| I'm installing a consumer box to a workshop/garage extension
| on a small shop to provide power for 4 fluorescent tubes lights
| and 6 double gang power sockets. Power sockets will only be used
| for drills, one shutter roller motor and hand lights and such
| like but not any heavy bench stuff.
| My local electrical retailer has supplied me with a 4 way CU with
| 32A and 6A MCBs for sockets and lighting.
One of each? Total load = 32+6 = 40A. What about RCD protection for the
| He has suggested to me that I need to run a 6mm T&E cable from the
| original CU and wire into a 40A MCB there.
That sounds sensible. (However the existing load on the consumer unit and
its supply must be considered.) It is possible 10mm might be required,
though, and might be wise to install now to allow for future increase in
load anyway. You may also need to run a parallel 10mm earth wire to the
secondary CU, as the circuit protective conductor in the 10mm cable may not
be adequate. This must be ascertained by calculation or test.
The secondary CU should normally be supplied from a non-RCD circuit on the
| I have conflicting info from an electrician friend of a friend who
| says I should run the meter tails at the original CU to a 100A
| junction box and then run 16mm cables to the Garage CU and original
| CU. This seems like overkill to me.
You cannot run unfused 16mm meter tails 12 metres. A switchfuse would be
required, and the meter tails into the switchfuse should be 25mm anyway
As this is business premises, the Electricity at Work Regulations 1989 SI
1989 No 635 will apply, and you may wish to acquaint yourself with the
Memorandum of Guidance HSE SH(R)25. It is reasonably safe to assume that
current IEE Wiring Regulations and appropriate British Standards have
'deemed to satisfy' status for compliance with the EAW Regs. The
installation will therefore have to have a full Design schedule signed off
by a competent person, along with the Installation itself, and the
pre-energisation inspection and live testing carried out, with full
compliance with the design requirements verified, before the full completion
certificate can be given. In the event of an accident or fatality, anything
less would probably not be acceptable as a defence in Court.