Cooker Circuit Design and Diversity: Cable, Breaker and Zs

A domestic cooker circuit rarely has to carry the full nameplate current of the appliance. The IET On-Site Guide allowance for household cooking appliances is the first 10 A of rated current, plus 30% of the remainder, plus 5 A if the cooker control unit has a socket-outlet. That is why a 12 kW cooker rated at 52 A designs out at about 28 A and sits on a 32 A breaker. The diversity sum is the easy part. The cable route, the RCD type, the manufacturer's instructions and the Zs at the connection unit are where cooker circuits actually go wrong, and this guide covers all of them.

How does the cooker diversity calculation work?

Diversity exists because a cooker never runs every element flat out for long enough to heat the cable to its limit. Thermostats cycle, rings get turned down once a pan boils, and induction hobs share power between zones. BS 7671 leaves the method to the designer (Regulation 311.1 requires the maximum demand to be assessed, taking diversity into account), and Appendix A of the IET On-Site Guide supplies the figures most of us use.

For household cooking appliances the allowance is: 10 A, plus 30% of the full-load current above 10 A, plus 5 A if the control unit incorporates a socket-outlet. Rated current is the total appliance wattage divided by 230 V. The On-Site Guide also makes clear that one circuit can feed two or more cooking appliances in the same room, which is the basis for the common built-in oven and hob on a single 32 A circuit.

Here are three worked examples that cover most of what turns up in a kitchen.

The third row is the one that catches people out. Large range cookers with two ovens, a grill and an electric hob can push the design current just past 32 A, and a 40 A device then changes the cable choice and cuts your Zs limit from 1.37 Ω to 1.09 Ω. Run the numbers on the actual appliance rather than assuming every cooker is a 32 A job.

When the manufacturer overrides diversity

Regulation 134.1.1 requires equipment to be installed in accordance with the manufacturer's instructions, and Regulation 510.3 repeats the point. Several induction hob makers specify a dedicated 32 A circuit for the hob alone, and some wide 90 cm hobs quote 40 A. When the instructions say dedicated, the diversity calculation does not let you share the circuit with the oven.

Many induction hobs also have a power-management setting that caps total draw at a chosen figure, often in steps such as 13 A, 16 A, 20 A or 32 A. That setting is useful when the existing supply is limited, but it is configured inside the hob's menu and is easily reset by the customer or the next fitter. If you rely on it in the design, say so on the certificate and tell the customer.

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 catch is that Table 4D5 rates the same cable very differently depending on how it is installed, and kitchen refits use nearly every reference method in the book.

Source: BS 7671:2018+A4:2026, Appendix 4, Table 4D5. Check the column against your own copy before relying on a single figure; ratings assume 30 °C ambient and no grouping.

What that table tells you on site:

• Chased into plaster or clipped to the back of units (Method C): 6 mm² has plenty in hand for 32 A and is fine for 40 A as well.
• In conduit or trunking within an insulated external wall (Method A): 6 mm² is exactly 32 A. That works with no other correction, but any grouping with other circuits or a high ambient tips it under.
• Across a loft under a modern 270 mm top-up (Method 101): 6 mm² is only 27 A. A 32 A circuit needs 10 mm² or a different route.
• In an insulated stud wall where the cable is not touching the plasterboard (Method 103): 6 mm² is 23.5 A. Even 10 mm² only just reaches 32 A. Re-route the cable or fix it against the inner face.

Regulation 523.9 requires the reduced rating where cables are surrounded by thermal insulation, and Table 52.2 gives derating factors for short lengths buried in insulation: 0.88 for 50 mm, 0.78 for 100 mm, 0.63 for 200 mm and 0.5 for 400 mm or more. So a 6 mm² cable rated 47 A clipped direct that passes through 100 mm of insulation for a short length drops to about 36.7 A for that section. Grouping factors from Table 4C1 apply on top, which matters where the cooker cable shares a notch in the joists with the shower and ring final feeds.

Volt drop is seldom the limiting factor because cooker runs are short. 6 mm² T&E is 7.3 mV/A/m. A 32 A design current over 20 m gives 32 × 20 × 7.3 ÷ 1,000 = 4.67 V, about 2% of 230 V, comfortably inside the 5% limit for other uses in Appendix 12. It only bites in a long run to an outbuilding kitchen or a large house with the board at the far end. The general method is in our guide to cable sizing and volt drop.

Type B or Type C?

Type B is the default for a cooker. A Type C is unnecessary unless the manufacturer specifies it or there is a proven inrush problem, and it halves the permitted Zs. For a 32 A device Table 41.3 gives 1.37 Ω for Type B and 0.68 Ω for Type C. Plenty of TN-C-S properties will pass at 0.68 Ω; a TN-S property with a long run often will not. See maximum Zs values for the full table.

Does a cooker circuit need RCD protection, and which type?

Three regulations normally bring a domestic cooker circuit under a 30 mA RCD:

• Regulation 411.3.3 requires additional protection by a 30 mA RCD for socket-outlets rated up to 32 A, so a cooker control unit with a 13 A socket puts the circuit in scope.
• Regulation 522.6.202 requires RCD protection for cables concealed in a wall at a depth of less than 50 mm without earthed mechanical protection, which describes almost every kitchen chase.
• On a TT system the RCD is providing fault protection, so it is not optional at all.

