Consumer Unit Loading

[BusBar]

New to this forum, although have 'observed' for a while. A point on which I'm confused: On-Site Guide states that consumer units should be rated sufficiently to handle total load connected to them, without applying any diversity! And domestic cu s tend to be rated at 100A max. So without diversity 2 X 6A light circuits = 12A, 2 X 32A ring circuits = 64A, Immersion heater = 16A. Fairly typical so far, no? Which in total is 92A. Not a lot left for separate ring for kitchen, cooker circuit or anything else really... Ok, in reality the load generally stays well below even the figure after diversity was applied, but it does beg the question as to why there are 12 Way units on the market, sometimes pre-loaded with breakers for 160A worth of circuits if you just add them up? And also whether this profusion of circuits is wise as it makes it potentially easier to overload the service fuse without any breakers tripping? Anyone able to enlighten me?

 

[andy]

the main fuse is only 80... pointless havin a CU with a 160A isolator etc

 

[BusBar]

Well, diversity can be applied to size the tails and therefore to gauge suitability of the main fuse, so for instance a 100A main fuse would quite possibly be fine in a scenario with circuits protected by more than 100A total MCBs (although as I said , I do wonder whether there is more potential to overload the main fuse with several circuits with loads of stuff running on them - you'd need to have some heavy appliances on though). My point is more that as far as rating the switch of the CU goes, the On-Site guide is very specific, in that it does not allow diversity to be applied. Which would seem to mean that there are a great number of CUs that don't conform to this, although with diversity applied and in practice they'd probably be fine...

 

[Damocles]

Like the man just said, you are looking at it from the wrong point of view.

The various mcb each protect their own circuits and have switches rated for the current they permit. The main switch must be matched to the supply fuse. So a nominal 100A main supply fuse requires a main switch also with a nominal 100A rating.

The switch is designed to cope with any temporary overload which the protective device might allow.


Having, say, 10 32A ring circuits in the CU increases the chance of blowing the main fuse, but the components are designed to cope with this. If you really did keep blowing the supply fuse someone would be round to ask some serious questions and get it sorted.

Diversity calculations are just informed guesswork. 10 32A circuits, say one for each room in a house, might work perfectly well and never ever give a problem. It would allow you to turn off room power individually, but most people would consider it overengineering.