Workshop Electrics

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.

Ref: Wiring Matters Issue 21 and Wiring Matters Issue 18

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.

Three Phase
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.

22 thoughts on “Workshop Electrics

  1. Having re-read the IET documentation the way the RCDs are wired up means that your workshop RCD should not actually trip the RCD in the house.

    “Where two, or more, RCDs are connected in series, discrimination must be
    provided, if necessary, to prevent danger (Regulation 531-02-09 refers). During a fault, discrimination will be achieved
    when the device electrically nearest to the fault operates and does not affect other upstream devices.”

    The electrician who’s installing your circuits should understand this requirement.

  2. Jamie Morgan says:

    It’s really important to stress, that installing electrics aren’t just a case of picking the cable from the van and throwing it in. Depending on the distance from the property and the loading the circuit will need to be designed, there’s a difference between a single light and a freezer in a small 6×4 close to the house, than a garden office with computers, lighting and heating 200ft from the house. We have also been involved in projects for garden gyms, saunas, spas and hot tubs.

    All works in the garden, kitchen and bathroom are classed as special locations under BS7671, so work must be carried out by a qualified, insured and licenced electrician and submitted to the local authority for approval.

    Jamie Morgan MIET BMEI
    Electrical Engineer
    ESI: Electrical Engineers & Contractors Ltd

  3. Jamie Morgna says:

    The prices for s-type Rcd’s would put most people off having any form of discrimination in their installation.

  4. Jamie, thanks for your comments. Does work inside the garden office also class as a special location?

  5. Yes it would class as work in a special location, as its outside of the usual safety protections of the ‘equipotential zone’ or in lay mans terms, in a house, more than one service are interconnected – gas, electrical, water, so that the pipework of these provides additional earth routes, these aren’t there in an outbuilding, so if the earthing fails in the garden, or an outbuilding, you become the shortest route for the earth fault current.

    The risk of electric shock when things go wrong in outbuildings or gardens is so much higher, thats why its been classified as a special location, requiring part-p notification.

  6. cheers, thanks for the clear explaination.

  7. […] Anyway, to summarise what I’m saying, it’s a good idea to have MCB’s connecting up the power in the workshop. For a similar reason a RCD is also a good idea, see article on workshop electrics. […]

  8. […] heavy rain and sun the lawn is almost back to normal. Following the digging of the trench for the workshop electrics we back filled the hole, watered the soil so that it would bed down and laided the turf back on […]

  9. […] also:Workshop ElectricsNo Volt Release SwitchThe RCD explainedTypes of […]

  10. Staci Bryans says:

    Nice read, I just passed this onto a colleague who was doing some research on that. And he actually bought me lunch because I found it for him smile Therefore let me rephrase that: Thanks for lunch!

  11. Felipa Wadkins says:

    This really answered my problem, thank you!

  12. Garry Bontemps says:

    Hello Andy! Great Blogpost! Thanks for this article, very good information, I will be forwarding this to some friends if you’re ok with that. Greetings from Germany!

  13. Dortha Briddick says:

    Very interesting information!Perfect just what I was looking for!

  14. Franco says:

    I want to run 200 amp tig welder from my shed what power would I need for this mainly for welding aluminium small stuff 2″ being the maximum I weld up to

    • Hi Franco, it does depend on the spec of your machine but typically a 200 amp TIG welder would typically need a 16A input supply. Check the manual for the peak input current. This is higher than domestic equipment is rated for so you’d need to get an industrial socket fitted. Your electrician should be able to fit a suitable socket and cable. They’d also likely fit the matching plug on your welder too.

      • Franco says:

        Thanks for the quick response
        I have a small timber shed but I will be upgrading when I can, would a small timber shed be suitable?

        • The key requirements for TIG welding are free of draughts, good lighting, none welders are shielded from arc and fire safe. You will also need space for the materials, work piece and equipment.

          So a wooden shed with a concrete floor and plenty of space could work. You could protect the walls from sparks with a welding curtain.

  15. robert brett says:

    fabulous write up thanks for posting. Rob

  16. dave cooper says:

    workshops can you installplastic trunking and conduit. or must it be metal trunking and conduit

    • Hi Dave,
      the intention is to protect the cables from likely damage. So if your activities are similar to a house/office then domestic trunking would be appropriate. If however you are completing woodwork or metal work and might have sparks, grinders, cutting tools etc then a heavier duty protection would be advised. In my case because we were running SWA down the garden it made sense to use that along with metal sockets to provide the power in the Workshop.

  17. Sam Jarvis says:

    Thanks for this. Came across this when looking for solutions to a voltage drop issue.

    I’ve just hooked up a dust extractor that pulls about 6A@230v. It runs fine on its own, but more often than not, when starting another machine with a high startup load, like the table saw (induction) or thicknesser (universal motor), the overload in the DOL starter on the dust extractor will trip. Almost certain this is due to voltage drop causing the extractor’s motor to draw over the overload’s rating. The lights certainly dim anyway. I wonder if implementing some form of power factor correction would aid with this? Would that aid with voltage drop (throwing capacitors into the circuit must have some relevance)? There is a long run out to the shed, approximately 30m of 6mm SWA, which I’m aware would be a source of voltage drop issues. I have the option of laying another length in parallel or replacing it with 10mm if that would help significantly enough.

    Thanks for any guidance you can…

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