Our Voltimum Experts answer your questions on a daily basis in our Technical Expertise area. This Question of the Day, on a client wanting new consumer units fitted to house and extension, is answered by SELECT:
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 requirements for additional RCD protection, which should be borne in mind when designing the sub-main and final circuit connections.
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