R1+R2 Calculator - BS 7671 Table I1

Calculate combined line and CPC conductor resistance (R1+R2) for earth fault loop impedance verification according to BS 7671:2018 and the IET On-Site Guide

R1+R2 Loop Impedance Calculator

Calculate the combined resistance of line and circuit protective conductors (R1+R2) for earth fault loop impedance verification per BS 7671.

Cable Details

Select the cable installation type

Affects temperature correction factor

Phase/line conductor CSA

Circuit protective conductor CSA

m

One-way cable run length

Common R1+R2 Values per Metre (20°C)

Reference values from BS 7671 Table I1 for copper conductors. These values can be used for design calculations and to verify test results.

Cable CombinationR1 (mΩ/m)R2 (mΩ/m)R1+R2 (mΩ/m)
1.0/1.0mm²0036.2
1.5/1.0mm²12.1030.2
2.5/1.0mm²7.41025.51
2.5/1.5mm²7.4112.119.51
2.5/2.5mm²7.417.4114.82
4.0/1.5mm²012.116.71
4.0/2.5mm²07.4112.02
4.0/4.0mm²009.22
6.0/2.5mm²07.4110.49
6.0/4.0mm²007.69
6.0/6.0mm²006.16
10.0/4.0mm²006.44
10.0/6.0mm²004.91
10.0/10.0mm²003.66
16.0/6.0mm²004.23
16.0/10.0mm²002.98
16.0/16.0mm²002.3
25.0/10.0mm²002.557
25.0/16.0mm²001.877
35.0/16.0mm²001.674
35.0/25.0mm²001.251

What is R1+R2?

R1+R2 is the combined resistance of the line conductor (R1) and the circuit protective conductor or CPC (R2) in an electrical circuit. This value is critical for calculating the earth fault loop impedance (Zs) which determines whether protective devices will disconnect quickly enough during a fault to prevent electric shock, as required by BS 7671 (the IET Wiring Regulations).

The R1+R2 value is obtained either by calculation during the design stage or by dead testing using a low-resistance ohmmeter during inspection and testing. The measured value is recorded on the Schedule of Test Results as part of initial verification or periodic inspection.

Design Calculation

During circuit design, R1+R2 is calculated from conductor resistance tables (BS 7671 Table I1) using the cable size, type and length. A temperature correction factor is applied to account for conductor heating under fault conditions.

Dead Testing

R1+R2 is measured with the circuit isolated using Method 1 (bridging line and CPC at the distribution board) or Method 2 (long lead from MET). The measured value at 20°C must be compared with the calculated value to verify the circuit.

Zs Verification

Zs = Ze + R1+R2 (at operating temperature). The calculated Zs must not exceed the maximum values in BS 7671 Tables 41.2-41.4 for the protective device fitted to the circuit to ensure disconnection within the required time.

How to Calculate R1+R2

  1. Step 1 - Find conductor resistance per metre: Look up the resistance per metre for the line conductor and CPC from BS 7671 Table I1 or the On-Site Guide Table B3. For example, 2.5mm² copper = 7.41 mΩ/m and 1.5mm² copper = 12.10 mΩ/m.
  2. Step 2 - Add R1 and R2 per metre: R1+R2 per metre = R1 per metre + R2 per metre. For 2.5/1.5mm² T&E: 7.41 + 12.10 = 19.51 mΩ/m.
  3. Step 3 - Multiply by cable length: Total R1+R2 at 20°C = R1+R2 per metre × length ÷ 1000. For 20m: 19.51 × 20 ÷ 1000 = 0.3902Ω.
  4. Step 4 - Apply temperature correction: Multiply by the correction factor for the insulation type. For PVC (70°C): 0.3902 × 1.20 = 0.4682Ω. For XLPE (90°C): multiply by 1.28.
  5. Step 5 - Calculate Zs: Zs = Ze + R1+R2 (at operating temperature). Compare with maximum Zs from BS 7671 Tables 41.2-41.4 for the protective device.

R1+R2 Values Table - BS 7671 Table I1

The following table shows resistance per metre at 20°C for common copper conductor sizes. These values are from BS 7671 Table I1 (Appendix I) and the IET On-Site Guide Table B3.

Conductor Size (mm²)Resistance at 20°C (mΩ/m)Common Use
1.018.10CPC in T&E
1.512.10Lighting, CPC in 2.5mm² T&E
2.57.41Ring finals, radials
4.04.61Cookers, immersion heaters
6.03.08Showers, EV chargers
10.01.83Large showers, sub-mains
16.01.15Sub-mains
25.00.727Distribution cables

Testing vs Calculated Values

There is an important distinction between calculated R1+R2 values and measured values obtained during testing. Calculated values are theoretical and based on conductor resistance tables. Measured values are obtained using a low-resistance ohmmeter during dead testing and should be close to but may differ from calculated values due to:

  • Joint resistance: Every connection, termination and junction box adds a small amount of resistance to the circuit.
  • Temperature: Resistance measurements are affected by the ambient temperature at the time of testing. Standard values are at 20°C.
  • Cable manufacturing tolerances: Actual conductor CSA may vary slightly from nominal values.
  • Test lead resistance: Always null (zero) test leads before taking measurements to eliminate lead resistance from readings.

