POWER SUPPLY TO WORKSHOPS

[halken]

My workshop will be built in the next couple of weeks and i thought i could just run an extension cable from the house to run my power tools but ive read that machines over a certain power rating should not be used with extension cables. This is going to be a problem as my workshop will be a 16 x 8 concrete garage about 20ft from my house, how much would it cost to get electric hard wired into the garage and does it make a difference to ex cables. Ive got 2 dewalt radial arm saws and a record band saw and a dust extractor that might be the problem, my other power tools ive run on extensions and never had a problem with them. i am concreting the base on monday should i put a duct to carry the cables in the base or will i not have a problem with ex cables
Kenny

 

[chippy1970]

I ran 6mm armoured cable to my workshop buried in the ground I then have a 2.5mm twin and earth circuit for the sockets and 1.5mm for the lighting coming from a small two way consumer unit with rcd.

The armoured isn't as expensive as it used to be, get an electrician to do it for you if you're not confident as it all needs to be done to regulations so it is safe.

 

[davy_owen_88]

Unfortunately too many people think that extension leads are the solution to these sorts of scenarios. The problem with extension leads is that the conductors used are far to small to combat the voltage drop caused when you start up a large machine.

When a motor starts is can draw in excess of 2x it's rated load, and with this comes a large drop in voltage. If this voltage drop is excessive the motor will probably just fail to start. If you keep trying the motor will overheat.

Luckily for you, 20ft/6m isn't too far and the SWA cable is what tends to bump up the price. If you want to do this work yourself (It is notifiable remember) then get yourself some:

10mm² 3-core SWA
Some external glands
Warning tape
A handful of 5mm brown, blue and green & yellow sleeving.
A nice metalclad consumer unit
Main switch
RCD for sockets
6A MCB for lighting
16A MCB's for any large machines
32A MCB for the socket circuit
An adaptable box
And some grommets

Oh, and some largish waste pipe for the duct.

Bury the duct at least 400mm deep, insert a few draw lines, backfill 3/4 and then lay the warning tape and cover over. Make sure you have enough SWA to go from somewhere within your house to where you intend on placing the consumer unit in the workshop. You can switch from SWA to twin and earth as soon as you get into the house to save a few quid which is where the adaptable box comes into play. Mount it somewhere suitable and terminate the SWA into it using the appropriate glands. Then using a suitably sized terminal strip join SWA to t&e and connect the t&e to your consumer unit using a 40A MCB.

I can talk you through the rest of the process if you wish but my hands are starting to hurt :lol:

 

[halken]

thanks for the replies. I think i need an electrician as i have no experience with electricty, one more stupid question does the feed have to be from the house consumer unit which is at the front of the house or could it be taken from a spur from the kitchen which is at the back of the house where the workshop is
Kenny

 

[Fecn]

does the feed have to be from the house consumer unit which is at the front of the house or could it be taken from a spur from the kitchen which is at the back of the house where the workshop is
Kenny

It'll have to come from the consumer unit because the wiring that currently goes to the kitchen won't be able to cope with the extra load.

 

[woodbutcher]

it will also need an rcd breaker unit because it is out side the house and the power could be used in the garden.


woodbutcher.

 

[MikeBarnett]

As I understand it- It's notifiable work- meaning the local council gets/should get notified.

