Cable calculation, step 4: diversityDoes a cooker really draw 41.7 amps?

41.7 A assumes everything is on full at once, and nobody cooks like that. You turn the hob down once the pan boils. The oven sits at 180–220 °C, not flat out. And thermostats in the oven and hob keep clicking the elements off and on. So we can apply diversity.

On-Site Guide · Appendix A · maximum demand and diversity · p.149 On-Site Guide · Table A1 · current demand to be assumed (incl. household cooking appliances) · p.150 On-Site Guide · Table A2 · diversity by type of premises · pp.151–152 Cooking appliances row, "individual household" column

Table A2's wording is famously clumsy: 10 A + 30% of the full load of connected cooking appliances in excess of 10 A + 5 A if a socket-outlet is incorporated in the control unit. Table A1 says it more clearly: the first 10 A, plus 30% of the rest, plus 5 A for a socket. (Lighting is easier: 66% in a household. Cookers are the most complicated diversity sum.)

Ib (full load)                   = 41.7 A
1) Take off the first 10 A:      41.7 − 10     = 31.7 A
2) 30% of the remainder:         31.7 × 0.3    = 9.51 A
3) Put the 10 A back:            9.51 + 10     = 19.51 A
4) Add 5 A for the socket:       19.51 + 5     = 24.51 A

New Ib (after diversity) = 24.5 A   (almost half!)

A smaller Ib saves time, effort and money: smaller cable is cheaper and easier to work with. But it's still your judgement. In an HMO, several tenants might cook at once. A family of five with a keen cake baker might have the oven on all day. Speak to the client. You could work it out with and without diversity and go somewhere in the middle. We work it both ways so you can see the difference (see section 14).