R1+R2 Calculator
Calculate total line conductor resistance (R1) plus circuit protective conductor resistance (R2) for electrical continuity testing and verification under BS 7671 wiring regulations.
BS 7671 R1+R2 Calculator
How to Use R1+R2 Calculator
Verifying the continuity of protective conductors (CPC) using the R1+R2 testing method is a basic inspection and testing requirement for verifying electrical safety in UK installations. Follow these steps to calculate and verify results:
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1Measure or enter R1 resistance. Input the line conductor's measured or design resistance.
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2Measure or enter R2 resistance. Input the circuit protective conductor (CPC) resistance value.
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3Select unit for each input. Choose between Ohms (Ω) or Milliohms (mΩ) depending on how your low-resistance ohmmeter reads.
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4Select circuit type. Choose the appropriate circuit topology (Radial Circuit, Ring Final Circuit, Lighting Circuit, or Distribution Circuit) to record the test configuration.
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5Calculate result. Click the Calculate button. The calculator will automatically perform unit conversion and compute the total combined continuity resistance.
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6Compare with test records and BS 7671 limits. Evaluate the testing status and record the values on your Electrical Installation Certificate (EIC) or Electrical Installation Condition Report (EICR).
Practical UK Continuity Testing Workflow
To perform this test on-site, a UK electrician first ensures the circuit is safely isolated from the main supply. At the consumer unit or distribution board, a temporary link lead is installed between the line conductor and the CPC (earth) of the circuit under test. The electrician then moves to the furthest point of the circuit (such as the last socket-outlet on a radial or the midpoint of a ring final) and takes a reading with a low-resistance ohmmeter between the line and earth terminals. This physical measurement is the R1+R2 value which should closely match your calculated design values.
How to Calculate R1+R2
The calculation of line plus protective conductor resistance (R1+R2) forms the backbone of pre-energizing verification. It ensures that protective earthing conductors are correctly installed and have sufficiently low resistance to trigger automatic disconnection during an earth fault.
The Continuity Formula
For a radial or distribution circuit, the combined continuity resistance is calculated by directly summing the individual conductor resistances:
Where:
- R1 = Resistance of the Line (Phase) Conductor (Ω)
- R2 = Resistance of the Circuit Protective Conductor (CPC) (Ω)
Worked Example: Twin & Earth Circuit
Let's calculate the combined continuity resistance for a domestic radial socket circuit using typical measured test values:
Step 1 — Identify Measured Conductor Values
An electrician measures the isolated conductors and obtains:
Step 2 — Sum Conductor Resistances
Substitute the measured values directly into the continuity equation:
Final Answer
The combined R1+R2 continuity resistance of the circuit is 0.70 Ω. This value is then recorded on the test certificate and added to the external loop impedance (Ze) to calculate the prospective loop impedance (Zs) for comparison against BS 7671 limits.
Relation to Earth Fault Loop Impedance (Zs)
The main purpose of calculating or measuring R1+R2 is to verify that the total earth fault loop impedance (Zs) will be low enough to ensure automatic disconnection of the supply (ADS) in the event of an earth fault. The relationship is defined by the following equation:
Where Ze is the external earth loop impedance measured at the origin of the supply network. Keeping R1+R2 low ensures that the prospective fault current is high enough to trip the circuit breaker or blow the fuse within the required BS 7671 disconnection time limit (such as 0.4s for standard final circuits).
R1+R2 Chart
This table lists typical R1, R2, and combined R1+R2 resistances for standard 2.5 mm² line / 1.5 mm² CPC copper twin-core-and-earth cables (BS 6004) at 20°C ambient temperature. Conductor resistance values are based on typical Guidance Note 3 parameters.
| Circuit Length (m) | Typical R1 (Ω) | Typical R2 (Ω) | Typical R1+R2 (Ω) |
|---|---|---|---|
| 10 m | 0.074 Ω | 0.121 Ω | 0.195 Ω |
| 20 m | 0.148 Ω | 0.242 Ω | 0.390 Ω |
| 30 m | 0.222 Ω | 0.363 Ω | 0.585 Ω |
| 40 m | 0.296 Ω | 0.484 Ω | 0.780 Ω |
| 50 m | 0.371 Ω | 0.605 Ω | 0.976 Ω |
| 75 m | 0.556 Ω | 0.908 Ω | 1.463 Ω |
| 100 m | 0.741 Ω | 1.210 Ω | 1.951 Ω |
| 125 m | 0.926 Ω | 1.513 Ω | 2.439 Ω |
Note: Values are illustrative examples only for reference and must not replace measured site test results. Actual resistance varies with ambient temperature and cable manufacturing tolerances.
R1+R2 Frequently Asked Questions
In BS 7671, R1 is the resistance of the line (phase) conductor, and R2 is the resistance of the circuit protective conductor (CPC or earth). Measured together during continuity testing, R1+R2 represents the total resistance of the conductors from the distribution board to the furthest point of the circuit.
R1+R2 is measured during electrical continuity testing to verify that all protective conductors are continuous and properly connected to earth. This measurement forms a critical part of initial verification and periodic inspection (EICR) to ensure that the earth fault path is low resistance.
No. R1+R2 is the resistance of the circuit's own conductors. Earth fault loop impedance (Zs) is the total loop impedance, which is the sum of the external supply impedance (Ze) and the circuit resistance: Zs = Ze + (R1+R2). Zs is measured live, whereas R1+R2 is measured dead.
Electricians measure R1+R2 by first isolating the electrical supply. At the distribution board, they temporarily connect the line conductor and the CPC together. Then, using a low-resistance ohmmeter at the furthest point of the circuit, they measure the resistance between the line and earth terminals.
Under UK standards, a typical R1+R2 value for a final circuit is between 0.1 Ω and 1.5 Ω, depending on cable cross-sectional area, length, and material. Values under 1.0 Ω are excellent, while values above 2.0 Ω indicate longer runs or potential high-resistance connections that require verification.
Yes, R1+R2 can be calculated using the formula R1+R2 = (mΩ/m × Length × Temp Correction) ÷ 1000, using tabulated conductor resistance values from Guidance Note 3. However, calculation is usually done during design, whereas physical measurement is required during testing.
