Understanding Cooker Circuit Diversity

Unlike electric showers or space heaters which represent a continuous maximum load, domestic electric cookers, ovens, and hobs never draw their full nameplate rating indefinitely. A four-ring induction hob and double oven may mathematically sum to 12kW, but you will rarely have all four rings on "Boost" and both ovens running at maximum temperature simultaneously for an extended period. Because ovens are thermostatically controlled, they constantly cycle on and off to maintain heat.

The Standard 10A + 30% Diversity Rule

To prevent over-engineering and installing unnecessarily thick, expensive cables, electrical guidelines (such as the IET On-Site Guide, Appendix A) permit a standardised diversity calculation for domestic cooking appliances.

The mathematical rule to determine the Design Current (Ib) is:

• Take the first 10 Amps of the total raw load.
• Add 30% of the remaining load.
• If the cooker control unit (the red isolator switch on the wall) incorporates a 13A socket outlet, add a flat 5 Amps to the total.

For example, a 10kW cooker draws approximately 43.5 Amps raw at 230V. Applying diversity: 10A + (30% of 33.5A) = 20.05A. Adding 5A for the socket outlet yields a final diversified design current ($I_b$) of 25.05 Amps. This massive reduction allows the use of a standard 32A MCB and a 6.0 mm² cable, rather than needing an industrial 50A supply.

Thermal Insulation and Cable Sizing

While diversity reduces the calculated load, the physical environment of the cable dictates its safe capacity ($I_z$). A 6.0 mm² Twin & Earth cable clipped directly to a joist (Method C) is rated to carry 47 Amps, easily accommodating a 32A cooker circuit. However, if that exact same 6.0 mm² cable is buried deep under loft insulation (Method 103), its capacity plummets to 23.5 Amps. In this scenario, running the oven for Christmas dinner would cause the cable to overheat, mandating an upgrade to a 10.0 mm² cross-section.