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Cable selection: the 7 stepsFrom the load to the cable size. Seven steps. Know them?

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  1. Work out the design current, Ib. Ib = P ÷ V. Also note the installation reference method now, from the question.
  2. Select the protective device rating, In (e.g. B40), so that In ≥ Ib.
  3. Identify the installation method (C, B, A, 100–103…).
  4. Apply any correction factors, e.g. ambient temperature, grouping or thermal insulation.
  5. Calculate the tabulated current, It, that the cable must be able to carry.
  6. Select the cable size from the tables (BS 7671 or the On-Site Guide).
  7. Check the voltage drop.
Ib
Design current of the circuit: the load. "Ib", not "ID".
In
Nominal rating of the protective device in amps, e.g. the "6" on a B6.
Iz
Current-carrying capacity of the cable under normal conditions once correction factors have been applied. The Z here has nothing to do with impedance.
It
Tabulated current-carrying capacity: the value straight from the tables.
Ib ≤ In ≤ Iz
With no correction factors, Iz = It, so you pick a cable whose tabulated rating It ≥ In.

Voltage drop: radial circuits

VD = (mV/A/m × Ib × L) ÷ 1000

÷1000 because mV/A/m is in millivolts

Voltage drop: ring final circuits

VD = (mV/A/m × Ib × L) ÷ 4000

÷4000: the ring is two legs in parallel,
each about half the length to the load,
so the drop is ½ × ½ = ¼ of a radial
(mV/A/m already covers line + neutral)
Max voltage dropSingle-phase 230 VThree-phase 400 V
Lighting: 3%6.9 V12 V
All other circuits (sockets, showers, cookers): 5%11.5 V20 V

The 3% and 5% are measured from the origin of the installation, not from the consumer unit a circuit happens to start at. If the voltage drop is too high, upsize the cable. You never downsize.

On-Site Guide · Appendix F: voltage drop explanation, radial formula and ring formula · p.184 Highlight the formula and the 3% / 5% paragraph

In real life
On most jobs a designer, your boss or the company has already done these calculations, and you install to the design. You still need to know the method for the exam. Higher-level design qualifications take it much further.

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