This Question of the Day, about a client wanting a new consumer unit to be fitted to a house extension, and the distribution system required, has been answered by SELECT (Theme – Distribution Equipment including Cable Management):
Question: Please can you advise the best way to help a client that wants to have a large extension to the house, and also wants to have a consumer unit for that section of the property?
This client currently has a TN-S system, with 16mm tails, and also wants to change the existing consumer, as it has re-wirable fuses.
Answer: Any electrical design will require an assessment of the intended connected load to determine whether the existing supply cable is adequate - is the load likely to below or exceed 100A?
If the existing supply cable is assessed as adequate, the earthing and bonding arrangement should also be confirmed as satisfactory - you mention there is connection to a TN-S earthing system, so this should be OK.
To supply the extension from its own consumer unit will require a sub-main to be installed, which should be like any circuit designed to meet the requirements of BS 7671:2008 - i.e taking account the likely load, suitable overcurrent protection and suitable cable size, with any relevant correction rating factors etc taken account of.
If you are installing a new main consumer unit, the best advice is to take the sub-main circuit supplying the extension from a dedicated circuit in the new consumer unit.
Also worth remembering is that BS 7671:2008 (2011) requirements for additional RCD protection, which should be borne in mind when designing the sub-main and final circuit connections.
To see many more Q & A in Voltimum UK's Experts Area, please click on the link:
www.voltimum.co.uk/consult.php?universe=consult.index.questions