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How big should a consumer unit be?

Arran Cameron

Alright, it's only a metal box. So is the bodyshell of a Mercedes Benz. It's what you put inside it that really matters. Is the biggest really the best, or is most of it just going to end up as dead space? In some countries domestic consumer unit are so large that you can almost walk inside them, whereas us Brits seem to be content with a puny little canister filled with just 4 or 5 MCBs or (heaven forbid) rewirable fuses.

Those mega sized consumer units aren't just for show. They are filled with all sorts of weird (to us backwards Brits) and wonderful (to local folk who can't live without them) control devices, alongside protection for wiring, semiconductors, humans, and curious pet cats. Yes, pet cats have sadly been killed by electrical fires and shocks resulting from outdated consumer units.

The consumer unit is the central control box for all the electrics in a house. How big should the box be in order to meet the requirements of today and tomorrow? Think - EV chargers, solar panels, more electric heating appliances, home automation etc. Do you recommend 2 (or more) row boxes over the single row boxes, which are the norm for British houses, if wall space permits?

Is there a recommendation for the layout of a 2 row box for a domestic installation? For example, should all MCB / RCBO be located on bottom row and other devices on the top row? Boxes with more than one row are currently used mainly for commercial buildings, or mansions, rather than average size houses.

Consumer units are relatively 'dumb' devices even if fitted with microprocessor controlled AFDD and timeswitches. There are few, if any, official components that offer the facility to remotely monitor a consumer unit or connect it to the IoT. This will potentially be the next stage of development of consumer unit technology. For example, the next generation of SPDs will report the times and the voltages of each transient, and make them available for remote monitoring. RCBOs will be remotely resettable as well as providing details of the time they tripped and the fault current - including that which flowed to earth via a curious cat.

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  • In some countries domestic consumer unit are so large that you can almost walk inside them, whereas us Brits seem to be content with a puny little canister filled with just 4 or 5 MCBs or (heaven forbid) rewirable fuses.

    The BS 1362 fuse does make quite a difference to UK CU sizing - the fused plugs allows us to use 30/32A final circuits so can cover a substantial floor area with one MCB (the original design was for 1000 ft² - i.e.both entire floors of a post-war 3-bed semi) - and the higher rating of the circuit allows us to accommodate many larger appliances (washing machines etc)  on the general circuit. Even small fixed appliances can be fed from the same circuit by FCUs and a 3A fuse. By comparison French are limited to I think 6 or 8 (single) sockets on a 16A or 20A circuit so need an MCB for the equivalent of every three or four double sockets, plus extra ones for washing machines, dryers etc. plus extra ones again for other small fixed appliances, so it all adds up rather quickly.

    Hopefully the abomination of split-load CUs will fade (using individual circuit protection (e.g. RCBOs/AFDDs) instead, so that'll free up 4 modules, SPDs typically can fit in a module or two, we might need a couple of extra ways for PV and a heat pump say, so mostly it looks like balancing out.

    There's also the issue of the allowed height above floor level of the toggles of devices for accessibility requirements - put in a two or three row CU and it's nigh on impossible to keep everything within the required band.

       - Andy.

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  • In some countries domestic consumer unit are so large that you can almost walk inside them, whereas us Brits seem to be content with a puny little canister filled with just 4 or 5 MCBs or (heaven forbid) rewirable fuses.

    The BS 1362 fuse does make quite a difference to UK CU sizing - the fused plugs allows us to use 30/32A final circuits so can cover a substantial floor area with one MCB (the original design was for 1000 ft² - i.e.both entire floors of a post-war 3-bed semi) - and the higher rating of the circuit allows us to accommodate many larger appliances (washing machines etc)  on the general circuit. Even small fixed appliances can be fed from the same circuit by FCUs and a 3A fuse. By comparison French are limited to I think 6 or 8 (single) sockets on a 16A or 20A circuit so need an MCB for the equivalent of every three or four double sockets, plus extra ones for washing machines, dryers etc. plus extra ones again for other small fixed appliances, so it all adds up rather quickly.

    Hopefully the abomination of split-load CUs will fade (using individual circuit protection (e.g. RCBOs/AFDDs) instead, so that'll free up 4 modules, SPDs typically can fit in a module or two, we might need a couple of extra ways for PV and a heat pump say, so mostly it looks like balancing out.

    There's also the issue of the allowed height above floor level of the toggles of devices for accessibility requirements - put in a two or three row CU and it's nigh on impossible to keep everything within the required band.

       - Andy.

Children
  • in reply to AJJewsbury
    AJJewsbury said:

    There's also the issue of the allowed height above floor level of the toggles of devices for accessibility requirements - put in a two or three row CU and it's nigh on impossible to keep everything within the required band.

    That is a very important point. A young contractor put his consumer unit at high level in the downstairs cloakroom of a clutch of 8 large, new dwellings. As I understand it, BC will not issue a formal completion notice until the arrangement complies with Technical Booklet R (similar to Approved Document M in England).

    The lad in question is only starting out so it’s going to be one hell of an expensive lesson for him!

  • in reply to AJJewsbury
    AJJewsbury said:
    The BS 1362 fuse does make quite a difference to UK CU sizing - the fused plugs allows us to use 30/32A final circuits so can cover a substantial floor area with one MCB (the original design was for 1000 ft² - i.e.both entire floors of a post-war 3-bed semi) - and the higher rating of the circuit allows us to accommodate many larger appliances (washing machines etc)  on the general circuit. Even small fixed appliances can be fed from the same circuit by FCUs and a 3A fuse. By comparison French are limited to I think 6 or 8 (single) sockets on a 16A or 20A circuit so need an MCB for the equivalent of every three or four double sockets, plus extra ones for washing machines, dryers etc. plus extra ones again for other small fixed appliances, so it all adds up rather quickly.

    That is a valid point. American consumer units are also bigger than than most of their British counterparts because of the way sockets are wired.

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