Installing Workshop Electrics archive

Tuesday 15 June 2010

Alan Holtham discusses the installation of your workshop power supply


Wherever your workshop is sited you will need an electrical supply, but the exact method of providing this varies with the location. The size of supply you are likely to need will determine the cabling required so list your machines and their loadings, and work out how many you are likely to be using simultaneously.

I sometimes have the saw and planer running together if I am, say, ripping consecutive strips off the edge of a board and re-planing each time, and have the big extractor running as well. Bear in mind, too, that if you anticipate employing people there could be even more demand, so the supply has to have plenty of spare capacity.

Also remember that although the machines will have a stated full-load current FLC at a given voltage on the motor plate, you will always exceed this during start-up.

You can calculate normal current with the formula: Amp A equals Watts W divided by Volt equals 5.3A which equates to 3.06A per phase if you take account of the RMS value.

On start-up, this motor will actually draw about 50A from the single-phase supply, but this depends on the initial load. It may only be momentary, but it could be longer and has to be factored in to the supply.

As a guide, a small workshop will probably have something like a 2.2kW saw, a 1.5kW planer, a 1.0kW bandsaw and a 0.75kW extractor. You will also need to allow 3kW for power tools and other small machines, 2kW for electrical heaters and 0.5kW for lighting, so you can see it is all now beginning to add up, although of course it will not all be running at once.

Extending supply

An existing attached garage will probably have a token 13A plug, but be careful of extending this as it is probably just a spur off the existing downstairs ring main and will need beefing up with a dedicated workshop circuit.

For a shed or outbuilding you will have to get the supply from the house. The normal recommendation is via a cable buried underground, but this must be of the armoured type SWA that is resistant to mechanical damage and must be buried sufficiently deep with warning tape on top of it.

A 4mm cable is the minimum recommended size, but I would recommend at least 6mm to allow for further expansion. The length of the cable will also have some bearing on the size, as longer cables will need to be thicker to account for the likely voltage drop.

The common solution is to take the power from a spare way in the existing house consumer unit, to a small two-way distribution board in the shed or garage.

A suitably sized circuit breaker needs to be fitted in the house consumer unit along with a residual current device RCD, usually at either end, to protect not only you but also the cable linking the workshop.

This RCD protection needs careful balancing as you do not want nuisance tripping in the house, and should be set up so that the trip for the sockets does not trip the lights.

Extending an existing ring main with a spur is not ideal, but if you go down this route bear in mind that the additional demand could easily overload the ring. If you do not have any spare ways in your domestic consumer unit you may have to fit a bigger one. You will also need to ensure that the earthing and bonding arrangements are up to standard so take proper advice on all this. It is not a DIY job.

Consumer units

The standard garage 2-way consumer unit normally has a main RCD along with a 6A breaker for the lights and a single 16A breaker for the socket circuit. Personally I would prefer to fit a bigger consumer unit so I can install separate 16A circuits for some of the bigger machines that are too big to start on a 13A plug. A 4- or 6-way consumer unit is better, with a 32A breaker for the socket ring main, one or two 16A circuits for bigger machines and a 6A breaker for the lighting circuit.

If you need 3-phase your workshop is usually much bigger and may have its own independent supply. In this case you should then have plenty of capacity. Separate single-phase rings can be taken off the different phases to spread the load equally. The alternative for one-off machines is to go down the converter route.

Circuit breakers

Whatever the type of supply, it should be fitted with miniature circuit breakers MCBs to protect the circuit and tools. Older workshops may still use either traditional wired fuses or cartridges in the distribution unit, but these should be backed up with some form of RCD protection.

RCDs are designed to protect people from shock or electrocution and work by detecting any imbalance between the live and neutral wires, cutting the supply within three milliseconds. RCDs have now replaced the old Earth Leakage Circuit Breakers ELCB which were not as reliable.

Modern domestic MCB consumer units will be fitted with type A or B breakers which trip if the current through them exceeds a preset threshold.

As we have already seen, however, most motors will momentarily draw a bigger current at start-up which is probably enough to pop the breaker, so you may have to fit a motor-rated or type D breaker that has a higher time threshold before it trips. Similarly, for an old distribution board you will need type D fuses.

Sockets and cabling

When you come to work out how many sockets to fit, start by listing out the power tools you are likely to use regularly, remembering that it is much easier if the popular ones can be left plugged in permanently. If you have to mess around finding a spare socket and plugging it in, the temptation is not to bother.

Even a small workshop may have a dozen or so power tools, so the more sockets you can fit the better. You can also then plug in where the cable is least likely to cause a trip hazard and avoid overloading sockets by using multiple adapters.

My own workshop has 18 double sockets with 15 tools and machines permanently plugged in, and this does not include the turning workshop.

To minimise confusion, label each plug to make sure you pull out the right one. It is easy to pull out the wrong one, when changing cutters for instance, and think the machine is isolated when it is not, particularly as most modern plugs all look the same.

Always ensure plugs are fitted with the right size of fuse. Many power tools are adequately protected by a 5A fuse and do not need a 13A one. Just match the rating to the power of the tool.

Overhead sockets are excellent for minimising trailing cables, particularly for jobs like routing or sanding where you need to work around the job. A line of overheads down the middle of the workshop is ideal and I have a few over my workbench as well.

Keep power tools accessible and yet easy to store by cutting the power cable about 12in from the tool and fitting a 3-pin connector. Then you can leave the wire in position permanently and just hook up to the tool you need to use.

Charging station

For inaccessible tools, or machines like an extractor that you may want to house outside the workshop, the plug can be operated remotely by a wireless control.

As the manufacturers of cordless tools have yet to settle on a standard battery configuration you end up with several different chargers. A dedicated charging station with plenty of plug sockets is the best way to ensure that batteries are kept fully charged.

If your workshop is small and you often work outside it is worth fitting an external waterproof socket to save having to use long trailing extensions.

Wiring between sockets

There are numerous options for the actual wiring between the sockets. Unless you are building a workshop from scratch and can bury the wiring in the plaster or behind any cladding you will probably have to run the wire externally, using cable in conduit for maximum protection. The circular type is very cheap, easily fitted and quickly altered if you change the layout.

Metal-clad sockets work well with this and are much stronger than standard domestic plastic sockets which are easily smashed in a tough workshop environment.

Whatever sockets you use I cannot over-emphasise the need to put in plenty, banking them up if necessary in busy areas like the workbench.

The only problem with lots of sockets is making sure everything is turned off. As a safety measure, I have a large isolator by the exit to my workshop that cuts the supply. I also have a small bypass circuit for the alarm.

Woodworkers Institute

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Alan Holtham , Shop Talk , workshop series , electrics

Useful Links

Building Regulations at
Health and Safety Executive at
British Standards Institution at


Screwfix at

Legal And Safe

In England and Wales most electrical work comes under the umbrella of Part P of the building regulations, so should be installed by a certified electrician. Part P demands that electirical work is carried out by a competent person. Professional electricians should be certified and can issue certificates guaranteeing that their work meets the required standard. Householders can carry out certain simple tasks without certification, like replacing fittings and cable, adding additional lighting to an existing circult, but not in kitchens and bathrooms, adding sockets and fused spurs to an existing ring or radial main, and installing earth bonding.