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Table B1, and calculating an expected resultWhat should a 10 metre conductor read?

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Table B1 in Guidance Note 3's Appendix B gives the resistance of conductors in milliohms per metre, and it's the table you'll live in.

Reading Table B1
The star ★Marks the twin & earth sizes — the combinations you actually buy. Worth highlighting
First columnOne conductor on its own
Second columnThe associated protective conductor, for two-conductor tests
✋For an R2 test, ignore the second column. It says "line conductor" at the top, and that throws a lot of people. On an R2 test you only have one conductor, so just read it as "conductor". First column only.
Expected R = (mΩ/m × length in metres) ÷ 1000

10 mm² conductor, 10 metres

Table B1 · 10 mm² = 1.83 mΩ/m

R = (1.83 × 10) ÷ 1000
R = 0.0183 Ω expected
What's the point of the calculation? It gives you an expected result before you put a meter on anything. If your measurement lands nowhere near it, you know to investigate rather than write it down. This is not the same as the appliance test — there's no 0.1 Ω added here. This is the cable alone.

And backwards: finding the length from the resistance

Length = (R × 1000) ÷ mΩ/m
Where this is genuinely useful. You cannot measure a cable that's under floorboards. But you can measure its resistance, and if you know the conductor size, Table B1 gives you the length. The by-the-book approach is to measure the drum before you install — measure a known 100 m drum, and you can work out any length off it. Almost nobody does; they run the cable in and test afterwards. Both get you there.

GN3 · Appendix B · Table B1 · mΩ/m · ★ = twin & earth ★

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