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.

Cable Sizes — Conductor Dimensions

Conductor area, diameter and DC resistance for AWG and kcmil cable sizes, with the nearest IEC metric equivalent for substitution.

Data verified 2026-09-29 · based on NEC 2023 edition

Quick Answer

Cable size is a cross-sectional area, not a diameter. Copper DC resistance falls as area rises: 14 AWG (2.08 mm²) is 10.1 Ω/km while 4/0 AWG (107 mm²) is 0.200 Ω/km — a factor of 50 for a 50× area increase. Use the resistance column, not the size label, when calculating voltage drop.

Find a size

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.

Value Sources

Each data column on this page is tied to the source it came from. The numbers in square brackets correspond to the table headers above.

#	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	standard	IEC 60228:2004
[3]	NEC Chapter 9, Table 8 — Conductor properties (uncoated copper, 20 °C)	standard	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

Cross-checked against:

Areas and resistances cross-checked against the NEC conductor-properties table
Metric mapping checked against IEC 60228 preferred conductor sizes
Conductor diameter recomputed from the area at build time and compared against the printed value
Both resistance columns checked against each other by exact unit conversion

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.

Accuracy and use. The values on this page are compiled from the published standards and cross-checked sources listed above. Where values are derived, the derivation is stated. No warranty, express or implied, is made as to the accuracy or completeness of this information, and no liability is accepted for any loss or damage arising from its use. Engineering reference data is provided for guidance in preliminary work — before a value is used for design, fabrication or acceptance testing, verify it against the current revision of the governing standard and against your own inspection. The user assumes all risk and responsibility in connection with the use of this information.

Cite This Page

Every value on this page is traceable to the sources listed above. If you use the data in a document, paper or report, cite it as:

Plain text

Engineering Reference. "Cable Sizes — Conductor Dimensions." https://refmanual.com/charts/cable-sizes/ (accessed 2026-09-29). Data from NFPA 70 (NEC) Chapter 9, Table 8 — Conductor properties, NEC 2023 edition.

BibTeX

@misc{cable-sizes,
  title        = {Cable Sizes — Conductor Dimensions},
  author       = {{Engineering Reference}},
  howpublished = {\url{https://refmanual.com/charts/cable-sizes/}},
  note         = {Accessed: 2026-09-29},
  source       = {NFPA 70 (NEC) Chapter 9, Table 8 — Conductor properties, NEC 2023 edition}
}

RIS (EndNote, Zotero, Mendeley)

TY  - ELEC
TI  - Cable Sizes — Conductor Dimensions
AU  - Engineering Reference
PB  - Engineering Reference
UR  - https://refmanual.com/charts/cable-sizes/
PY  - 2026
DA  - 2026-09-29
N1  - Source: NFPA 70 (NEC) Chapter 9, Table 8 — Conductor properties, NEC 2023 edition
ER  -

Each row in the tables above also has a permanent link — hover a row and use the # link to cite a single value rather than the whole page.