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The eleven steps of cable selection: design current and InA 9 kW shower. What's the design current?

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The eleven steps of cable selection

From a kilowatt rating to a verified cable, CPC and protective device. Most people stop around step 7, at volt drop — which is halfway.

StepFindWhere
1Ib — design currentP ÷ V
2In — device ratingTable 41.3 / B6 / A6
3Reference methodBS 7671 Appendix 4, Table 4A2
4Correction factorsAppendix 4 — Ca, Cg, Ci, Cf
5It needed — required capacityIn ÷ the factors
6It — tabulated capacity → cable sizeTable 4D5 for twin and earth
7Volt dropSame table — mV/A/m
8Expected R1+R2GN3 Appendix B / OSG Appendix I
9Zs, checked against the maximumZe + (R1+R2)
10IpfU0 × Cmin ÷ Zs
11CPC size — adiabaticBS 7671 Chapter 54
The worked example that follows: a 9 kW shower, 25 m of twin and earth, ambient 35 °C, run over the joists in a ceiling under thermal insulation, protected by an RCBO. Ze given as 0.21 Ω.
From a kilowatt rating to a verified cable, cpc and device 1Ib — design current P ÷ V 2Iₙ — device rating Table 41.3 / B6 3reference method Table 4A2 4correction factors Cₐ Cg Cᵢ Cf 5It needed — required capacity Iₙ ÷ the factors 6cable size Table 4D5 7volt drop mV/A/m most people stop here — which is halfway 8expected R₁ + R₂ GN3 App B 9Zₛ, checked against the maximum 10Ipf — Uₒ × Cmin ÷ Zₛ 11cpc size — adiabatic BS 7671 Chapter 54 the worked example 9 kW shower · 25 m twin and earth ambient 35 °C · in insulation Ib 39.13 A → 40 A device → 10 mm² → volt drop 4.3 V → Zₛ 0.40 Ω → cpc 1.5 mm² minimum
Steps 1 to 7 size the cable. Steps 8 to 11 prove it will actually disconnect in time and that the cpc will survive the fault — which is the part that makes it a design rather than a guess.

Step 1 — design current

Ib = P ÷ V = 9000 ÷ 230 = 39.13 A

Step 2 — In, and whether diversity applies

In = 40 A Type B RCBO

Check Table A2 before assuming diversity. For an instantaneous water heater or shower, the entry is either 100% of the largest appliance or no diversity allowed — so either way, the full 39.13 A stands. Nothing to reduce.

A cooker would be different. That is where the Table A1/A2 arithmetic earlier comes in. A shower does not get it.

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