In practice a new cooker circuit in a dwelling goes on a 30 mA RCBO. The type matters more than people think. Induction hobs and many modern ovens contain rectifiers and inverters that can produce pulsating DC fault currents. Regulation 531.3.3 limits Type AC RCDs to fixed equipment known to be free of DC components, so an induction hob should normally be on a Type A device or whatever the manufacturer specifies. If you find an induction hob on a Type AC RCD on an EICR, record it and explain why. If you are deciding between protecting circuits individually or in groups, our guide on RCBO vs RCD split-load boards sets out the trade-offs.

Where should the cooker switch go, and is one required?

BS 7671 requires isolation (Section 462), switching off for mechanical maintenance (Section 464) and functional switching (Section 463), but it does not say every cooker must have a wall-mounted cooker switch. The 45 A double-pole cooker switch, with or without a 13 A socket, is a convention backed by an On-Site Guide recommendation: the cooker control switch should be within 2 m of the appliance, and where two cooking appliances are installed in one room, one switch can serve both provided neither is more than 2 m from it.

Position matters more than the switch itself. It must be reachable in an emergency without leaning across a hob, so not directly above the hob and not hidden behind the built-in oven. For a built-in oven and hob on one circuit, the tidy arrangement is one 45 A double-pole switch on the wall above worktop height, feeding a cooker connection unit for each appliance or a single dual-outlet unit. Connection units behind base units must stay accessible for maintenance, which usually means a cut-out in the back of the adjacent cupboard rather than sealing them in behind the carcass. Our guide to kitchen electrics regulations covers the rest of the room.

Is a cooker circuit notifiable under Part P?

In England, installing a new circuit is notifiable building work under Part P. A new cooker circuit from the consumer unit is therefore notifiable: you self-certify through a competent person scheme or notify building control before starting. The certificate is an Electrical Installation Certificate.

Replacing a cooker on an existing circuit, adding a connection unit or moving the switch along the same wall is not notifiable in England. Kitchens stopped being special locations for Part P when the 2013 changes came in. That work goes on a Minor Electrical Installation Works Certificate. Wales keeps a broader list of notifiable work, so a Welsh job that only adds to an existing kitchen circuit can still be notifiable. Scotland works under the Building (Scotland) Regulations instead. The full picture, with the GOV.UK Approved Document P position, is in our Part P notification guide.

What readings belong on the certificate for a cooker circuit?

Test it like any other radial and record the conductor sizes (6 mm² line and neutral, 2.5 mm² cpc for standard flat cable), reference method, protective device type and rating, RCD type, and the measured R1+R2, insulation resistance, polarity and Zs.

Continuity. For 6 mm²/2.5 mm² twin and earth, 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, IET On-Site Guide Table I1). A 12 m run should read around 0.13 Ω. A reading well above that points to a loose termination in the cooker switch or connection unit, which on a 30 A load is a fire risk long before it is a shock risk. The method is in our R1+R2 continuity guide.

Insulation resistance. Disconnect the appliances first. Elements and electronic controls drag the reading down, and the electronics can be damaged at 500 V. If you have to test with the appliance connected, test with line and neutral joined to earth, record the limitation, and do not record the appliance's leakage as a wiring fault. Table 64 sets 1 MΩ as the minimum, but a new circuit reading under about 2 MΩ deserves a closer look; see the insulation resistance guide.

Zs. Measured Zs at the connection unit must suit the device. Table 41.3 values are at conductor operating temperature, so readings taken cold should be compared with roughly 80% of the table figure. Here is the same 12 m 6/2.5 circuit on two supplies:

That TN-S fail at 40 A is exactly why the range-cooker example above needs a design check, not just a bigger breaker. Options include 10 mm² cable (R1+R2 drops to about 6.44 mΩ/m with a 4 mm² cpc), using an RCD for fault protection where BS 7671 permits that in a TN system, or a shorter route. On a TT system the RCD provides fault protection, and the check is the electrode resistance against Table 41.5 (1,667 Ω for a 30 mA device), backed by a satisfactory RCD test. More on the reading itself in Zs explained.

What are the common cooker circuit observations on an EICR?

The same handful of faults appear over and over. The coding below follows the usual position in Electrical Safety First Best Practice Guide 4 and NICEIC guidance, but the code is always the inspector's judgement for the installation in front of them.

The undersized-cable case deserves emphasis. Kitchen refits sometimes leave a 2.5 mm² spur off the ring feeding a 7 kW induction hob through a cooker switch, or a 2.5 mm² radial re-terminated onto a 32 A device in a new board. The breaker will never protect that cable, and the fault stays hidden until the first big family meal. When you replace a hob, trace what actually feeds it rather than trusting the label on the switch. For the wider coding logic see EICR observation codes C1, C2, C3 and FI.

How Certio helps

Certio produces the EICR, EIC, Minor Works certificate and PAT records on the BS 7671 model forms and turns them into branded PDFs. Photograph the consumer unit and the circuit schedule drafts itself; point the phone camera at a Megger MFT-X1 and the reading goes straight into the schedule of test results, or dictate it while your hands stay on the probes. Suggested observation wording with the regulation reference is there for you to check and confirm. The judgement stays yours; the typing goes. Starter is £4.99 a month, charged when you sign up.

Written by the Certio team. Last checked 30 October 2026.