 AWG is 3.31 mm², 10 AWG is 5.26 mm², 8 AWG is 8.37 mm², and each three-gauge step doubles the area. Above 4/0, the kcmil figure converts directly: 1 kcmil = 0.5067 mm², so 250 kcmil is 127 mm².
- Is 2.5 mm² cable the same as 12 AWG?
- No — 2.5 mm² is 24% less copper than 12 AWG, which is 3.31 mm². The nearest IEC size above it is 4 mm², which is 21% more. This gap exists because the metric and AWG series were designed independently, and it means any substitution is a compromise: undersize and accept more voltage drop and heating, or oversize and accept the cost and the larger bend radius.
- Why does this chart not show ampacity?
- Because ampacity is not a property of the conductor size alone. The same 2.5 mm² conductor carries 20 A with 60 °C insulation, 25 A with 75 °C insulation and 30 A with 90 °C insulation, in a raceway at 30 °C ambient — and different figures again when free air, bundled, or in a hotter location. Publishing a single number next to a size would be wrong for most installations. The ampacity chart gives the values with their installation basis stated.
Related
Value Sources
References and calculation methods used for this page are listed below. Numbered markers connect table columns where present. A reference listing does not itself establish that every value has been independently checked.
| # | Source | Type | Revision / method |
|---|---|---|---|
| [1] | Conductor diameter computed from the cross-sectional area | derived | d = √(4A/π), treating the stranded conductor as a solid round of equivalent area. Real stranded conductors are slightly larger in diameter than the equivalent solid because of the gaps between strands — typically by 2–5% depending on stranding class. |
| [2] | IEC 60228 — Conductors of insulated cables | reference | IEC 60228:2004 |
| [3] | NEC Chapter 9, Table 8 — Conductor properties (uncoated copper, 20 °C) | reference | NFPA 70 National Electrical Code, 2023 edition |
Data Sources
| Standard | Revision | What it covers on this page |
|---|---|---|
| NFPA 70 (NEC) Chapter 9, Table 8 — Conductor properties | NEC 2023 edition | the area and DC resistance columns for uncoated copper at 20 °C |
| IEC 60228 — Conductors of insulated cables | IEC 60228:2004 | the metric conductor cross-sectional areas in the mapping column |
| ASTM B258 — Standard nominal diameters and cross-sectional areas of AWG conductors | ASTM B258-18 | the AWG size definitions themselves |
Verification scope
No page-specific independent source review is recorded here. The listed references and formulas explain the basis of the compilation; automated consistency checks do not verify the original sources or establish product conformity.
Derived values — the following values on this page are calculated, not taken directly from the standard:
| Value | How it is derived |
|---|---|
| Conductor diameter | d = √(4A/π) from the metric area. Checked at build time against every row to within 2%. Stranded conductors are 2–5% larger in diameter than the equivalent solid. |
Dimensions and resistances are for the bare copper conductor at 20 °C. Outside diameter depends on insulation and voltage rating and is not given here. Resistance rises about 0.393% per °C, so a conductor at its 75 °C rating has roughly 22% more resistance than the table shows. Ampacity is intentionally excluded — see the ampacity chart for current ratings with their installation basis stated.
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