Our Voltimum Experts answer your questions on a daily basis in our Technical Expertise area. This one, concerning the application of diversity reasoning, is answered by the NICEIC:
In a particular case, we have a TT system where the householder has made provision for two new electric showers both on 50A breakers on an 80A 30mA RCD in their own unit. Alongside, is the original E7 6 x 16A breakers storage radiator unit, and then a 16th Edition (ie single RCD) normal household unit looking after the immersion, cooker, lights and sockets for the home.
Applying reason: a) the radiators are governed by the E7 times, b) it is unlikely that the showers are all on at the same time (and not for long anyway), c) immersion is not used except when the boiler isn't working etc - is my diversity reasoning in the 'spirit' of the regulations, or should we apply Table 1B of the OSG strictly. Or should it be a balance of both?
Answer: In BS7671: 2008 (17th Edition) - Regulation 311.1 gives requirements for the assessment of requirements of maximum demand. The methods for determining the maximum demand have to consider diversity.
As per the OSG, it applies diversity in the sense of the amount of current flowing through a circuit during normal use. It does state at the top of page 96 in the OSG that other methods of calculation are not precluded. A formal review should be held, and the 'worst case' should always be considered.
A competent person will know that two showers, a cooker and an immersion heater are likely to take the loading over its designed threshold, so - therefore - it would be essential to consider either a larger single supply or a three-phase supply, and adequate ratings of switchgear and cables need applying.
These issues will be the responsibility of the designer, and future demands should always be taken into account for these particular issues.
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