A domestic cooker circuit rarely needs to carry the full nameplate current of the appliance. The IET On-Site Guide diversity rule for a single household cooking appliance is the first 10 A of rated current, plus 30 % of the remainder, plus 5 A if the cooker control unit includes a socket-outlet. That is why a 12 kW electric cooker rated at 52 A sits comfortably on a 32 A breaker and 6 mm² cable. This guide walks through the calculation, the cable and protective device choices, isolation, the Part P position, and what to record on the certificate.
- The diversity formula from the On-Site Guide (Table A1) is 10 A + 30 % of the remaining current + 5 A for a socket in the control unit.
- Most ovens and hobs combined land between 25 A and 32 A after diversity, which is why 32 A on 6 mm² is the standard domestic circuit.
- 6 mm² flat twin and earth is 47 A clipped direct but drops to 34 A in an insulated wall and 26 A under thick loft insulation, so check Table 4D5 against the actual route.
- Manufacturer's instructions for induction hobs override the general rule; if they call for a dedicated 32 A supply, give them one.
- A new cooker circuit is notifiable in England and needs an EIC; a replacement appliance or an added connection unit on an existing circuit is a Minor Works job.
- Maximum measured Zs for a 32 A Type B device is 1.37 Ω; for Type C it is 0.68 Ω.
How does the cooker diversity calculation work?
Diversity exists because a cooker never runs every element at full power at the same time for long enough to heat the cable to its limit. Thermostats cycle, rings are turned down once a pan boils, and the grill and oven are rarely both on for an hour. BS 7671 leaves the method to the designer under Regulation 311.1, and Appendix A of the IET On-Site Guide provides the figures most of us use.
For household cooking appliances the guide states: the first 10 A of the rated current, plus 30 % of the remaining current, plus 5 A if the cooker control unit incorporates a socket-outlet. Rated current is total appliance watts divided by 230 V.
Worked example. A freestanding cooker is rated 11.5 kW. That is 11,500 ÷ 230 = 50 A. Apply diversity: 10 A, plus 30 % of 40 A (12 A), plus 5 A for the socket in the control unit. Design current Ib = 27 A. A 32 A protective device satisfies Ib ≤ In, and 6 mm² cable clipped direct, with Iz of 47 A, satisfies In ≤ Iz with room to spare.
Second example, the one that causes arguments. A built-in single oven at 3.2 kW and a four-zone induction hob at 7.4 kW total 10.6 kW, or 46 A. Diversity gives 10 + 30 % of 36 A (10.8 A) = 20.8 A, plus 5 A if there is a socket, so 25.8 A. A 32 A circuit covers both appliances. The On-Site Guide is explicit that one circuit may supply two or more cooking appliances in the same room, and that is the basis for the common oven-plus-hob arrangement on a single 32 A circuit.
The catch is the manufacturer. Regulation 134.1.1 requires that installations are erected in accordance with the manufacturer's instructions, and Regulation 510.3 repeats the point for equipment. Several induction hob makers specify a dedicated 32 A circuit for the hob alone, and some larger 9 kW hobs quote a 40 A or even 45 A supply. When the instructions say dedicated, the diversity calculation does not let you share. Follow the instructions, and if the customer's kitchen fitter has already run one 6 mm² for both appliances, that conversation needs to happen before the worktop goes on.
What cable size and breaker should a cooker circuit use?
The starting point for most kitchens is 6 mm² flat twin and earth on a 32 A Type B MCB or RCBO. The table below gives the current-carrying capacity of common cable sizes from Table 4D5 of BS 7671 for the reference methods that actually occur in houses. Check the current edition of the table before relying on any single figure, because Appendix 4 has been revised more than once.
| Cable (flat T&E) | Method C: clipped direct | Method A: in conduit in insulated wall | Method 100: ceiling, insulation ≤ 100 mm | Method 101: ceiling, insulation > 100 mm |
|---|---|---|---|---|
| 4 mm² | 37 A | 26 A | 27 A | 20 A |
| 6 mm² | 47 A | 34 A | 34 A | 26 A |
| 10 mm² | 64 A | 46 A | 45 A | 35 A |
Two conclusions fall out of that table. First, 6 mm² on a 32 A breaker is fine clipped direct, in a stud wall with the cable touching the plasterboard, and in the common chase-in-plaster method, all of which give Iz at or above 34 A. Second, if the cable runs across a loft under 270 mm of mineral wool for any real distance, 6 mm² is only good for 26 A and the circuit should be 10 mm² or the route should change. Regulation 523.9 requires the reduced rating where cables are surrounded by thermal insulation, and Table 52.2 gives the derating factors for shorter lengths in insulation.
Volt drop is seldom the limiting factor on a cooker circuit because the runs are short, but it is worth a moment. 6 mm² T&E is 7.3 mV/A/m from Table 4D5. A 32 A design current over a 20 m run gives 32 × 20 × 7.3 = 4.67 V, or 2 % of 230 V, well inside the 5 % limit for power in Appendix 12. It only becomes a problem in a long outbuilding or a large house with the board at the far end. Our guide on Cable Sizing and Volt Drop Basics: Ib, In, Iz and Table 4D5 covers the general method.
On the breaker: Type B is the default for a cooker. A Type C is unnecessary unless the manufacturer specifies it or there is a real nuisance-tripping problem, and moving to Type C halves the maximum permitted Zs to 0.68 Ω, which many TN-C-S properties will pass but plenty of TN-S ones with long runs will not. Whichever type you use, check the measured Zs against the corrected values in Table 41.3; see Maximum Zs Values BS 7671: Tables, 80% Rule and Ze.
