DIY Doctor

Loft Supply

Postby brooky333 » Mon Feb 25, 2008 1:34 pm

Would it be OK supply a loft conversion with a 2.5mm feed backed up by a 20A RCBO to a sub board wth a 16A MCB for power and a 6A MCB for lights.
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Postby sparx » Mon Feb 25, 2008 9:17 pm

Hi, probably not due to volt drop, although 2.5 mm2 is rated above 20A it would need to be a very short run to allow any sensible load not to exceed volt drop limits.
I would suggest running feed in 4.0mm2 to be on safe side or do sums for 2.5, ie Amps (max load) X length (metres) X mV figure from tables (18mV)EG: ignore lighting load as likely to only be a few 100w, but power could be full 16A, distance up and along could be say 20Mtrs,
16 X 20 X 18 = 5760mV /1000=5.7v at sub board only leaving 1.2v drop on lights & 5.8v drop for power circuit.
Presume you are well aware of 'part p' for new circuits even if not in 'special locations',
regards SPARX
NB if you want to measure exact run, how installed ie clipped surface, in insulation, ect. will run it through tables for compliance.
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