Cooker and shower on the same circuit

But that's a non-sequitur. Things burn down all the time even where the load, cable and OCPD are all correctly rated, e.g. due to poor terminations. You'd have to show me an example of something burning down specifically due to a load being applied that is greater than the OCPD and cable rating, as opposed to the MCB just tripping or whatever.

As I keep saying, diversity is about time as well as load.

Agreed. If it help the discussion, here's a snapshot from my home energy monitor. The "comb" shape in the middle is the oven being used to bake a cake - the initial vertical solid area on the left is the oven initially coming up to temperate, after which the thermostat cycles it on and off to maintain temperature - as you can see after the first 7 or 8 minutes it spends more time off than on. (Obviously for higher temperature cooking the mark:space ratio would be higher, but I've yet to see it anything like solid). The narrow peak in the middle is I think the kettle.

   - Andy.

Such as a 30 amp fused switch supplying a 8.5 kW electric shower blowing up and disappearing in a cloud of smoke, just leaving a burn mark on the wall where it used to be?

Design current (of a circuit). The magnitude of the current (rms value for AC) to be carried by the circuit in normal service.

So assume 37 amps for a shower add 20 amps for the cooker allowing diversity adds up to 57 amps.

The highest current rating for 6.0 mm twin and earth is 47 amps when it’s clipped direct, so it cannot have a 50 amp MCB. Generally the next available MCB rating is 40 amps.

How long will a 40 amp MCB carry a 142.5% overload for, bearing the 142.5% is a conservative figure allowing diversity on the cooker load?

Bear in mind that you need to take account of the MCB manufacturers MCB derating factors, something that some people have not considered when attempting to justify that this combined circuit arrangement is acceptable in similar previous discussions.

www.hagerelectro.com.au/.../TECHINFO_MCBS.PDF

Sparkingchip said:

How long will a 40 amp MCB carry a 142.5% overload for, bearing the 142.5% is a conservative figure allowing diversity on the cooker load?

About half an hour according to Fig 3A4. Your point being?

From the Hager MCB derating guide, assuming an ambient temperature of 40°C which is quite realistic given the MCB is running overloaded for an extended period, a 40 amp MCB is under rated just for a 8.5 kW shower, never mind running a cooker at the same time.

So the first challenge is keeping the overloaded MCB cool.

• From the Temperature Derating table,
The nominal trip current (In) of 40A MCB operating at 40°C is derated to 36.8A.
• From the Grouping Factor table,
Three MCBs installed side by side has a grouping factor K(g) = 0.95
• Combined effect of temperature derating & grouping factor, 36.8 x 0.95 = 34.96A.

In a fully filled consumer unit the tripping current of the 40 amp MCB could easily be down 31.28 amps with a derating factor of 0.85 and an ambient temperature of 40 C.

You're double-counting the ambient temperature of the CU and the MCB. Initially both the CU and the MCB will be at ambient (20°C say), then say the shower and oven are switched on. The MCB starts heating up as it's supposed to, since the bimetallic strip is designed to heat up and eventually trip on long overload. So after half an hour it trips. During this time the CU will have started to warm up a bit, but is unlikely to have reached 40°C. Now if on the other hand we were having a heat wave and the CU started off at 40°C  ambient, then yes, the MCB might trip after 4 minutes rather than 30 mins. But so what? The home owner learns not to run the shower and cooker simultaneously in a heat wave. It's still safe, still nothing has set on fire.

Unfortunately comments like “I'm afraid I agree with Wally above. It is perfectly true (unfortunately) that this may not be understood by many "electricians", but that is because they are not suitably qualified for the job.” are far from helpful and add nothing to the discussion.

we need to avoid a chocolate teapot scenario.

I say that there is no point in making teapots out of chocolate, because the handle will melt when someone holds it and the complete teapot will melt when hot water is poured into it, making it unfit for purpose and potentially dangerous as someone may be scalded when the hot water spills out.

Given your style of presenting an argument I could imagine you coming back saying, if you don’t hold the teapot for very long and only use it for iced tea it will be absolutely fine; and there are not any regulations that say teapots cannot be made out of chocolate, also there’s no reason why anyone should get hurt using a chocolate teapot so long as they know its limitations.

Well, my stance is that, while it is far from ideal to share the circuit,

(1) from a safety viewpoint, I have not yet seen any argument which convinces me that there is additional danger in doing so.

(2) It will provide satisfactory service for the vast bulk of time - maybe a very occasional trip.

While I would be exceedingly unlikely to install such a circuit myself, if doing an EICR I don't see that it merits a C1/C2. A C3 perhaps, with suitable commentary.

This situation is completely unlike a chocolate teapot, in that such a teapot is completely useless, while the shared circuit will likely allow the house owner to shower and cook to their heart's content for years without ever being aware that there might be an issue.

With the oven cutting in and out of circuit via the thermostat.

Z.