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Applying diversity

When calculating the load to size a cable for a cooking appliance, do you apply diversity?  For clarity , I do but not sure I should.

  • It depends upon the type of cooking appliance.

    A domestic type electric cooker cooker consists of several boiling rings, at least one oven, and often a grill. The assumed load is less than the total consumption of each component, on the grounds that simultaneous use of all rings, ovens and the grill is most unlikely.

    Common practice is to take the first 10 amp, plus 30% of the remainder. Example -total loading is 50 amps. take the first 10 amps and 30% of the remaining 40 amps which is another 12 amps. Add 5 amps if the cooker outlet includes a socket outlet. Most common types of cooker will need a 32 amp MCB and cable to suit. Some larger models may need a 40 amp MCB and cable to suit.

    Do not reduce the cable size below the rating of the OCPD.

    For a specialist or industrial cooking appliance the above is not generally applicable. For example a large oven in a food factory may operate continually at full load. If it says 41 amps on the rating plate, then in my view it means 41 amps and not some lesser figure. Needs a 45 amp MCB and cable suit. 

  • indeed it is OK, for a multi element machine unless it is credible that all the elements will be on full chat for a sustained period - and here sustained means like more than 15 mins.Given the way thermostats operate, this is most unlikely in a domestic setting.

    Also true if the oven and rings are separates but share a feed.

    But something that has a single element that perhaps operates a pressure cooker, or heats a serving counter, maybe not a safe assumption.

    Mike

  • Note that the OSG has a table of rule-of-thumb calculations for diversity, including one for cooking appliances. On a cooker, even if all rings are used, once the pans on them have come to the boil, they will likely be turned down considerably. Similarly, once the oven set temperature has been reached, the heating element gets turned off for a while. So a multi-thinggy oven is a good candidate for applying diversity.

    A short-term overload is perfectly acceptable, hence why the OL part of an MCB is a bi-metallic strip that gradually warms up, crudely mimicking the behaviour of the overloaded cable warming up.

    The thing about diversity is that it's a guess about likely usage patterns, in order to avoid hugely over-engineering everything. So, if diversity is done correctly, the worst that should happen is something tripping if you guessed wrong. If it trips frequently, the customer may need to upgrade the wiring. What should never happen is that  if you guessed wrong, the cable overheats or something sets on fire.

    Or to put it another way, diversity is a gamble with convenience, but never a gamble with safety.

  • On Christmas Day when all the bits and bobs might be used, you wouldn't put your peas on at the same time that you put the turkey in the oven now would you?

  • My mother would!

    And the sprouts Rolling eyes

  • LOL. Well our parents and grandparents did have a tendency to boil things to death for days not minutes !

    Once you left nursery and started at infant school then the occasion was marked by cooking the carrots and would only be taken off heat once you left senior school to start an apprenticeship or university . Then served before they went cold