For verification purposes, the measured R1+R2 at ambient temperature should not exceed the calculated value by more than approximately 10%. Significantly higher readings indicate poor connections, damaged conductors, or incorrect cable.

Temperature Correction - The 80% Rule

BS 7671 maximum Zs values (Tables 41.2-41.4) are given at the conductor's maximum operating temperature. When testing at ambient temperature (typically 20°C), the measured Zs will be lower than the operating temperature value. The industry standard approach is:

  • PVC insulation (70°C): Apply correction factor of 1.20 to R1+R2 at 20°C, or use the 80% rule - measured Zs must not exceed 80% of the BS 7671 tabulated value.
  • XLPE insulation (90°C): Apply correction factor of 1.28 to R1+R2 at 20°C.
  • Formula: R(t) = R(20) × [1 + 0.004 × (t - 20)] where t is the operating temperature in °C.
  • Copper coefficient: The temperature coefficient of resistance for copper is 0.004 per °C (also written as 0.004/K).

R1+R2 for Common Twin & Earth Cables

1.0/1.0mm² (Lighting)

R1+R2 = 36.20 mΩ/m. Used for lighting circuits protected by 6A MCB. At 20m length: total R1+R2 = 0.724Ω.

2.5/1.5mm² (Ring Finals & Radials)

R1+R2 = 19.51 mΩ/m. The most common domestic cable. At 20m: total R1+R2 = 0.390Ω. Used for socket circuits and 32A radials.

6.0/2.5mm² (Showers & Cookers)

R1+R2 = 10.49 mΩ/m. For higher-powered circuits. At 20m: total R1+R2 = 0.210Ω. Used for electric showers up to 9kW and cooker circuits.

10.0/4.0mm² (Large Loads)

R1+R2 = 6.44 mΩ/m. For high-power circuits. At 20m: total R1+R2 = 0.129Ω. Used for large electric showers and sub-mains.

R1+R2 Testing Methods - BS 7671 Guidance Note 3

Method 1: Link at Distribution Board

The most common method for initial verification. A temporary link (short, low-resistance conductor) is placed between the line and CPC terminals at the distribution board. R1+R2 is then measured at the furthest point of the circuit using a low-resistance ohmmeter.

  • • Circuit must be isolated and proved dead
  • • All loads must be disconnected
  • • Null (zero) test leads before measurement
  • • Record reading on Schedule of Test Results

Method 2: Long Lead from MET

Used when access to distribution board terminals is limited. A long test lead is connected from the Main Earthing Terminal (MET) to the furthest point. One probe on the line conductor, the other on the long lead connected to the CPC via the MET.

  • • Subtract the long lead resistance from the reading
  • • Less accurate than Method 1 due to lead length
  • • Useful for testing individual sockets on ring circuits
  • • Also measures continuity of the CPC back to MET

Ring Circuit R1+R2

For ring final circuits, the measured R1+R2 at each socket should be approximately one quarter of the end-to-end R1+R2 readings (r1 and r2) if the ring is continuous and has no interconnections. Significantly higher readings at any socket indicate a break in the ring or high-resistance joint.

Common R1+R2 Questions

What is R1+R2 used for?

R1+R2 is used to calculate the earth fault loop impedance (Zs) of a circuit using the formula Zs = Ze + (R1+R2). The Zs value determines whether the circuit's protective device (MCB, fuse, or RCBO) will disconnect fast enough during an earth fault to prevent electric shock, as required by BS 7671 Regulation 411.

What is a good R1+R2 reading?

There is no single “good” value — it depends on cable size, length, and the protective device. The measured R1+R2 should be close to the calculated value (within ~10%). For a typical 2.5/1.5mm² ring final circuit at 40m, expect R1+R2 readings around 0.19–0.20Ω at each socket (one quarter of the end-to-end values). Significantly higher readings suggest poor connections.

Why is my R1+R2 reading higher than calculated?

Common causes include: joint resistance from connector blocks, crimp joints, and junction boxes; poor terminations at socket outlets or the distribution board; damaged conductors from incorrect stripping or over-tightened terminals; test lead resistance not zeroed before measurement; or ambient temperature significantly different from the 20°C standard. A reading more than 10% above the calculated value warrants investigation.

Do I apply the 1.2 correction factor to my measured R1+R2?

It depends on how you're checking compliance. If comparing Zs against the BS 7671 tabulated maximum values (which are at operating temperature), then yes — multiply the measured R1+R2 by 1.20 (PVC) or 1.28 (XLPE) before adding Ze. Alternatively, use the 80% rule: compare your measured Zs (at ambient) against 80% of the tabulated maximum. Both approaches account for the same temperature difference.

What is the difference between R1+R2 and Zs?

R1+R2 is the resistance of the circuit's own conductors only (line + CPC). Zs is the total earth fault loop impedance including the external supply impedance (Ze). The relationship is Zs = Ze + (R1+R2). You can control R1+R2 by selecting different cable sizes or lengths, but Ze is determined by the DNO supply and is outside your control.

Where do I find R1+R2 values per metre?

R1+R2 per metre values are found in BS 7671 Table I1 (Appendix I) and the IET On-Site Guide Table B3. These tables give individual conductor resistance per metre at 20°C. Add the R1 value (line conductor) and R2 value (CPC) together to get R1+R2 per metre for your specific cable combination.

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