Need your help guys.
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. He has suggested to me that I
need to run a 6mm T&E cable from the original CU and wire into a 40A
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.
Who is right here?
The garage CU is 12 metres from the original CU and the garage is
connected directly to the main building.
IMHO the retailer. The cable to the new CU needs to protected somehow.
With your friend's suggestion it wouldn't be...
Also 16mm is as you say, way overkill...
Is he really an electrican or just pretending to be one??
This is one way of doing it. However, it would suffer from lack of
discrimination. This means that a fault on the socket circuits is as likely
to blow the 40A MCB as the 32A one. This would knock out the lights at,
perhaps, a critical moment. (Large dangerous power tools still spinning
down). You might get slightly better discrimination fitting a Type C 40A MCB
to the master consumer unit, although it won't be guaranteed at all. The MCB
there is only protecting the submain to the garage, not the equipment
This is a "better" solution, but might not be required depending on your
attitude to the lack of discrimination and the ease and expense of running
such large cables. You should consider this if there is the slightest
possibility that the garage might require a beefier supply in the future.
Also, I would strongly recommend using an 32A RCBO instead of an MCB for the
socket circuits. Power tools are crying out to be so protected.
P.S. I forgot to mention that the 16mm tails would have to be protected by a
suitably rated switchfuse after the Henley block. You can only count on the
electricity company allowing about 2m of cable to be protected by their
| 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.
Thanks for the advice guys. I am placing a RCBO in as well now.
I want to do this job properly so am taking my time. The consumer box
and ring circuit, lighting etc have all been done. I'm now working on
electrical stuff in the shop itself. The state of the wiring is a real
mess and the last supposed electrician who was supposed to have
cleaned up the wiring and added new lighting did a really bad job. He
had run 1.0 mm cable direct from the mcbs and just left it hanging
over the consumer box itself (no clipping etc). He also left his
business sticker on the consumer box, maybe I should give him a call!
Someone also decided to leave a live junction box without its lid
amongst a whole mess of cables which was a real nice touch. I've now
replaced the 1.0mm with 1.5mm and clipped it up properly.
Regards the guy who gave advice about running 16mm tails from a 100A
junction box, he works for a council doing electrical work in council
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