Sum of MCB ratings > CU rating ?

10 Aug 2007
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United Kingdom
My consumer unit is rated at 100 amps. If I add up the ratings of all the fuses/MCB's installed in the consumer unit , can these exceed 100 amps ?

Put another way, if I want to have an extra radial circuit with an additional MCB, is it permitted to apply 'diversity' and go over the 100 amp limit of the CU ?
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The total load of the board is the sum of the estimated max demand, not the sum of the MCbs.

A technique known as diversity is used to come up with the total demand.

Will be able advise more if you post what is connected at your property (including rating of cookers, showers, etc) , & a pic of your fuseboard.
Also remember that the supplier's fuse will often be 80A or less (even if the fuseholder says 100A) so the CU can't actually carry more than that for long.

My own house has a 60A suppliers fuse which never blows, even though my MCBs are
32+32+32+32+32+16+6+6+6+6+20 = ever such a lot

Bercause, obviously, you put a Socket Ring on a 32A; a lighting circuit or an alarm on a 6A, an immersion on a 16A, and so on.

Actual load (averaged) my supplier tell me is about 1.3kW winter and 0.75kW summer
Thank you gents for your guidance.

Reading between the lines, that sounds like a "yes".

i.e. I can safely add more MCB's to the CU taking the new sum of MCB ratings above 100 amps, *provided* I am sure the new expected max load does not exceed the supplier's fuse rating.
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