What size cable for a cooker?
Most home cookers run on a 32 A circuit. Clipped direct to a wall, that needs at least 4 mm² twin and earth. Run through insulation, it needs 6 or 10 mm². Many electricians fit 6 mm² as standard for the headroom.
- V – Voltage
- I – Current
- R – Resistance
- P – Power
Step 1: work out the current with diversity
A cooker rarely has every ring and the oven on full at once, so the On-Site Guide lets you apply diversity to a household cooking appliance:
Ib = first 10 A + 30 % of the rest (+ 5 A if the cooker switch has a socket)
Worked example: a 10 kW cooker on 230 V draws 10000 ÷ 230 = 43.5 A flat out. With diversity: 10 + 0.3 × (43.5 − 10) + 5 for the socket = 25.0 A. The next standard breaker up is 32 A.
A 32 A circuit covers a cooker of up to about 15.3 kW with a socket on the cooker switch, or 19.2 kW without. Bigger range cookers need a 40 A or 45 A circuit: follow the maker's instructions.
Step 2: choose the cable for a 32 A circuit
The cable must carry at least the breaker rating after correction factors. How it's installed makes the biggest difference. Run lengths use the 25.0 A design current from the example:
| How the cable is run | Smallest twin and earth for 32 A | Longest run (voltage drop) | Longer run? Next size up |
|---|---|---|---|
| Clipped direct to a wall or joist | 4 mm² (rated 37 A) | up to 41 m | 6 mm²: up to 62 m |
| Above a plasterboard ceiling, insulation up to 100 mm | 6 mm² (rated 34 A) | up to 62 m | 10 mm²: up to 104 m |
| In an insulated stud wall, touching the inner face | 6 mm² (rated 35 A) | up to 62 m | 10 mm²: up to 104 m |
| In an insulated stud wall, not touching the inner face | 10 mm² (rated 32 A) | up to 104 m | 16 mm²: up to 164 m |
Ovens and hobs on one circuit
A separate built-in oven and hob in the same kitchen are often treated as one cooking appliance: add their ratings, then apply diversity once. A second oven or a range elsewhere should be assessed on its own. Check each appliance's installation instructions, as some specify their own circuit.
Other things to check
- The On-Site Guide recommends a cooker control switch within 2 m of the cooker, not directly above the hob.
- Socket-outlets on the cooker switch need 30 mA RCD protection like any other socket.
- Check the maximum Zs for the breaker: a B32 is 1.37 Ω (Max Zs table).
Assumes 70 °C flat twin and earth (6242Y), 230 V, an MCB or RCBO, 30 °C ambient, the circuit not bunched with others, and no thermal insulation round a clipped cable. Voltage drop limit 5 % (11.5 V), BS 7671 Table 4D5 values. Grouping, heat or insulation can need a bigger cable: check with the cable size calculator. For study and estimating only; the installing electrician must design and certify the circuit.