Reference

Cable sizes and conductor resistance

Metric and AWG conductors compared by actual area, with resistance per kilometre at 20 °C and at working temperature. Computed from geometry, with the standard's own figure beside it and the gap between them explained.

Written againstIEC 60228IEC 60364-5-52NEC Ch. 9Checked 29 August 2026

What is deliberately not here

There is no current-carrying capacity table on this page, and that is not an omission to be filled in later.

Ampacity is the safety number — it decides whether a cable overheats — and it is not a property of the conductor alone. It depends on the insulation, how the cable is installed, how many circuits run beside it, the ambient temperature and the protective device, and it is published as a set of tables with correction factors that only mean anything together. A single column of amps copied onto a website is exactly the artefact that gets used without its conditions.

So it stays in the standard and in the cable maker's data sheet, where the conditions travel with it. Everything on this page is what follows from geometry and a stated resistivity, which can be published honestly.

Metric and AWG do not line up

Charts that print "2.5 mm² = 14 AWG" are stating an equivalence that does not exist. The two ladders were built independently — one on round millimetres of area, the other on a geometric series of wire-drawing steps — and the nearest gauge to a metric size is routinely a sixth smaller or larger. Area is what carries current, so a sixth is not a rounding difference.

The column below gives the nearest gauge and how far off it is, rather than an equals sign. Above 4/0 there is no gauge at all: American practice sizes larger conductors in thousands of circular mils, so those rows are blank rather than matched to something that is not close.

Metric conductor sizes against their nearest AWG gauge, with the difference in area
Metricmm²	Nearest AWG	That gaugemm²	Difference
1	18	0.82	-17.7%
1.5	16	1.31	-12.8%
2.5	14	2.08	-16.8%
4	12	3.31	-17.3%
6	10	5.26	-12.3%
10	8	8.37	-16.3%
16	6	13.30	-16.9%
25	4	21.15	-15.4%
35	2	33.63	-3.9%
50	1/0	53.48	+7.0%
70	2/0	67.43	-3.7%
95	3/0	85.03	-10.5%
120	—	—	—
150	—	—	—
185	—	—	—
240	—	—	—

Areas are the nominal cross-section. The AWG figure is computed from the gauge definition — 0.127 mm at gauge 36, multiplied by the thirty-ninth root of 92 for each step down — not read from a chart.

Resistance per kilometre

Two columns, because there are two honest answers.

The geometric figure is resistivity divided by nominal area — copper at 1/58 Ω·mm²/m at 20 °C. It is exact for an ideal solid conductor of exactly that cross-section.

The standard's figure is a guaranteed maximum, and it runs a few per cent higher for two reasons: the strands of a stranded conductor spiral, so each is longer than the cable, and the number has to hold across manufacturing tolerance rather than describe a typical sample. Design work uses the maximum, because that is the direction that is safe to be wrong in.

The third column is the ratio between them, and it is there so you can check the second column rather than trust it.

Copper conductor resistance per kilometre, computed and as specified
Metricmm²	ComputedΩ/km at 20 °C	ComputedΩ/km at 70 °C	Specified maxΩ/km at 20 °C	Ratio
1	17.24	20.63	18.1	1.050
1.5	11.49	13.75	12.1	1.053
2.5	6.90	8.25	7.41	1.074
4	4.31	5.16	4.61	1.070
6	2.87	3.44	3.08	1.072
10	1.72	2.06	1.83	1.061
16	1.08	1.29	1.15	1.067
25	0.6897	0.8252	0.727	1.054
35	0.4926	0.5894	0.524	1.064
50	0.3448	0.4126	0.387	1.122
70	0.2463	0.2947	0.268	1.088
95	0.1815	0.2172	0.193	1.063
120	0.1437	0.1719	0.153	1.065
150	0.1149	0.1375	0.124	1.079
185	0.0932	0.1115	0.0991	1.063
240	0.0718	0.0860	0.0754	1.050

The 70 °C column is the same geometric calculation at the temperature PVC insulation is rated to — resistance rises about a fifth between 20 °C and 70 °C, which is why a voltage drop check that passes cold can fail warm. Aluminium runs about 60% higher than copper throughout.

Using this for a voltage drop check

Take the specified maximum rather than the computed figure — it is the conservative one, and it is what a design check is expected to use. Correct it for temperature if your conductor will be running warm, remembering that the correction is about a fifth between 20 °C and 70 °C.

Then size the cable for current-carrying capacity first, using the standard's tables, and check the drop second. Doing it the other way round is how a cable ends up too small for the current it carries.

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