An electrician would take the feed from the consumer unit in your house. A new circuit would be added to that unit if there is space for it and if it is safe to do so. If not you may need a new consumer unit.
The new circuit will be protected by a breaker fitted in the house consumer unit. This breaker will be selected according to the load characteristics. There are different types of breakers. If you are running hobby workshop stuff it will be a normal domestic breaker rated according to the expected load. Eg you can run roughly 5kW absolute max from a 32 amp circuit. You may need a higher rated breaker if you are going to run more (or if the design of the circuit demands it), and you may just possibly need a different type of breaker that won't trip if you have the kind of (industrial) motors that create large starting currents.
Cable selection- Steel Wire Armoured (SWA) cable can be buried in the ground. The depth needs to be deeper than any forseeable disturbance. The size of cable (cross sectional area) depends on the current to be drawn and a volt drop calculation. (longer cable runs cause volt drops- volt drops above 5% aren't allowed- solution, fit a bigger cable, less resistance, less volt drop, but maybe now you have a bigger cable you need to do other stuff too)
At the workshop end, you'll need a small consumer unit selected for the environment- steel clad, in other words. Inside will be an RCD and two or three breakers for the circuits in the workshop. You need the RCD because you are outside the equipotential zone (the house).
So you have a breaker in the house feeding an RCD and breakers in the workshop. It all has to work together, be carefully protected by the correct breakers and RCD and comply with the regulations (BS7671, a big book about an inch thick). If you change cable from SWA to twin and earth when you get to the house as has been suggested above my understanding is that you need to fit circuit protection (a small CU and breakers, maybe an RCD, not sure) there too, at the point where the cable changes.

There are two ways to do it- 1. Get an electrician who is registered with one of the trade bodies and will give you the proper test certs and notify the work properly.

2. Do it yourself, but legally. It means getting advice from your local building control dept at the council- usually they will approve your installation plans, and test it for you after it's fitted. They charge though. If you want to read up on it try Basic Electrical Installation Work by Trevor Linsley.
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[davy_owen_88]

you can run roughly 5kW absolute max from a 32 amp circuit.

Nope, 32A x 230V = 7360W.

volt drops above 5% aren't allowed

It's actually 4% - 230v*0.04 = 9.2V maximum drop.

If you change cable from SWA to twin and earth when you get to the house as has been suggested above my understanding is that you need to fit circuit protection (a small CU and breakers, maybe an RCD, not sure) there too, at the point where the cable changes.

No need so long as you keep the cable size the same. Fusing down only needs to occur when you change to a cable with a lower current carrying capacity. You also would not need RCD protection at the connection.

 

[OJ]

when you're calculating your design load, make sure you account for diversity. You will require less current than you think, ie it's not just the sum of all the loads that you have. I would just duct under the walls as the SWA will be derated if it is in a duct the whole way. It is normal to lay the cable directly on an inch of blinding sand (above and below) to protect it from sharps. cheers, O

 

[davy_owen_88]

SWA has the same current rating whether buried direct or buried within a duct so it is always better to fit the ducting.

 

[MikeBarnett]

you're right about the volt drop it's 4 not 5%, it was a late night post- otherwise as I understand it when the method of installation changes in that way, ie from twin +earth to SWA you need circuit protection even if the cable cross section stays the same, as the cable rating will change. I could be wrong though.
I think you'd be asking for trouble running 7Kw from a 32A circuit. In practice cable rating factors based on installation methods and type of cable reduce allowed currents drastically. I don't think you can do that simple calculation (which in any case is for DC) and start nailing cable up or burying SWA.

Maybe I'm too cautious.

 

[davy_owen_88]

otherwise as I understand it when the method of installation changes in that way, ie from twin +earth to SWA you need circuit protection even if the cable cross section stays the same, as the cable rating will change. I could be wrong though.

You don't need to fuse down as long as the current carrying capacity of the cable you are changing to is equal to, or higher than the last protective device in the chain. If you have a 32A MCB in the consumer unit, and the CCC is 32A or more then you don't need local protection when changing between cables. Only when the CCC drops below the MCB rating does further protection become necessary.

I think you'd be asking for trouble running 7Kw from a 32A circuit. In practice cable rating factors based on installation methods and type of cable reduce allowed currents drastically. I don't think you can do that simple calculation (which in any case is for DC) and start nailing cable up or burying SWA.

A 32A circuit is rated at 32A because the cables and fittings are selected to provide that load. If it wasn't suitable a smaller MCB or a larger cable would have been used.

Ohms law applies directly to any purely resistive load, not just DC.