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Upgrade to existing fusebox for loft conversion

Hi, We are planning a loft conversion and want to know the best way to find out if our existing consumer unit/fusebox will need to be replaced or not.

We currently have a Crabtree Starbreaker (C1 x 80A 30mA RCD) 7-way unit with 3 spare slots.

I expect we will need a lighting feed, ringmain power and a powershower connection to the loft extension. We will only have 2 new rooms and one shower room in the conversion.

Currenlty we have 3 x 32A MCBs and 1 x 6a MCB.

Here is a pic of the current unit

We don't have much space in the existing cupboard for a wider unit so it would be good to know in advance if we need to upgrade it and then work out what the options might be.

6 Answers from MyBuilder Electricians

Best Answer

Don’t see any reason to upgrade consumer unit. Plastic doesn’t need to be upgraded just because you are adding new circuits, providing that the current set up isn’t at load capacity. Your power shower may only require a plug top if it’s simply a digital system drawing hot water from a combi boiler. Potentially two new circuits required for ring final and lighting. Don’t forget to maybe add some new smoke detection to new story.


Answered 11th Nov 2018

The power shower would need its own feed. the lights and sockets could possible be taken from the existing circuits in the board depending on existing load on the circuits.
Looking at the photo you supplied the board does not meet the current standards as it is plastic not metal. If you were not doing any additional work and if the electrics tested ok it would not need changing.
For the work you are planning to undertake we would probably recommend upgrading the board for which you would probably be looking at an additional £200 - £300 on top of your loft conversion.
We would need to carry out an EICR (Condition report) to check the electrics before undertaking this kind of work to confirm exactly what was needed.
Hope that helps


Answered 11th Nov 2018

Best thing to do is leave the existing alone add a few new mcbs for the new attic
Power shower sockets smoke detectors radio linked and lighting
To comply

The board won’t need changing as long as it’s tested safe and noted on the certificate as a departure from amendment 3 of the 17th edition




Answered 11th Nov 2018

Exactly what Jon has said - you do not need to upgrade as long as it is still in test as it will pass the previous regs from which the date it was installed. It is always nice to have it all on individual breakers for each floor but not essential. Hope this helps !


Answered 12th Nov 2018

No need to replace consumer unit. If practical then new circuits would be ideal for sockets and maybe lighting (although lighting could be fed from another lighting circuit as the loading tends to be quite low). No need for separate circuit for power shower as it's just a pump and normally runs just off a plug top, or a supply less than 13amps so can be fed off sockets in loft.
One thing you should consider.....usually the loft is insulated, therefore circuits feeding sockets in loft extension will be wired in lagging. Certain cable sizes/MCB ratings need to be considered to comply with current regs. (This doesn't normally affect domestic lighting circuits). This means you cannot wire 2.5mm cables buried in more than 100mm of insulation on a 32amp circuit. EITHER the cables should upgraded to 4mm, OR MCB should be reduced to 20amp or less, OR cables should be kept up out lagging. Just something to bear in mind!


Answered 12th Nov 2018

A single RCD for a dwelling no longer complys to the BS7671 18th edition 2018 .


Answered 5th Dec 2018

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