Free notes
Worked example: 8.5 kW showerWhat cable does an 8.5 kilowatt shower need?
Free from the notes: Sparky’s video on it, the section word for word as members see it, and a PDF to keep.
The question at the end: What cable does an 8.5 kilowatt shower need? The answer is in the notes below.
The notes
Exactly as they are on the site.
The question
An 8.5 kW electric shower is to be installed in a domestic property. • Wired in PVC/PVC flat twin and earth • Reference method 102 • Cable run 20 m • Protective device: BS EN 60898 circuit breaker • No rating factors Select a suitable cable and protective device. Show all working.
Working
1) Design current Ib = P ÷ V = 8500 ÷ 230 = 36.96 A (keep the decimals: don't round as you go) 2) Protective device (domestic, so Type B) Next standard size ≥ 36.96 A → In = 40 A, Type B, BS EN 60898 Check: Ib ≤ In → 36.96 ≤ 40 ✓ 3–4) Method 102, no correction factors → Iz = It, and it must be ≥ In = 40 A 5–6) On-Site Guide Table F6 (70 °C flat T&E), method 102 column: 6 mm² → 35 A ✗ (less than 40) 10 mm² → 47 A ✓ → select 10 mm² twin and earth 7) Voltage drop (not lighting, so max 5% = 11.5 V) Table F6, 10 mm² → 4.4 mV/A/m VD = (4.4 × 36.96 × 20) ÷ 1000 = 3.25 V 3.25 V ≤ 11.5 V ✓ Answer: 10 mm² T&E on a 40 A Type B circuit breaker
On-Site Guide · Appendix B · Table B6: circuit-breaker ratings (BS EN 60898 / RCBOs) · p.159 Electrician's Guide to the Building Regs · Table 4.1.2a: standard Type B final circuits — cable, device, maximum length, maximum measured loop, method 101 size · p.75
On-Site Guide · Appendix F · Tables F4–F6: current-carrying capacity and voltage drop · from p.187 On-Site Guide · Table F6: 70 °C thermoplastic flat twin and earth (It and mV/A/m) · p.193
Table F4 is single-core cable and Table F5 is multicore round cable. Neither is T&E. Table F6 says flat thermoplastic sheathed cable, and that's the twin and earth table. It gives you both It and mV/A/m.
The Electrician's Guide has a standard final circuits table for Type B domestic circuits (Table 4.1.2a, p.75). For a shower on a 40 A device it gives 10 mm² — the same answer this example works out — along with a maximum cable length, a maximum measured loop impedance, and the bigger size you'd need in reference method 101 (insulation). Use it to check a design, never as a replacement for the seven steps: the exam wants the method, and the table only covers standard circuits with no rating factors.
Free section from Sparky Study Notes. The full Level 3 notes, 1,463 practice questions and the video lessons: sparkystudynotes.co.uk
Test yourself on it
The tests are in the members’ area. Get one wrong and it shows you why, and where to revise.
Get new notes and exam tips from Sparky
About one email a fortnight. Unsubscribe any time. Privacy noticeThat was one section. There are 583.
Sparky Study Notes is the whole Level 3 syllabus written up like this: the explanation, the worked example, and where it is in your book. With a video lesson for every part and a practice paper for every exam.
- 583sections of written notes
- 35video lessons
- 1,463practice questions
- 146diagrams, redrawn
- 18th Edition
- Building Regs
- PAT
- Fundamental Inspection & Testing
- Initial Verification
- Periodic Inspection & Testing