small workshop - new ringmain

[digger1410]

I am creating a 3 new ringmains for a workshop.

Each will have about 5 double sockets and up to 3 small machines wired through a 20amp fuse.

Silly question time.

Although not all machines will be running all the time, there will be a small occasion when they will be. 5 mins max.

If 2.5mm T&E only carries 20amp on a 32A fuse in CU, what cable would be safe?

 

[OwainDIYer]

You cannot have machines on 20A fuses wired onto a standard 32A ring final circuit.

You do not wire ring final circuits on 20A fuses.

Please tell us exactly what you want to do, or get someone qualified to do it for you.

If this is a commercial premises then the Electricity at Work Regulations apply along with power machinery rules. Have you considered emergency switching, power braking if the mains supply fails, and emergency lighting?

 

[ericmark]

The ring main is normally considered as type tested and normally one considers that the load will be distributed so 20A is drawn at centre and 12A even throughout the ring main and the 2 x 2.5mm cables are considered to be rated at 2 x 20A or 40A so using a 32A MCB or a 30A fuse is considered to protect them.

There are exceptions and if the ring main had the full 32A load placed on it very close to the origin then it could result in an overload on the short leg but the outlets would have to be grouped very close to the origin and normally is outlets are required close to the consumer unit then spurs or radials would feed then and they would not be part of the ring main load.

To help prevent this type of problem any items of 2Kw or over are not permanently connected to ring mains. Immersion heaters for example will have a radial to feed them.

So with a garage the lathe, welder, compressor and vehicle lifts will be connected to radials. In fact since most plug in items will be 110v split phase one often has no ring main in a garage. Since most garages are three phase often sockets are split across the phases but consideration has to be given to how close each phase is and often split so each phase feeds an area where it will be unlikely items used in one area will be used in another but there are no rules as such it is just common sense.

Where high power is used there are many considerations including power factor and really the wiring of a garage is not a DIY job. However often they are wired before all the machinery is installed so the designer has to do some guessing as to what is required but as long as the cable has a greater current carrying capacity to the protective device worse that can happen is the protective device opens. (Fuse blows)

 

[big-all]

how far is the workshop from the house distribution board ??

 

[ban-all-sheds]

I am creating a 3 new ringmains for a workshop.

Is the workshop:

■ in or attached to a dwelling;

or

■ in the common parts of a building serving one or more dwellings, but excluding power supplies to lifts;

or

■ in a building that receives its electricity from a source located within or shared with a dwelling;

or

■ in a garden or in or on land associated with a building where the electricity is from a source located within or shared with a dwelling

?

If so, when you applied for Building Regulations approval what did you say would be the way that you'd comply with Part P?

Each will have about 5 double sockets and up to 3 small machines wired through a 20amp fuse.

If you were allowed to do that (you're not), what sort of 20A fuse would you use, and where/how would you install it?

If 2.5mm T&E only carries 20amp on a 32A fuse in CU, what cable would be safe?

Where are you going to get a 32A fuse? What sort of fuse is it, and what sort of CU uses them?

If a circuit is protected by a 32A device why can it only carry 20A? What would limit it to that?

Do you know what tests you would carry out on the circuits - what sequence you'd do them in and at what point you would energise them? For each test do you know what is being measured, why it is important, how you would carry out the test, and with what equipment, and what sort of results you would expect to get if everything was OK?

Do you understand how the way in which you install cables affects how much current they can carry?

What are the rules concerning cables concealed in walls, partitions and under floors?

Does the workshop have its own DNO supply, or does it come from another building elsewhere? If the latter, do you know the rules for cables run outdoors, buried in the ground or overhead, and do you know what type of supply you have and whether you need to consider the issues of exporting a TN-C-S earth to a building with extraneous-conductive-parts?

 

[Stoday]

When you applied for Building Regulations approval what did you say would be the way that you'd comply with Part P?

Why do you think part P applies? Why do you think he should he obtain BR approval?

There's nothing to suggest that the OP is not renting a workshop that's independently supplied or is supplied from a non-domestic supply.

 

[ban-all-sheds]

You're quite right - that was a leap too far, assumption-wise, particularly given my later question about the supply.

As the OP has not returned to the topic I'll change it - not to cover up my mistake (it's still there in your quote) but to avoid distractions over the rest of the questions, which are valid and important wherever and however the workshop is supplied.