20°C values from OSG Table I1; ×1.2 approximates conductor operating temperature. Guidance only — always verify against BS 7671 and the On-Site Guide for the actual installation method.
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This calculator predicts the expected R1+R2 for a circuit from the line conductor size, CPC size and route length, using the mΩ/m resistance values in Table I1 of the IET On-Site Guide. UK electricians use it to sanity-check continuity test results, estimate Zs at design stage and spot wiring errors before they become certification headaches.
Select the line and CPC sizes — for twin and earth these usually differ, such as 2.5mm² line with a 1.5mm² CPC — enter the circuit length, and the tool combines the per-metre resistances over the run. An optional multiplier of 1.2 adjusts the 20°C tabulated values to approximate conductor operating temperature for use in hot Zs calculations.
R1+R2 = (rLine + rCPC) × L × F / 1000
rLine and rCPC are the conductor resistances in milliohms per metre at 20°C from OSG Table I1 — for example 7.41 mΩ/m for 2.5mm² and 12.1 mΩ/m for 1.5mm² copper. L is the circuit route length in metres. F is the temperature factor: 1 for values at 20°C (to compare with cold test readings) or 1.2 to approximate the resistance at conductor operating temperature. Dividing by 1000 converts milliohms to ohms.
Choose the line conductor CSA from 1mm² to 25mm². The tool applies the matching 20°C resistance from OSG Table I1.
Choose the CPC size, which for flat twin and earth is usually one size smaller than the line conductor (for example 1.5mm² CPC in 2.5mm² T&E).
Enter the route length in metres from the board to the furthest point. The combined mΩ/m figure and the total R1+R2 are calculated for the run.
Leave the adjustment off to compare with a cold continuity test reading, or switch on the ×1.2 multiplier to approximate operating temperature for a hot Zs calculation.
The resistance values are the 20°C figures from Table I1 of the IET On-Site Guide for copper conductors. Your measured R1+R2 should land close to the calculated value; a reading well above it points to a loose termination, an unexpectedly long route or an undersized conductor, while a reading well below it may mean a parallel path.
The ×1.2 multiplier is the standard approximation for bringing 20°C values up to conductor operating temperature for 70°C thermoplastic cables. Use it when feeding the result into a Zs calculation compared against full design limits; leave it off when predicting what your low-resistance ohmmeter should read on a cold circuit.
For ring final circuits, remember this tool gives the end-to-end resistance of a radial run — the effective R1+R2 at the sockets of a healthy ring is a quarter of the sum of the end-to-end line and CPC readings, which the ring final calculator handles. Guidance only; verify against BS 7671 and the On-Site Guide.
20°C values from OSG Table I1; ×1.2 approximates conductor operating temperature. Guidance only — always verify against BS 7671 and the On-Site Guide for the actual installation method.
From OSG Table I1, 2.5mm² is 7.41 mΩ/m and 1.5mm² is 12.1 mΩ/m, a combined 19.51 mΩ/m. So a 20m radial run gives an expected R1+R2 of about 0.39Ω at 20°C. Multiply by 1.2 if you need the value at operating temperature.
The usual causes are loose or dirty terminations, a longer actual cable route than estimated, or a smaller CPC than assumed. High individual readings at one point on a circuit particularly suggest a poor connection there. Investigate before certifying — the calculation tells you what a healthy circuit should read.
Use the 1.2 factor when you need conductor resistance at operating temperature — typically when calculating Zs to compare against full (100%) design limits. If instead you compare cold measured values against 80%-adjusted device limits, use the 20°C figures without the multiplier. Do not apply both corrections at once.
It calculates the resistance of a single run of conductor, which matches the end-to-end readings (r1, r2) you take on a ring. The expected value at the sockets of a cross-connected ring is (r1 + r2) / 4 — use the ring final expected readings calculator for that step.
Calculated Zs for a circuit from external loop and conductor resistances.
Maximum earth fault loop impedance for BS EN 60898/61009 devices.
Cross-connected test values from end-to-end ring measurements.
Volt drop for twin & earth over a run, checked against BS 7671 limits.
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