Worked example: a 10 kW cookerA 10 kilowatt cooker, start to finish. Can you size it?

Free from the notes: the section word for word as members see it, and a PDF to keep.

The notes
Get the full notes

The notes

Exactly as they are on the site.

Download these notesPDF · free to keep

Build this one from scratch — it ties this whole part together.

A 10 kW load on a 230 V TN-S system

1 · Design current
Ib = P ÷ V
Ib = 10 000 ÷ 230
Ib = 43.48 A   (the full load current — everything on at once)

2 · Protective device
The rule:  Ib ≤ In
The next size up from 43.48 A is In = 50 A, type B circuit breaker

3 · Disconnection time
Final circuit over 32 A supplying only fixed equipment
→ Reg 411.3.2.3: up to 5 seconds (TN)
(If the cooker control unit has a socket-outlet, it's a ≤ 63 A
circuit with a socket → Table 41.1, 0.4 s.)

4 · Maximum Zs
Table 41.3, type B, 50 A (one value for both times, Reg 411.4.202)
0.87 Ω tabulated (circuit in use)

5 · What you should measure on a new circuit
0.87 × 0.8 = 0.70 Ω   (or read it straight from OSG Table B6)

6 · Fault current needed to trip it
Type B = 5 × In (worst case)
50 × 5 = 250 A

✓If the Zs of that purple fault path in service does not exceed 0.87 Ω (0.70 Ω measured on the new, cold circuit), the breaker will operate inside the permitted disconnection time, and ADS is achieved. That's the entire chain, in one sentence.

A reality note on Ib. 43.48 A assumes every ring, the grill and the oven all running flat out at once — a Christmas dinner, not a Tuesday. In real design you'd apply diversity (maximum demand), which is why the On-Site Guide's Appendix A will happily let you put a 15 kW domestic cooker on a 32 A breaker. That's a later module.

Ib ≤ In · device rating must be at least the design current 411.3.2.3 · 5 s (TN) · or Table 41.1, 0.4 s, with a socket on the control unit Table 41.3 · 0.87 Ω On-Site Guide · Appendix A · diversity

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 access

Get new notes and exam tips from Sparky

About one email a fortnight. Unsubscribe any time. Privacy notice

That 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

Get access — £100 12 months. One payment. See what’s inside