Cooker and shower on the same circuit

Question

Hello everyone, 
 A few months back, a qualified electrician told me that a cooker and a shower can both be put on the same circuit; that doesn't sound right, surely loads using such a large amount of power must be on their own individual circuits? 
 I haven't been able to ask a question about this until now because I had difficulty logging into my IET account and had to get a new username and password for it. 
 Thank you, 
 Dasa

Sparkingchip · Answer

The IET ELECTRICAL INSTALLATION DESIGN GUIDE Calculations for Electricians and Designers advises in 3.1: 
 &rdquo;The demand of a circuit or an installation is the current taken by the circuit or installation over a period of time, say 30 minutes&rdquo; 
 and references BS7671 132.3 and 311.1. 
 So: 
 132.3 Nature of demand 
 The number and type of circuits required for lighting, heating, power, control, signalling, communications and information technology, etc shall be determined from knowledge of: 
 (I) location of points of power demand 
 (ii) loads to be expected on various circuits 
 (iii) daily and yearly variations demand 
 (iv) any special conditions, such as harmonics 
 (v) requirements for control, signalling, communication and information technology, etc. 
 (vi) anticipated future demand, if specified. 
 
 311.1 Maximum Demand and Diversity 
 For economic and reliable design of an installation within thermal limits and admissible voltage drop, the maximum demand shall be determined. In determining the maximum demand of an installation or part thereof, diversity may be taken into account. 
 
 So given that guidance, consider this. 
 Last night I pulled off the motorway and stopped at a M&S shop and bought food for our evening meal, I got home and turned on the oven then having started to cook the food I went and had a shower leaving it cooking, then I returned to finish cooking the meal whilst my wife had a shower, the main course needed to cook for 35 minutes, but I left the oven on for the melt in the middle chocolate puddings, so the oven was on for around 45 minutes plus the veg needed cooking as well. 
 Therefore if we had a 6 mm circuit protected by a 40 amp MCB supplying both an electric shower and the cooker it would not comply with those regulations, in particular 132.3 (i), (ii) and (iii) because the total demand should be anticipated by the designer, also 311.1 because the thermal limits of the equipment and cable will be exceeded, quite possibly the voltage drop as well. 
 Overload should be prevented by design and the circuit protection is not there to compensate for incorrect design.

Legh Richardson · Answer

If you design a circuit with a protective device the protection is there to disconnect either where there is a fault or/and overload condition. 
 I was under the belief that long term expected overloads in domestic appliances are not to be encouraged as ebee has stated designing a circuit where the load current exceeds the rating of the device will cause the device to disconnect, the greater the difference between the protective device rating and the full load current will shorten the length of time that the circuit stays operative. The device is now generating heat which will have a heating effect on surrounding protective devices particularly in small domestic consumer units. This term is known as running hot. 
 Lets take a simplistic example 
 A final circuit protected by a B32A MCB that is used to supply a 10kW cooker, connecting a smallish 8.5kW shower will add a 36.5 A loading to a 20A FLC including diversity for the effects of the rheostats. 
 The B32A breaker will disconnect in approximately 200secs with 56.5A FLC. Only a clown would attempt to reconnect the circuit as the protective device is now warm and will trip even earlier. 
 Now we have to consider the size of the cable conductors supplying this setup. If the circuit was designed for the cooker then the cable size is likely to be 6.0mm&sup2;; if designed for a the shower and the cooker is an add-on then the cable may be 10.0mm&sup2; but with the prevailant design attitude shown here it is more likely to be 6.0mm&sup2;. 
 There are various installation method options here. Lets assume method C with a 6.0mm2 Best case here is 46A. 
 So now the cable will begin to get warm. probably not too serious if out in the open but will be a heat generator in the consumer unit.

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