Ring Final Circuit Calculator - BS 7671

Calculate continuity test values and verify ring final circuit design according to BS 7671 Section 433, Appendix 15, and IET Guidance Note 3

Ring Final Circuit Continuity Test

Calculate expected values for ring final circuit continuity testing according to IET Guidance Note 3. Enter your end-to-end resistance measurements (r1, rn, r2) from Step 1 of the ring test.

Select the Twin & Earth cable size used

Ω

End-to-end resistance of line conductors

Ω

End-to-end resistance of neutral conductors

Ω

End-to-end resistance of CPC (earth) conductors

Ring Final Circuit Testing and Design

Ring final circuits are a common wiring configuration in UK domestic and light commercial installations for socket outlets. This calculator helps electricians perform continuity tests and verify circuit design according to BS 7671:2018 and IET Guidance Note 3.

Continuity Testing

Calculate expected R1+R2 values from your end-to-end resistance measurements (r1, rn, r2). Verify ring integrity and identify any faults or anomalies in the circuit.

Circuit Design

Verify circuit design parameters including Zs (earth fault loop impedance), voltage drop, and maximum permissible circuit length for compliance with BS 7671.

BS 7671 Compliance

Ensures compliance with Regulation 433.1.204 (ring final circuits), Section 433, and Appendix 15. Includes all relevant BS 7671:2018+A2:2022 requirements.

Ring Final Circuit Testing - Step by Step

  1. Step 1 - End-to-End Testing: At the consumer unit, measure end-to-end resistance of line conductors (r1), neutral conductors (rn), and CPCs (r2). Values should be similar - r1 and rn within 0.05Ω.
  2. Step 2 - Cross-Connection at Far End: At the furthest point of the ring, cross-connect the line and CPC conductors together.
  3. Step 3 - Socket Testing: Test between line and CPC at each socket outlet. Reading should be approximately (r1 + r2) / 4, with maximum value at ring mid-point.
  4. Step 4 - Neutral Testing: Cross-connect line and neutral at far end. Test between line and neutral at each socket. Readings verify neutral continuity.

BS 7671 Requirements for Ring Final Circuits

  • Regulation 433.1.204: Maximum floor area served shall not exceed 100m²
  • Cable Requirements: Minimum current-carrying capacity of 20A (typically 2.5mm² Twin & Earth)
  • Protective Device: Typically protected by 30A or 32A device (MCB or fuse)
  • Socket Outlets: Unlimited number of 13A socket outlets permitted on ring
  • Spurs: Non-fused spurs permitted from socket outlets or junction boxes (maximum one single or one twin socket per spur)
  • Voltage Drop: Maximum 5% (11.5V) from origin to any point
  • Zs Compliance: Earth fault loop impedance must be within limits for the protective device used

Important Testing Notes

  • ⚠️ IET Guidance Note 3: r2 reading should be approximately 1.67 times r1 reading for 2.5/1.5mm² cable
  • ⚠️ Tolerance: R1+R2 readings at sockets should be ±10% of calculated value
  • ⚠️ Spurs: Readings significantly higher than expected indicate a spur - verify this is intended
  • ⚠️ Parallel Paths: Lower than expected readings may indicate unintended parallel paths
  • ⚠️ Broken Ring: If readings don't reduce around the ring, circuit may not be continuous

Typical Cable Resistance Values (20°C)

Cable SizeLine Conductor (mΩ/m)CPC (mΩ/m)Typical Use
2.5/1.5mm²7.4112.1032A ring (most common)
4.0/1.5mm²4.6112.10Long ring circuits
6.0/2.5mm²3.087.41High load or long distance
10.0/4.0mm²1.834.61Commercial installations

Common Issues and Solutions

r1 and rn don't match

Check for loose connections, damaged conductors, or incorrect cable type.

r2 ratio incorrect

Verify cable specification - may have wrong CPC size or mixed cable types.

Readings don't reduce around ring

Ring may be broken - check for disconnected conductors at accessories.

One socket reading much higher

Likely a spur - verify it's wired correctly (max one twin or one single socket).

Frequently Asked Questions