R1+R2 Calculator for Ring Final Circuits
Calculate expected R1+R2, check ring continuity readings, and estimate the ring cable length.
Ratio: line/CPC = 2.5/1.5 = 1.67
End-to-End Readings
Line-to-CPC Cross-Connection Reading
R1+R2 formula for a ring final circuit
For a ring final circuit, the expected maximum R1+R2 reading is calculated as (r1 + r2) / 4 after taking end-to-end resistance readings. Enter r1, rn, r2, and the measured R1+R2 to check whether the measured value is within the expected guidance limit.
End-to-End Test (r1, rn, r2)
With the ring safely isolated and separated at the distribution board, measure the resistance of each conductor from one end to the other. Line (r1) and neutral (rn) should be substantially similar. The CPC (r2) will be higher where a reduced CPC is used.
Line-to-CPC Cross-Connection Test
Cross-connect line and CPC conductors at the distribution board, then measure R1+R2 at socket-outlets. The maximum value should not exceed (r1 + r2) / 4 plus an allowance for measurement tolerance. Higher readings can indicate a spur, loose termination, or high resistance joint.
Polarity Pattern Check
Line-neutral cross-connection readings are used as part of the polarity and conductor pattern checks. They are separate from the line-to-CPC R1+R2 calculation shown above.
BS 7671 Edition Note
BS 7671:2018+A4:2026 has been published. The IET states that Amendment 3:2024 remains valid until 15 October 2026 during the transition period.
Open Circuit
Open circuit on one or more conductors. Often caused by loose terminations, incorrect separation, or cable damage.
Unbalanced Ring
Difference between r1 and rn exceeds the guidance tolerance. Check interconnections and conductor continuity.
Spur or High Resistance Joint
Elevated R1+R2 readings at socket-outlets can point to a spur, loose termination, or high resistance connection.
Cable Selection Mismatch
r2 should broadly follow the selected line/CPC conductor ratio. Unexpected values can mean the cable size or readings need checking.
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