Timothy Daley asks this question, which concerns the possible use of a larger cable for a final circuit than the consumer unit feed at a farm installation. It has been answered by Prysmian (Theme - Cables, Wiring & Wiring Accessories):
Question: Is it possible to use a larger cross-sectional area cable for a final circuit than the consumer unit feed?
I have been asked to supply an outbuilding at a farm with power. The trouble is, it is 100m away from a sub-distribution board in a garage, which is 15m from the main board. The supply to the sub-board is a 6mm SWA. The outbuilding is to have 2 X 13A sockets, 2 X fluorescent lights and 2 X 2D exterior lights. Using 6mm SWA would exceed the maximum voltage drop. Could I up this to 10mm SWA to overcome the voltage drop?
Answer: Yes, it is possible to use a larger sized cable for the long run to the outbuilding. The key issue is to minimise voltage drop, which can be achieved using a larger CSA cable. Appropriate protection should be applied to protect the smallest cable in the circuit.
I know of an application similar to yours where the run is about 2km!
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