Monday, 13 February 2012

Garden Office and Garden Studio Electrical installations

Electricity in Garden Offices and Garden Studios

There are always some questions about the electrical supply to Garden Buildings from our prospective clients.

I am a qualified domestic installer “electrician” with experience of designing and installing more than 500 electrical systems specifically for Garden Structures.

Firstly let me explain what electricity is:

  • It is sub-atomic particles (electrons) flowing in a conductive material.
  • It is invisible it has no smell and it can kill.
  • It forms the backbone of modern industrial society and probably will do for some time to come.
  • It powers our I-phones, the internet and it makes our homes comfortable with heat, light and entertainment.
  • It is used in our factories, vehicles, transport systems and hospitals in every corner of the planet and beyond.

Electricity comes in various strengths measured in volts and various volumes measured in amperes. A high number of volts can hurt a lot and kill if it flows through the heart. A high number of amps are required to create lots of heat or start a heavy motor.

At this point I propose to stop explaining electricity but ohms law can be read at:'s_law .

Noella can still recite it word for word having learned it at school!

Here are two practical examples I hope you find easy to understand:

  1. Homes use 240 volts of alternating current.
  2. Cars are started with 12 volts of direct current.


Electricity travels better down thinner wires using wavy current (AC) at high voltage than straight current (DC) at low voltage. As you are aware our cars are started with 12 volt (DC) batteries but need massive cables connected to the starter motor to turn the engine.
The 12 volt battery system used in cars is safe to touch and travels with the vehicle waiting to start it again for the return journey. The 240 volts used in our homes can provide lots of power down thin wires but needs to be safely installed because a short exposure to 240 volts flowing through your heart can be fatal.


Low voltage with heavy demand requires big wires and high voltage is dangerous.

Electricity in Garden Offices, Garden Studios and Granny Annexes

Our garden buildings Offices, Studios, Garages, Annexes etc need 240v ac to operate all our usual domestic stuff and therefore must be installed “as safe as houses”. For some years now it has been outside the law to use any 240v appliance outside the home i.e. in the garden without the appliance being protected by an RCD. This device (full name: residual current detector), switches off the current flow if any electricity flows to earth e.g. if electrical current flows through your body to ground it will switch off the current before you are badly hurt.

I, like most electricians would prefer these devices to be fitted on every domestic circuit, however 60 % of dwellings in the UK still do not have them. I am pleased to say that EVERY Garden Building installation I have made is fully RCD protected.

Safety is paramount.

Essential elements of a safe electrical system for “Garden Buildings”

  1. None of the conductors should be exposed. This means that mechanical protection must be provided (switch covers and socket plates etc.). Every electrician is responsible for ensuring that the installation passes the “British Standard Finger Test”. It must not be possible to insert anything into any enclosure and touch a live conductor.
  2. Secondly all devices lights, sockets, appliances etc. must be sufficiently protected against any external part becoming live.

Essentials of an efficient “Garden Building” electrical system.
The Garden Office or Garden Studio is normally wired in exactly the same way as a typical home as follows:

There are lighting circuits, power socket circuits and circuits for high demand items. The wires for different circuits are of different sizes. Lights use less power than heaters therefore thinner wire is adequate. To ensure a circuit cannot be overloaded a fuse / trip (referred to an over-current device) sized appropriately and certified by the installer is fitted. If the wrong size is fitted and the wire is overloaded fire could result.

So – the correct wires and over current devices must be installed to ensure total safety inside the new building. The next job is to make sure that the building has the correct feed. This feed should not affect the main house and fulfil its job providing safely a reliable supply to the new building.

Correct connection of a Garden Office or Garden Studio to the electrical supply.

By far the most common way of providing this connection is to use a “Steel Wired Armoured cable” (SWA). The size / capacity of this cable depends on two factors a) the distance it needs to carry the electricity and b) the likely demand for amperes at peak. This is an assumptive calculation made by the installer in compliance with (Building Regulations Part P) – the armoured cable must always be overload protected at source but must be sufficient in capacity to make sure that under heavy use (like fan heaters, showers and kettles are on at the same time) the supply will not trip causing disruption to the occupants.

It is very important to note that disruption and safety are not related as safety is paramount and must be assured by the installing electrician. Each electrician is legally bound to certify each and every circuit he or she works on.

The armoured cable needs to travel from the main property to the new building. As the cable is mechanically protected against contact with the conductor and earthed (the surrounding steel strands are connected to ground) an SWA cable is electrically safe on the surface, in the air or buried at any depth.

This important point is the subject of some debate amongst electricians and quoted incorrectly on many industry websites.

(updated 11/12/2015) "Don't need to change anything here electrons are still electrons and safety is still the main thing" There have been a few challenges from electricians wanting to bury SWA  Armour (steel wired armoured cable) at high cost to the client but I assume by now no-one else wants to challenge me on this. SWA is safe above ground or below ground that is all. I think the comments are good enough - it's still a boring subject but safety sometimes is - it doesn't make it less important because safety remains VITAL.

What is the difference between Earth and Neutral (blue and green)?

Both go to the same place but earthing is done for the protection of humans and neutral takes back the electron count. So when you switch on a light the electrons used are is counted by the meter so you must pay. If you send it back down the earth the light will still come on but you may may pay with your life as correct earthing is a life saver.

Earthing is provided to an outbuilding in the swa armour by the outer steel sheathing and sometimes an inner core as well. Earth rods are not necessary if good connection to earth is made at the main property. The armour can be on the surface or underground it is a safe cable protected by steel connected to earth.

This post written by Richard Grace Project Director Garden Structures Ltd

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