This calculator uses the standard BS 7671 20°C copper conductor resistance values (IET On-Site Guide, Table I1) to cross-check and fill in expected ring final or radial circuit results — Zs, Ze, R1, R2, R1+R2, end-to-end conductor readings, and cable length — from whichever readings you already have. For training and estimation only; always verify against on-site measurements.
Circuit & Conductor Setup
Ring final: end-to-end readings taken around the whole ring, crossed over at the board.
Auto-set from standard T&E pairing
Expected Results
Values you entered are tagged Measured; everything else is derived and tagged Expected.
Estimates only, based on 20°C copper resistance values with no correction for conductor operating temperature, cable grouping, or ambient temperature. Always confirm against on-site test results and follow IET Guidance Note 3 when completing a formal EICR or Electrical Installation Certificate.
These tools are quick guides for a ballpark figure. They are not certified design or test software, and a result on this page is not a design, a certificate, or a substitute for the current edition of BS 7671. Check the working and cross-reference it before you apply it to a real installation. Thanet Electrical Ltd does not accept liability for loss or cost arising from reliance on these figures, except where the law does not allow that liability to be excluded, including death or personal injury caused by negligence.
Frequently asked questions
What is R1+R2 on a ring final or radial circuit?
R1+R2 is the combined resistance of the line conductor (R1) and circuit protective conductor (R2) at the point furthest from the consumer unit. On a ring final circuit it equals one quarter of the total end-to-end (r1+r2) loop resistance measured with the ring crossed over at the board. On a radial circuit R1 and R2 are simply the one-way end-to-end resistances of the line and CPC.
Can I enter just one or two readings instead of a full set?
Yes. Enter whatever you already know — for example just Zs and Ze, or just a cable length, or just a Line-Line reading — and the calculator derives every other expected value it can from BS 7671 20°C copper resistance values and the conductor sizes you select. It also flags any readings that don’t agree with each other.
What is the difference between a ring final and a radial circuit here?
On a ring final circuit, the end-to-end conductor resistance (Line-Line, Neutral-Neutral, Earth-Earth) is four times the R1, R2 figure measured at the furthest socket, because current can flow two ways around the ring. On a radial circuit there is only one path, so R1 and R2 equal the full end-to-end resistance. Select the correct circuit type so the calculator applies the right relationship.
Why does Twin and Earth cable auto-select the CPC size?
UK Twin & Earth (T&E) cable is manufactured with standard factory-paired conductor sizes — for example 2.5mm² line with a 1.5mm² CPC. Selecting “Singles / separate CPC” allows any CPC size to be entered independently, as used with separate single-core conductors.