Inspection, test & fault finding
UK cable resistance, R1+R2, R1+Rn and length from resistance
Every standard UK conductor size with its resistance in milliohms per metre, a forward query that turns a run length into expected R1+R2 and R1+Rn values, and a reverse query that turns a measured reading back into a distance — the quickest way to work out roughly how far along a circuit a fault or a junction actually sits.
Query 1 — length ➝ R₁+R₂ and R₁+Rₙ
Expected readings for a run
Pick the cable, enter the one-way route length and the calculator returns the expected end-to-end R1, Rn, R2, R1+R2 and R1+Rn for a radial circuit.
Results
Query 2 — measured reading ➝ length
Distance from a measured reading
Enter the reading you actually measured (R1+R2 or R1+Rn), pick the cable type and the calculator returns the equivalent cable length. Useful for locating a buried joint, an unexpected spur, or confirming whether a reading is plausible for the run you can see.
Results
A reading noticeably higher than the visible run length usually means a loose or high-resistance connection, an undisclosed spur, or a smaller conductor than assumed. Re-null the leads and repeat before drawing conclusions.
Reference table
UK conductor resistance at 20 °C (mΩ/m)
| Size | Copper mΩ/m | Aluminium mΩ/m | Copper Ω per 10 m |
|---|---|---|---|
| 1.0 mm² | 18.1 | — | 0.1810 |
| 1.5 mm² | 12.1 | — | 0.1210 |
| 2.5 mm² | 7.41 | — | 0.0741 |
| 4.0 mm² | 4.61 | — | 0.0461 |
| 6.0 mm² | 3.08 | — | 0.0308 |
| 10 mm² | 1.83 | — | 0.0183 |
| 16 mm² | 1.15 | 1.91 | 0.0115 |
| 25 mm² | 0.727 | 1.2 | 0.0073 |
| 35 mm² | 0.524 | 0.868 | 0.0052 |
| 50 mm² | 0.387 | 0.641 | 0.0039 |
| 70 mm² | 0.268 | 0.443 | 0.0027 |
| 95 mm² | 0.193 | 0.32 | 0.0019 |
| 120 mm² | 0.153 | 0.253 | 0.0015 |
| 150 mm² | 0.124 | 0.206 | 0.0012 |
| 185 mm² | 0.0991 | 0.164 | 0.0010 |
| 240 mm² | 0.0754 | 0.125 | 0.0008 |
| 300 mm² | 0.0601 | 0.1 | 0.0006 |
Values are single-conductor resistances at 20 °C. For a circuit, add the relevant pair of figures together to get the loop resistance per metre (R1+R2 or R1+Rn). Aluminium is not normally used below 16 mm² in UK fixed wiring.