Maximum demand

Is this my maximum demand?

Or is the data logger on since just before Christmas which shows a short spike of just over 26A, correct? My house. 

The NICEIC now require all approved contractors to justify their assessment of maximum demand when they are responsible for the design of a new installation or alteration or addition. Reference to the OSG is in chocolate fire guard territory!

  • If you used the 9.5 kW shower in that period ... would you be comfortable using a design current of 26 A for the final circuit feeding the shower?

    I think to answer the question 'What is the maximum demand?' one must first ask 'Maximum demand for what purpose?'

    Regulation 311.1 accepts that it may be necessary to determine maximum demand for the whole installation, or a part of it.

    I think there are also differences between the maximum demand (as used by the DNO) and the maximum demand (used to rate the consumer unit).

    Now, if you were recording 15 minute values, and you had a 10 minute shower, it would of course be reasonable to see a 15-minute demand value of 26.3 A (calculated for shower rated at 240 V) if little else was being used at that time. It may well be reasonable for the DNO to use this value to rate their supply max demand, because it takes a while for cable to heat up  ... but we need to be careful because this assumption is not necessarily applicable to switchgear, for example.

    Other appliances (except lights) shown in the list don't necessarily use all their current all of the time, and may well show well below their maximum ratings when looked at in terms of 15 minute or half-hour values. The immersion is perhaps the big exception - if it's used much at all.

  • I fear that lyledunn is being mischievous.

    I have never quite understood what the DNOs mean by ADMD. Over what period?

    128 A per hour = £13/hour or £320/day.

  • I fear that lyledunn is being mischievous

    That may (or may not) be the case, but the question comes up in different ways ... for different purposes ... quite frequently.

    Overall, perhaps best to air the issues?

  • well for a ten minute or shorter shower you could run that 40A shower on 2,5mm cable... But if my daughter were to visit (long shower territory) then cable damage may well ensue.

    I presume you have not showered, or the data logger has been set up with far too long an integration period to catch it..

    But there is a non-flippant point to this AMD is very much a roll of the dice -

    1) the breaker total is wildly misleading and a serious over estimate,

    2) Dividing the annual bill by the no of hours in a year misses the short duration  peaks and under estimates.

    3) if you were to assume the OSG advice was right, then half the streets in the UK ought to be on fire...

    Short of supplying the property via an instant acting trip there is no way to ensure any particular maximum demand  - a typical  DNO  fuse after all is capable of  supplying a load of twice its rating  for 15 mins or so several times a day with no ill effect.

    Even the DNOs do not know that accurately  - in those cases where substation monitoring has been installed, it emerges that some transformers are running at up to twice capacity and others are almost unloaded.


  • I'd add that demand is caused by loads, rather than circuits.  Circuits are typically designed to meet a large number of requirements, load only being one of them. Unless you're in a fully populated medium sized mansion you probably don't need  4x32A rings plus a 20A radial as far as load is concerned (traditionally that lot would serve around 450m² of floor space - i.e. between 4 and 5 times a typical 3-bed semi, so including 4 or 5 kitchens). It would be equally valid to come up with a theoretical design with the minimum size/number of circuits that could supply the actual loads, and base you MD on that.

       - Andy.