Garage Supply - Help needed

1 Jun 2005
Reaction score
United Kingdom

I need to run a supply to the garage at the end of the garden, approximately 30 meters from the house. The load on the garage will be just a tumble dryer and a light and there is no water supply in the garage.

The main building supply is PME and theres is no spare ways in the CU. The DNO have already fitted an isolator after the main fuse and before the CU. A number of questions arise:-

1. I propose to take the supply to the garage by fitting a Henry block after the DNO isolator and introducing a second pair of meter tails 16mm into a small metal switched CU with a 20A MCB, thereby protecting the cable to the garage with the 20A fuse..

2. Run a three core 6.0mm SWA from the new CU to the garage in a trench 600mm deep and terminate in the garage with another metal or insulated CU fitted with a 30mA RCD and two fuseways 16A & 6A for power and lighting.

Would the above method be compliant? Equally inportant is do I export the earth from the PME supply in the house and earth the SWA armour and the earth conductor in the cable to the PME earth or do I make the garage a TT system not connect the SWA to the PME earth.

Opinions on the above seem to differ greatly, can anyone shed some light on what is the definitive answer.
Sponsored Links
20 amp you can have 4mm cable on that distance.

with 6mm you can have a 32 amp MCB. You dont want to have to dig that trench twice. Plump for 10mm if you can, someone might want a kiln or hot tub or LINAC down there one day. ;)
To answer the last bit on earthing - its always easier using three core SWA and using one as CPC rather than relying on a good earth through the armour alone as I suspect you may not be too experienced in SWA termination. Always be super safe by connecting the banjos to earth as well at both ends.
Thanks for the above, but its still not clear to me if I need to export the PME earth or not., and exactly what earth connections are made at each end.
Sponsored Links
Thanks Spark123, am I reading this article correctly:

If the garage does not have an extraneous conductive part (which it does not) its just a bare garage with a concrete floor, then its OK to export the earth on a PME supply.

If the garage does have an extraneous part eg. a water pipe then the garage must be made a TT system and the armour of the 3 core SWA together with one of the cores can be connected to the main earthing system in the house. The earth in the TT system must be kept separate from that in the house.

Another scenario.
Could I run a 2 core 6 mm SWA together with a separate 10mm earth to the garage and use the PME earth thereby avoiding having to make the garage a TT system with the invevitable high resistance value this would give.?
If you have no extraneous conductive parts it doesn't matter about increasing the size of the bonding to main equipotential bonding sizes, (unless you can see in the future the need to install a water supply utilising metal pipework.)
If you are in England/Wales I take it you are aware that installing this is notifiable to your LABC under part P of the building regs?
Thanks everyone for your input. It looks like I'll export the PME using
6 mm SWA on a 20a fuse with the potentiall to increase this to 32A if needed later.

The LABC are already involved with an ongoing side extension so I will use this notification to check out/approve the garage circuit.
so if there is exposed metallic parts....

SWA and single core of three to Earth Spike?
extraneous conductive parts you mean?

And the presence of them doesn't affect whether or not you can export the TNC-S equipotential zone , but it does mean the earth conductor in the submain cable is a main bonding conductor as well as a CPC and therefore must be sized in accordance with 54H

The above link posted by spark123 should explain it.

It may not be wise to export TNC-S all the time, metal framed buildings and other structures, anything involving animals, supplies taken a long distance, supplies taken for use not in a building, etc sometimes mean TT is preferable

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Sponsored Links