Does the cooker circuit need an RCD? Regulation 411.3.3 requires 30 mA RCD protection for socket-outlets rated up to 32 A, so a cooker control unit with a socket brings the circuit into scope. Regulation 522.6.202 requires RCD protection for cables concealed in a wall at less than 50 mm depth without earthed mechanical protection, which describes almost every kitchen chase. On a TT system the RCD is doing the fault protection anyway. In practice a new cooker circuit in a domestic property is going to be on a 30 mA RCBO or a shared RCD, and a modern consumer unit change should put it there.
Where should the cooker switch go and does it need one?
BS 7671 requires isolation (Section 462) and switching for mechanical maintenance and functional switching (Sections 464 and 463), but it does not say a cooker must have a wall-mounted cooker switch. The 45 A double-pole cooker switch, or the cooker control unit with a 13 A socket, is a convention that the On-Site Guide supports with a recommendation: a cooker control switch should be within 2 m of the appliance, and where two cooking appliances are installed in one room, one switch may serve both if neither is more than 2 m from it.
The important practical detail is position. The switch must be reachable in an emergency without leaning across a hob. Kitchen designers love to hide the switch behind the built-in oven or in a base unit, which defeats the purpose. A switch inside a cupboard is acceptable if it is accessible, but a switch behind the appliance it controls is not a means of isolation anyone can use.
For a built-in oven and hob on one circuit the neat arrangement is one 45 A double-pole switch on the wall, feeding two cooker connection units, one for each appliance, or a single dual cooker connection unit. Under-worktop connection units must be accessible for maintenance, which usually means a cut-out in the back of the adjacent base unit rather than sealing them behind the carcass.
Are cooker circuits notifiable under Part P?
In England, Schedule 4 of the Building Regulations 2010 (as amended in 2013) lists installing a new circuit as notifiable work. A new cooker circuit from the consumer unit is therefore notifiable, and the electrician either self-certifies through a competent person scheme (NICEIC, NAPIT, and so on) or notifies building control before starting. The correct certificate is an Electrical Installation Certificate, covered in Electrical Installation Certificate (EIC) Explained.
Replacing a cooker on an existing circuit, or moving the cooker connection unit along the same wall, is not notifiable in England and is recorded on a Minor Electrical Installation Works Certificate; see Minor Works Certificate: When to Use It (and When Not To). Adding a new outlet to an existing circuit is also non-notifiable unless it is in a special location, and a kitchen is no longer classed as one since the 2013 amendment removed kitchens from the Schedule. Wales retains a broader definition of notifiable work under its own version of Part P, so a Welsh job that adds to an existing kitchen circuit can still be notifiable. Scotland works under the Building (Scotland) Regulations rather than Part P. Our Part P guide at Part P notification: when and how electricians notify building work sets out the differences.
What tests and readings belong on the certificate?
The cooker circuit is tested like any other radial. Record the cable csa (6 mm² live, 2.5 mm² cpc for standard flat cable), reference method, protective device type and rating, and the measured R1+R2, insulation resistance, polarity and Zs. For 6 mm²/2.5 mm² twin and earth, the combined R1+R2 is about 10.49 mΩ per metre at 20 °C (3.08 for the 6 mm² line plus 7.41 for the 2.5 mm² cpc, from Table I1 of the On-Site Guide), so a 15 m run should read around 0.16 Ω. A measured value far above that points to a loose termination in the cooker switch or connection unit, which on a 32 A load is a fire risk before it is a shock risk.
Insulation resistance on a cooker circuit is usually tested with the appliance disconnected, because ovens and hobs with electronic controls and heating elements pull the reading down. If you must test with the appliance connected, test between live conductors joined together and earth at 500 V, note the limitation, and do not record a low reading that belongs to the appliance as a wiring fault. The minimum acceptable value is 1 MΩ, but anything under about 2 MΩ on a new circuit needs an explanation.
Zs at the cooker connection unit for a 32 A Type B device must not exceed 1.37 Ω. If the property is TT, the RCD provides fault protection and the relevant check is the RCD trip time and the 50 V touch-voltage limit against the earth electrode resistance.
What are the common cooker circuit observations on an EICR?
Inspectors see the same handful of faults repeatedly. The table gives the usual coding position, drawing on the Electrical Safety First Best Practice Guide 4 and NICEIC guidance, though the code is always the inspector's judgement for that installation.
| Observation | Typical code | Why |
|---|---|---|
| Cooker circuit has no local means of isolation | C3 | Improvement recommended; not a danger in itself |
| Cooker switch positioned directly above hob | C3 | Poor practice, but still operable |
| 2.5 mm² cable feeding a 32 A cooker circuit | C2 | Overload risk; cable rating below device rating |
| Heat damage or scorching at cooker connection unit | C2 | Evidence of overheating, potentially dangerous |
| Cooker circuit in TT system without RCD protection | C2 | Fault protection cannot be assured |
| Socket in cooker control unit without 30 mA RCD | C3 | Does not comply with 411.3.3 for a new install; usual coding for existing |
| Exposed live parts at connection unit with cover missing | C1 | Danger present, immediate action |
The undersized-cable case deserves emphasis. Kitchen refits sometimes leave a 2.5 mm² spur from the ring feeding a 7 kW induction hob through a cooker switch. The 32 A breaker in the board will never protect that cable, and the fault is invisible until the first Christmas dinner. When you replace a hob, look at what is actually feeding it, not just the label on the switch. For the wider coding logic see EICR observation codes explained: C1, C2, C3 and FI.
How Certio helps
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