Ring Final Circuit Calculator
Calculate expected ring continuity values and verify measured readings per BS 7671
Reference Info & Formulas
This calculator helps with:
- Expected R1, Rn, R2 values
- r1+r2 at furthest point
- Zs estimation
- Verification of measured values
Reference: GN3, BS 7671 Appendix 14
Step 1: Measure L-L, N-N, E-E end-to-end
Step 2: Cross-connect L-N at DB
Step 3: Measure at each socket
Values should be consistent (within 0.05Ω)
Data: BS 7671:2018+A2:2022 — Table 4F1, IET GN3
For guidance only. The responsibility for any electrical installation lies with the qualified person carrying out the work. Always verify calculations independently and apply professional judgement.
How This Calculator Works
Ring final circuits use 2.5/1.5mm² T&E on a 32A MCB with the cable forming a complete loop from and back to the consumer unit.
The three-step ring continuity test verifies the integrity of the ring. First, the end-to-end resistance of line, neutral, and CPC conductors is measured (r1, rn, and r2). Second, line and neutral are cross-connected at the board and measured at each socket — the figure-of-eight test. Third, line and CPC are cross-connected, giving the R1+R2 value at every point on the ring.
For a healthy ring, the cross-connected reading at each socket should be approximately (r1 + r2) / 4 at the midpoint, and should remain broadly consistent around the entire ring. Any significant deviation indicates a wiring fault.
Quick Reference — Expected Ring Circuit Readings
Typical resistance values for 2.5/1.5mm² T&E ring circuits
IET Guidance Note 3| Measurement | 20m Ring | 40m Ring | 80m Ring |
|---|---|---|---|
| End-to-end L or N (r1) | 0.26Ω | 0.52Ω | 1.04Ω |
| End-to-end CPC (r2) | 0.48Ω | 0.97Ω | 1.94Ω |
| Cross-connected midpoint ≈ (r1+r2)/4 | 0.19Ω | 0.37Ω | 0.75Ω |
Based on conductor resistance of 7.41 mΩ/m for 2.5mm² and 12.1 mΩ/m for 1.5mm² at 20°C.
Understanding the Three-Step Test
Step 1 — End-to-end resistance: Measure line to line, neutral to neutral, and CPC to CPC at the consumer unit with the ring disconnected. These readings (r1, rn, r2) establish the total loop resistance of each conductor.
Step 2 — Line-neutral cross-connection: Link one end of the line conductor to the opposite end of the neutral conductor at the board. Measure between L and N at each socket. This creates a figure-of-eight path that proves both conductors form a continuous ring.
Step 3 — Line-CPC cross-connection: Repeat the cross-connection between line and CPC. The reading at each socket gives the R1+R2 value used for earth fault loop impedance verification.
Practical Notes
Figure-of-Eight Is Essential
Compare Line and Neutral Readings
Maximum Floor Area per Ring
Frequently Asked Questions
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