Calculate cable size, MCB rating, and diversity for cooker circuit installations to BS 7671 using the IET On-Site Guide formula
Calculate cable size, MCB rating, and diversity for cooker circuit installations per BS 7671 and the IET On-Site Guide
The IET On-Site Guide provides a standard formula for calculating the diversified demand of domestic cooking appliances. This recognises that all heating elements rarely operate at full power simultaneously — thermostats cycle on and off to maintain temperature, reducing the average current draw over time.
Diversified demand =
First 10A of full load current (100%)
+ 30% of the remainder
+ 5A if CCU has a 13A socket outlet
This formula applies to domestic installations only. For commercial kitchens, hotels, and restaurants, different rules apply (100% of largest appliance, 80% of second largest, 60% of remainder per IET Guidance Note 1, Table H2).
| Step | Calculation | Result |
|---|---|---|
| Full load current | 12,000W / 230V | 52.2A |
| First 10A at 100% | 10A | 10.0A |
| 30% of remainder | 0.3 x (52.2 - 10) = 0.3 x 42.2 | 12.7A |
| Socket allowance | CCU with 13A socket | +5.0A |
| Total diversified demand | 10 + 12.7 + 5 | 27.7A |
A 12kW cooker with 52.2A full load diversifies to just 27.7A — easily within a 32A MCB and 6mm² cable capacity (47A clipped direct). This is why 32A/6mm² has traditionally been the standard cooker circuit.
On peak usage occasions, all elements may operate from cold simultaneously. A 15kW cooker drawing 65A full load diversifies to ~31.5A — marginally within a 32A MCB. The thermal characteristics of a Type B MCB allow short-duration moderate overloads, but many professionals now recommend a 45A circuit with 10mm² cable for large range cookers to provide operational headroom.
Cable selection depends on the diversified demand (not full load), cable length, and installation method. The cable must satisfy both current-carrying capacity and voltage drop requirements per BS 7671.
Traditional standard, adequate when clipped direct. But capacity drops to just 23A if cable passes through thermal insulation.
Increasingly the professional standard for new installations. 32A capacity even when fully surrounded by insulation.
| Cable Size (T&E) | Clipped Direct | In Conduit | In Insulation |
|---|---|---|---|
| 4mm² | 37A | 30A | 18.5A |
| 6mm² | 47A | 38A | 23.5A |
| 10mm² | 64A | 52A | 32A |
| 16mm² | 85A | 69A | 42.5A |
Values from BS 7671 Table 4D5 for flat 70°C PVC twin & earth copper cable. Derating factors for grouping, ambient temperature, and insulation contact must also be applied.
Modern homes with high thermal insulation can reduce a 6mm² cable's capacity from 47A to just 23.5A (Method 103 — entirely surrounded by insulation for >0.5m). At 23.5A, even a diversified demand of 27A exceeds the cable's rating. Always check the cable route for insulation contact — 10mm² is the safer choice for new installations.
A cooker circuit is a dedicated radial circuit from the consumer unit. BS 7671 and the IET On-Site Guide set specific requirements for protection, isolation, and cooker control unit positioning.
Type B MCB rated at or above the diversified demand. Standard ratings: 32A (most domestic cookers) or 45A (large range cookers or when extra headroom is needed).
Mandatory if CCU has a 13A socket or if cable is concealed in a wall at <50mm depth. 30mA Type A RCBO recommended. Type F required for some induction hobs.
Installing a new cooker circuit is notifiable work under Part P of the Building Regulations (England & Wales). The work must be carried out by a member of a Competent Person Scheme (NICEIC, NAPIT, etc.) or notified to Local Authority Building Control. An Electrical Installation Certificate (EIC) must be issued on completion.
Induction hobs use high-frequency power electronics that create unique challenges for circuit design, particularly around earth leakage and RCD selection.
Induction hobs contain EMI filters with capacitors connected between live/neutral and earth. This produces a steady-state protective conductor current (earth leakage) of up to 10mA per hob. When combined with other circuits on a shared RCD, this can cause nuisance tripping.
Detects AC and pulsating DC fault currents. Minimum requirement for most induction hobs. Suitable for standard installations.
Also detects composite fault currents from inverter-driven loads. Required by some premium induction hob manufacturers. Check installation manual.
Best practice: install a dedicated RCBO so the hob's earth leakage doesn't affect other circuits. Prevents nuisance tripping of the entire board.
BS 7671 permits a single CCU to supply multiple cooking appliances in the same room. Apply diversity to the combined total load. For example, a 7.2kW induction hob + 3kW oven = 10.2kW (44.3A full load) diversifies to approximately 20.3A — well within a 32A circuit.
However, check the manufacturer's installation manual. Some high-end induction hobs require a dedicated circuit. For premium installations, two separate radial circuits provide better reliability and independent isolation.
Most domestic cookers (up to 15kW) can be served by a 6mm² twin & earth cable with a 32A MCB when clipped direct. However, if the cable passes through thermal insulation, or you want extra headroom, 10mm² cable with a 45A MCB is the professional recommendation. Always apply the IET diversity formula to determine the actual demand.
The IET On-Site Guide diversity formula for domestic cookers takes the first 10A of the full load current at 100%, adds 30% of the remainder, and adds 5A if the cooker control unit has a 13A socket. This recognises that thermostats cycle on and off, so the average current is much lower than the peak rating. A 12kW cooker (52.2A) diversifies to approximately 27.7A.
A 32A MCB is sufficient for most domestic cookers after applying diversity. Even a 15kW cooker diversifies to ~31.5A. However, a 45A MCB with 10mm² cable is increasingly recommended for large range cookers to provide margin for peak “all elements from cold” scenarios and to future-proof the installation.
Yes, BS 7671 permits a single cooker control unit to serve multiple cooking appliances in the same room. Apply diversity to the combined total. However, check the induction hob manufacturer's instructions — some require a dedicated circuit. For a separate oven rated under 3kW, it can alternatively use a standard 13A plug on a ring circuit.
Yes. Installing a new cooker circuit (or modifying an existing one in a kitchen, which is not a Special Location but involves new circuit work) is notifiable under Part P. The work must be done by a Competent Person Scheme member or notified to Building Control. An Electrical Installation Certificate is required.
A typical 7.2kW induction hob draws 31.3A at 230V, requiring a minimum 32A MCB. After diversity (if combined with an oven on the same circuit), the demand is lower. Use a Type A or Type F 30mA RCBO to handle the DC earth leakage from the inverter electronics and prevent nuisance tripping of shared RCDs.
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