Diversity

[teracir]

could someone please point out to me when diversity factor is to be applied? does it apply to cirtain circuits or to the whole of wiring.

the reason why i ask this is because there are alot of threads on this forum which are leading to confusion. for example, in a 10.8kw cooker circuit, it would require approx 50 amps to protect and would be wired with 10mm2 cable. but if you apply the diversity factor (10amps +30% of remaining current +5 amps (attached socket if any)), the figure is reduced to less than 30 amps and if this is correct then you can do with a 4mm2 circuit instead of 10mm2. this makes a huge difference.

please comment of what is appropriate.

cheers

 

[Adam_151]

basically diversity is the understanding that not every thing is running at the same time, take the cooker for example, even if you did have all the hobs and all the ovens and grills on together, they'd be cycling on and off via the thermostat, couple that with the fact that a 32A breaker doesn't trip at 32.0001A (though if you have got deversity right, it should be very rare that the diversified rating is exceeded) some people consider it bad practice to apply diversity on cookers circuits to the extent allowed by the regs though.

When it really comes into its oen though is applied across DBs/CUs, thats why you can have a board with 3 rings, some lights and a cooker connected to a 60A supply, even if you have a 10kw shower on it as well, while probably exceeding 60A a lot more than is good to, actually causes very few problems in reality because it'll take a good 100A for a half hour.

(Actually the guidelines for diversity tend to err on the side of caution and a clamp meter on the tails is much more realistic)

 

[sparkacus]

Quite possibly you could manage the circuit with 4mm2 but wiring regs (although not statuary) recommend 10mm2. See BS 7671 (requirements for electrical installation) for more details.

 

[jondiy]

sparkacus, The regs to NOT specify cable types for individual appliances!!!

Basically, you have to use a design procedure, which involves working out the DESIGN CURRENT.

DESIGN CURRENT is the assumed current , and is calculated using diversity. You DO NOT have to use MAXIMUM DEMAND for cookers.

You then select a breaker that is the next available size above the design current, in this case 32amps , as it is the next rating over the 30 amps design current.

The cable type is calculated using correction factors for Thermal Insulation, Grouping, Ambient Temperature, and a Rewirable Fuse rating (if you are using one) . This then gives you a nominal current , from which you select the cable type , and the current carrying capacity of the cable must be greater than this value.

You then need to check the voltage frop , which varies depending on the length of the cable, and the cable type , and also the Earth fault loop impedance (Zs) , using the figures for your selected cable for R1 and R2. These Zs values need to be checked with the tables on 41B1 41B2 etc to make sure your values are below the specified ratings.

You can then work out the earth fault loop current , which is 230 divided by your Zs value. And also calculate the required size of the CPC using the equation! Also you are required to assess the Prospective Fault Current to make sure that the breaking capacity of the breakers is up to the job.

So my answer to this is - Depends on the factors above , but YES, diversity is for certain circuits, in this case yours. I'd be leaning towards a 6mm cable - I don't know your house setup etc ! Cookers have been run on 4 and 6mm cable for many many years.