R1+R2 calculator: R1+R2 values and expected Zs

R1+R2 is the resistance of a circuit's line conductor plus its protective conductor, from the board to the furthest point, in ohms. A 2.5/1.5 mm² cable is 19.51 mΩ for every metre of circuit at 20 °C, and Ze + (R1+R2) is the earth fault loop impedance, Zs, the circuit will present.

Line and protective conductor sizes, the length of the run, and the Ze at the origin. Out comes the R1+R2 you should read, the Zs the circuit will present, and how long it can go before it reaches the device's limit.

Conductor resistances from BS EN 60228; Zs = Ze + (R1+R2) as Regulation 411.4.4 and Appendix 14 of BS 7671 apply it. Checked against BS 7671:2018+A4:2026, updated 3 September 2026. Free, and nothing to sign in to.

Cable (line / cpc, mm²)

Board to the furthest point, following the cable's route rather than the straight line.

Conductor temperature
Ze at the origin

The chips are the declared maxima for each earthing arrangement; a measured value is always better.

Device type
Device rating

R1+R2 for 0 m of 2.5/1.5 at function at() { [native code] } °C

0.00 Ω

Zs inside the limit

Zs of 0.35 Ω against a B32's 1.09 Ω.

R1+R2 per metre at function at() { [native code] } °C 7.41 + 12.10
19.51 mΩ/m
R1 (line)
0.00 Ω
R2 (protective conductor)
0.00 Ω
Expected Zs with Ze 0.35 Ω Ze + (R1+R2)
0.35 Ω
B32 limit at function at() { [native code] } °C 80% figure, conductor cold
1.09 Ω
Longest run for a B32 Before the earth fault loop exceeds the limit; check volt drop too.
43 m

How is R1+R2 worked out, and what does it say about Zs?

Every copper conductor has a resistance per metre fixed by its size: BS EN 60228, the conductor standard, gives the maximum at 20 °C for each: 7.41 mΩ/m for 2.5 mm², 12.1 mΩ/m for 1.5 mm². R1 is the line conductor's figure times the length of the circuit; R2 the protective conductor's; and their sum is what an instrument reads end to end with the two linked at the far point. For 2.5/1.5 mm² that is 19.51 mΩ for every metre of circuit, the figure the On-Site Guide tabulates.

Copper's resistance rises about 0.4% per degree. A test reading is taken at roughly 20 °C, but the Zs limits are set for a conductor at its 70 °C operating temperature, so a design figure is the cold one times 1.2, and a cold reading is judged against 80% of the limit rather than corrected, which is the same adjustment made the other way round.

From R1+R2 to Zs

The earth fault loop the circuit presents at its furthest point is the supply's external impedance plus the circuit's own: Zs = Ze + (R1+R2). Compare that with the device's maximum Zs and you know before the cable is run whether the circuit will disconnect in time. Turn the sum around and it says how far the circuit may go: the headroom between Ze and the limit, divided by the pair's mΩ/m at 70 °C.

Worked example

A 25 m radial in 2.5/1.5 mm² on a PME supply with a measured Ze of 0.35 Ω, protected by a B32.

  1. Pair at 20 °C: 7.41 + 12.1 = 19.51 mΩ/m.
  2. R1+R2 = 19.51 × 25 ÷ 1000 = 0.49 Ω, which is what the wander-lead test should read.
  3. Zs = 0.35 + 0.49 = 0.84 Ω.
  4. A B32's limit is 1.37 Ω; a cold figure is judged against 1.09 Ω. 0.84 Ω passes with room to spare.
  5. Longest run: (1.37 − 0.35) ÷ 0.02341 = 43 m before the loop alone would fail it, though at 32 A the voltage drop on 2.5 mm² gives out well before that.

Questions the trade asks

The resistance of the line conductor plus the resistance of the protective conductor, from the board to the furthest point of the circuit, in ohms. It is measured with the two linked together at the far end and recorded in column 18 of the schedule of test results. Add the Ze at the origin and you have the circuit's Zs.

From BS EN 60228, the conductor standard, which gives the maximum resistance of each copper size at 20 °C: 7.41 mΩ/m for 2.5 mm², 12.1 for 1.5 mm². Add the two conductors of the pair, 19.51 mΩ/m for 2.5/1.5, and you have the figure the On-Site Guide tabulates. Nothing here is copied from a table; it is the conductor's own property.

Copper's resistance rises about 0.4% per degree. A reading is taken at roughly 20 °C, but the Zs limits are for a conductor at its 70 °C operating temperature, fifty degrees warmer and so about a fifth higher. Either multiply the reading up by 1.2, or compare it against 80% of the limit, but never both.

The declared maxima for the earthing arrangement: 0.35 Ω for TN-C-S (PME), 0.8 Ω for TN-S and 21 Ω for TT. They are design figures. Measure Ze at the origin before anything is certified, and use the measured value.

Take the device's maximum Zs, subtract Ze, and divide by the pair's resistance per metre at 70 °C. A B32 (1.37 Ω) on a 0.35 Ω PME supply in 2.5/1.5 gives (1.37 − 0.35) ÷ 0.0234 ≈ 43 m of cable, as far as the earth fault goes. Check the voltage drop as well; on a long run it usually bites first.

The figures go straight onto the certificate

In Pascal the maximum Zs is filled in for every circuit as you pick the device, the schedule of test results carries all 32 columns, and the observation library suggests the wording, code and regulation as you type. EICs, EICRs and minor works, from £25 a month.

See the certificate software

A calculator applies a method to the numbers you give it. It does not know the installation in front of you, and it is no substitute for BS 7671, the guidance or the judgement of the person signing the certificate. Check anything you rely on.