The changes to the UK electrical regulations made in 2005 are designed to reduce the number of people electrocuted by their own incompetent wiring and to eliminate cowboy electricians. The “part p” regulations basically specify that a competent person (electrician) should install and self certify your shed wiring. These notes are here to help you plan and discuss your requirements with an electrician and hence not over engineer or under engineer your workshop supply and to avoid unnessary work by making the right choices.
Please note that other countries may have different regulations and these regulations may change.
Q: What are the regulations for?
A: Part P schemes should ensure that electrical work in dwellings is designed, installed, inspected and tested to the standard required by BS7671. The regulations are British law, not just guidelines.
Q: What if I ignore this law?
A: You could be fined upto £5000, be required to remove your or have your work altered and if you sell your home the purchasers solicitors will require you to have certification for work done.
Q: Why does this apply to my shed or workshop?
A: The regulations apply to a building or garden that receives its electricity from a dwelling. So if you powered your shed from a wind turbine or from batteries then you could in theory do the work yourself but you might then find that you are affected by a whole raft of other regulations…
Q: What about modification of an existing setup?
A: The regulations apply to major involving one or more complete new circuits. You can complete non notifiable work such as changing a socket or adding lights or replacing a damaged cable. Check the regulations as there are exclusions for these works meaning some things must be done by an electrician, these include kitchens, bathrooms and gardens.
Q: What if I still want to do this myself?
A: This may be possible but you will need to discuss it with your building control office. There are options such as having the building control office inspect you work, however they don’t have to accept your plan.
Cabling to the workshop
The cabling to the workshop is typically connected to a spare way on your consumer unit and then via special cable to the workshop, it is buried in the ground. A warning message on a tape is buried on top of the cable.
Q: What kind of cable is used?
A: An underground cable needs to be protected. Typically this is done with a steel armour around the cable. The cable requires special glands at the ends to terminate the armour.
Q: How deep should the cable be buried?
A: This depends on the your circumstances but is typically 18 inches. If you are putting the cable below flowerbeds or a vegetable patch then you may wish to go deeper and/or provide additional protection to the cable. Special considerations may be required if your area is populated by burrowing animals or rodents.
Q: Is there a maximum distance?
A: Yes, the distance is determined by the current requirements the size of the cable and the allowed voltage drop. Your electrician will work this out for you but to put it simply, you will need fatter cables the further the distance and fatter cables to supply a higher current. Most workshops / gardens should not have a problem but if you are supplying high currents over 50m then you may have an issue.
Q: I don’t have any spare capacity in my home’s consumer unit, will I need a bigger one?
A: Not necessarily, the electrician may be able to spur your supply from your ring main.
Q: Can I put other items such as network cables in the same trench as my electrical cable?
A: It is recommended that you have your network cables at least 15-20cm away from your power cable. There is a version of Cat5 cable that can be directly buried or you can put it into waterproof PVC conduit.
Cabling in the workshop
Q: What kind of cables are used in the workshop?
A: The same as in the house.
Q: Do the cables need to be put in conduit?
A: The wires need to be physically protected, in a house this is normally by keeping them inside the walls. In a workshop this may not be possible hence you need to protect them with condit or by routing the cables away from the areas of any work. Please note that if you use metal conduits then you will also require additional earth bonding to be done.
Q: Once the workshop is wired up, can I make my own modifications?
A: The regulations still apply so you can only make non-notifiable changes, see above.
Q: Do I need an RCD?
A: Yes, your electrician should provide one either via a socket or in a small consumer unit.
Q: I don’t want the lights to go out when my supply trips.
A: Hopefully your supply won’t be tripping if it’s setup correctly but it’s possible for the electrician to add a small consumer unit in your workshop so that over current problems with the sockets won’t trip the lights. Be aware though that tripping the RCD on the workshop may also trip the RCD on the house, this is typically unavoidable.
Other special considerations?
Ref: Wiring Matters Issue 16
Q: My workshop is steel framed or I have pipes / large metal items in the workshop, do I need to advise the electrician?
A: Yes, this will require extra work as they will need to earth bond these items. Small portable appliances such as a power drill should not need bonding, larger lathes and mills will likely need earth bonding.
Q: I’m thinking of buying a large machine will this cause additional issues?
A: Yes, an inductive load such as an inductive motor can affect the power factor of the supply, this will have potential effects on the your workshop supply such as increased voltage drop over the cables and increase your power consumption. This can be resolved with additional components called Power Factor Correction which are typically in the form of capacitors added to the circuit.
Q: Can I use an extension cable?
A: It’s acceptable to use an extension cable for temporary use or for portable tools but not for a permanent installation.
Q: My drill/lathe/mill advises using a slowblow fuse but I’ve got MCB (minuature circuit breakers), what do I need?
A: There are different types of MCBs, advise your electrician of your requirements and they can fit a type C or other appropriate breaker.
Q: My lathe/mill is three phase how can I supply it?
A: You have a couple of options with regards to three phase supplies. If you have a large workshop then you could spend a lot of money and get the supply company to provide you three phase to your property. The alternative is to get a box called an inverter.
Q: What’s the choices for inverters?
A: There are different kinds, some can vary the output hence providing speed control. Static inverters or capacitors can make your equipment work from a single phase supply but are not really powering all phases. The alternatives generate the other phases either using clever power electronics (digital phase converters) or via a small motor and switches (rotary phase converters).
Q: What factors should I think about?
A: Things to consider, cost, speed control, current, space requirement, number of devices to be powered.
If you want to know the full detail of what is allowed then you will need to read
IEE Wiring Regulations 17th Edition : (BS 7671: 2008) but there are also many simpler guides to the wiring regulations.