(UK) Adding additional circuits into old fuse box

[OverheadCatenary]

Hi all,

First post here, hope I'm compliant with the rules! I did do a quick search before asking this, but I couldn't quite come up with the answer so I thought it best to ask!

I'm currently going through and renovating some of my property, and the garage wiring is of some slight concern to me. Currently, the garage appears to be wired off of a spur upstairs which then exits through the external wall, is not at all supported in its 2m journey over the gap between the house and the garage, enters the garage, and is spurred into a lighting circuit and 3 sockets. This is clearly far from ideal, and as I'm a hobbyist engineer and intend to run some reasonably beefy motors in the garage I would like to upgrade this installation.

My intention, subject to your validation, is to use one of the two spare slots I have on my consumer unit and take a new circuit off into the garage into it's own small consumer unit. I have a number of questions regarding this:

1) The house was constructed in the mid-70's, and has an old Wylex fuse wire based consumer unit. This unit is obviously old, but completely functional, and has two unused spaces that I could put a new circuit on. My plan is to complete all the wiring work myself, and then have a qualified electrician inspect my work and, if deemed worthy, have them connect it into the consumer unit. Is it still legal/within regulation to add a new circuit into this rather old unit? I understand that this is far from ideal, but I would ideally very much like to avoid the expense of replacing the consumer unit at this stage. It is my intention to install a rotary switch below the consumer unit to seperately isolate the garage circuit.

2) The consumer unit is located in the centre of the house, and in order to enable this installation to take place I will have to run the new cable for the garage to the outside somehow. My plan is to run the cable in hard conduit down from the consumer unit, through the wall, and then around the kitchen under the kitchen cabinets attached to the wall with cable clips, exiting through a hole in the outside wall, where I can run it in solid conduit up the side of the house and across to the garage. I've found a minimum specified requirement of 3.5m in height for a catenary, but I'm unclear if this applies for conduit installations as well.

3) Once inside the garage, I would like to install a small, 3 RCCB consumer unit from which the garage power can be taken using the following rated RCCB's:
a) 6A - Lighting. Powers two fluorescent lights and two LED panels.
b) 10A - Sockets. Used for powering small power tools, 12v battery chargers etc.
c) 10A - Hard wired machine tools & additional sockets. Wired to emergency stop button & momentary pedals, eventually reaching a lathe & bench drill, with a spur for additional sockets to power coolant pump & lighting.

Any advice would be greatly appreciated. I'm an experienced electronic & mechanical engineer so I'm confident I can do a good job of the installation work, but of course as above would get everything inspected and connected professionally.

Many thanks!

 

[Rockingit]

Interesting one, this. Whilst most people would start screaming for a consumer unit change, if your new cable from A-B is entirely surface clipped or in mechanical protection of some other form then you don't need RCD protection for it, which means it can in theory be fed happily from an old Wylex BS3036 board which is what I assume you have. If it's then going straight into an RCD/RCBO arrangement for final circuits then those circuits also comply with current regs.

You will, though, have created a new circuit which does fall under Part P so you'll need someone competant and registered for third party sign-offs to come and test and certify afterwards.

 

[Lucien Nunes]

As above, if the distribution circuit is installed such that it does not need additional protection against shock, and the sub DB provides such protection to the final circuits that need it, then you can launch from a BS3036 fuse. When calculating the cable size, don't forget the additional factor Cf (=0.725) for coarse overcurrent protection that accounts for the difference in curve between a BS3036 and either a 1361 or an MCB. In some cases this can increase the minimum permissible Iz to the point of needing a larger cable simply on account of the fuse curve.

using the following rated RCCB's

You mean RCBOs. RCCB is an old term for RCD and does not signify overcurrent protection.

b) 10A - Sockets.

I would not protect the socket-outlet circuit at 10A. General-purpose socket-outlets are expected to supply 13A and connecting such a load would result in a long-term low overload of the protective device which would not be expected to trip, a poor design approach. It will also considerably increase the chance of nuisance trips on inrush. If you can co-ordinate the OCPD with the distribution circuit there is much to be said for allowing 20A (ideally) 16A or at the very least 13A.

c) 10A - Hard wired machine tools & additional sockets. Wired to emergency stop button & momentary pedals,

Beware the possible interruption of supply to machines with controlled deceleration. E.g. cutting the supply to a woodworking bandsaw with injection braking (thereby disabling the brake) could increase the risk of injury compared with stopping it using its own button. Although this is a home workshop, it's a good idea to design safety provisions in compliance with best practice for the workplace.

I would ideally very much like to avoid the expense of replacing the consumer unit at this stage.

There is no requirement to do so but we are in an era when RCD protection is the norm and (unless outboard RCDs have been added) you are denying yourself this valuable contribution to electrical safety by retaining the old CU.

Finally, what equipotential bonding requirements apply in your garage if any?

 

[telectrix]

i would split he tails and fit an additional 2/3 way CU to supply the garage/whatever. that way you could feed in SWA, installung RCDs as and if needed, without mucking about withe the existing arrangement.