Ring final circuits and loadingWhy is the ring volt drop worked out over 4000?

A ring final circuit (on a 32 A device, 2.5 mm² T&E, up to 100 m²) is really two legs of cable from the same breaker. That lets the load spread across both legs, which is why you shouldn't plug a kettle and a toaster into one double socket.

That sharing is why 2.5 mm² is allowed on a 32 A device. The cable must have a current-carrying capacity of at least 20 A, and the ring must be designed so that no part of it carries more than that. If one leg breaks, every socket still works, but you now have a 2.5 mm² radial on a 32 A device. Nothing shows it until step 1 of the ring test.

• At the consumer unit, both line conductors go in the same terminal of the one breaker (or RCBO), both neutrals in the same neutral terminal, and both CPCs in the same earth terminal.
• The same happens at every socket. It's still parallel wiring: every socket gets 230 V.
• The load current splits between the two legs, and each leg is only about half the length to the load, so the drop is ½ × ½ = ¼ of a radial of the same length. That is why ring volt drop divides by 4000 instead of 1000 (mV/A/m already covers line and neutral).
• A radial has one leg and ends at the last point. Many kitchens now use a 32 A radial in 4 mm² instead of a ring.

On-Site Guide · Appendix H · Table H2.1 socket-outlet circuits · p.218 BS 7671 · Appendix 15 · Figure 15A ring final circuit arrangements (and the radial figure on the next page) · p.583 Worth highlighting: ring, junction box, FCU spurs, fixed equipment

Electrician's Guide to the Building Regs · Figure 4.2.2 radial final circuit arrangements · p.78 Shows the radial arrangements