I have been looking at "Tri-rated wire". This is wire that ostensibly meets the British BS6231 Type CK, US UL 758 Style 1015, and Canadian CSA 22.2.

I have noticed that the metric wire size (mm²) and imperial size (AWG) do not match properly. I know that there can never be an exact match, but they differ a lot.

For example, I have some wire that has both "0.75mm²" and "20AWG" printed on the wire. A search online shows that 20AWG is about 0.52mm². That is almost down to the metric 0.5mm² wire size. A closer gauge is 19AWG, though that is still off by about 0.1mm².

I have other wire that has both "1.00mm²" and "18AWG" printed on the wire. Again, they do not match - 18AWG is about 0.82mm². 17AWG is much closer to 1mm² (1.0378mm²).

I have looked at both the US and British standards, and I can see no mention of this discrepancy. I also looked at the relevant harmonised standard (EN 50525) which also does not seem to mention it.

Which measurement can I trust? Is there a prescribed way of interpreting the measurements? Is it down to the manufacturer? Are wires with metric measurements always metric, with the AWG being a rough guide?

Here is an image of the 0.75mm² wire:

A section of 0.75mm² wire

  • \$\begingroup\$ Have you measured the wire to see which specification is closest? It sounds like someone at the factory simply got it wrong. \$\endgroup\$
    – JYelton
    Jun 7, 2022 at 20:30
  • \$\begingroup\$ I thought that could be the case, too, but I have different brands of wire with the same issue. Unfortunately my cheap callipers are not accurate enough. \$\endgroup\$
    – SDB
    Jun 7, 2022 at 20:38
  • 1
    \$\begingroup\$ #17 and 19 AWG wire sizes are extremely rare, if not unknown, in North America, so the manufacturer labeled the wire with the nearest (or "best") common AWG size. 1.0 mm2 wire will safely replace #18 AWG, but wouldn't meet specs for #17 if it existed. \$\endgroup\$ Jun 7, 2022 at 21:24

2 Answers 2


Either the wire is labeled incorrectly or you are misreading it.

You mentioned 0.75 mm2 wire, but if it's actually 0.75 mm diameter, then it makes sense to also label it 20 AWG, which has a diameter of 0.81 mm and a cross-sectional area of 0.52 mm2.

18 AWG wire has a diameter of 1.02 mm and a cross-sectional area of 0.82 mm2. The diameter matches what you were given (or read) as the cross-sectional area.

So either both manufacturers have used diameter and cross-sectional area incorrectly, or you're not seeing the unit correctly (cable jacket printing can be abysmal to read; not your fault!).

My figures are from The Engineering Toolbox, and I don't know where they sourced their data, so your numbers may vary.

  • \$\begingroup\$ Thanks for your help. I've added a picture. It does seem to have the squared measurement, so I think that you might be correct about the manufacturer either making a mistake, or just selecting the closest smaller even-numbered AWG. \$\endgroup\$
    – SDB
    Jun 8, 2022 at 20:29

Yes, I confirm this mess. But AWG, as I understand, only says how often an AWG 0 reference wire has been pulltruded through smaller and smaller contraction chokes. This is not a precise description anyway. As you said, it's a guess.

  • \$\begingroup\$ No, AWG is a very precise series. dn = 0.005" * 92^(36-n)/39. So AWG 36 is 0.005". Stranded are equivalent cross-sectional area to a circular cross-section solid wire (give or take some tolerance). \$\endgroup\$ Jun 7, 2022 at 21:36

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