Workshop electrics. What do I need?

[OLD]

My suggestion protects the swa cable yours keeps lights etc on when house rcd trips .Note shed rcd trips at lower current than house so no interaction.Above is covered by discuss with sparks or is this a diy.

 

[garywayne]

Thanks Old.
This is a DIY job, but I am going to get it checked by a Part P sparks.
I am having trouble finding out how to calculate what size MCB to use in the house CU, what size T&E cable, and SWA, and what size RCD in the shed CU.
In the previous link that I mentioned before, the bloke suggests using waterproof junction boxes on the outside wall to connect the T&E to the SWA.

 

[jlawrence]

To find out what size cable you need use this calculator
Likely though you'll be fine with 6mm.

 

[flying haggis]

I am going by what I have read.

"Armoured cable is expensive and has to be terminated at a special junction box at each end of its run, where it can be joined to ordinary PVC-insulated cable".

not true

What are you saying is not true?

armoured cable does not have to be terminated at a special junc box at each end, it can be terminated into the CU, and it is not that expensive per metre although it is more expensive than T+E.

As an aside if this cable is going under ground consider adding a conduit in the trench about 3 to 4 inches away from the SWA for cat5/6 cable for computer use

 

[SemiSkilled]

waterproof junction boxes on the outside wall to connect the T&E to the SWA.

Why d'you want to connect t&e to the swa?

I use metal adaptable boxes, fix the gland to the box and leave the cable long enough to go through the box and connect to your consumer unit.

I'd prefer to use a henly block, into a switch fuse, SWA out to consumer unit in the garage and use individual RCBO's in the CU.

leaves the house on the original set up and if owt trips you don't take out circuits that don't have faults, ie if a dodgy drill trips the sockets you'll still have lights

You need to work out your maximum demand in the shed to size the mcb in the house,

Talk to the spark, and its a good idea to get him to terminate the swa

Off the top of my head using a biggish saw 16A, a Dust collector 16A, some strip lights and a bit in reserve you'd need a 45 A supply
( The motors don't take a lot when they run its the starting up, oh and when you calculate power requirements for discharge lighting ie strips, you use a multiplier of 1.8
Talk to the spark as you might not have a big enough incoming supply

If that 45 A sounds a lot, I had a 16A to my garage, fine, then I bought some 3 phase gear the converter needs a 32A supply :shock:

If you buy a bigger saw or PT better to have it and not need it than to nee...........................

Lee

 

[WoodnDesign]

Hi,

I've to get my workshop wired later in the year, I'd had a price for a cable runout, in the trench I'm to dig.. CU, 2 double sockets, double 6' light and fuse..
As a turner, I could or will need those sockets in full use.
I contacted the sparks with a list of all my equipment an there ratings, and that a lathe, vac, worklamp and a powertool would be used all at once, lights and amb. filter, if not something else.. He has now gone to 10mil cable.. no idea of how many extra sockets/lights... :!: .... and about twice the first price... :shock: ....

Thanks to you all and the links, I'll have a better idea as to what will be needed and what he maybe doing or not, when you have to rely on someone to do the job due to the new P regs.. I have in the past worked and done some electrics, but for this I want to be safe...

Regards. David.

 

[Jake]

waterproof junction boxes on the outside wall to connect the T&E to the SWA.

Why d'you want to connect t&e to the swa?

Because it is (a) cheaper and (b) much more pliable for internal runs.

Edit: just checked I am wrong on (a) - at 6 and 10mm SWA is cheaper or than T&E at TLC! Even 10mm2 three core (to do a belt and braces job if exporting the earth is viable) is only 25p/m more per metre.

(b) still stands, depending on where the cable is running (though it might be irrelevant if it is a straight surface mounted run).

 

[Jake]

10mm sounds sensible to me, but if DIYing you need to start with a sensible assessment of the max current you are going to want to draw (in winter, all lights blazing, any electric heating on and your biggest machine plus extractor plus anything else that you might have on at the same time) and the actual distance the cable will run and do the voltage drop calcs properly. You also need to work out the earth loop impedance calcs to work out your earthing arrangements.

This is not simple and you'll need to do some proper research and thinking - please do not go with a 50A breaker on 6mm cable for starters.

 

[Hobbyshop]

I am also planning a workshop build for this year and I have been researching the electrical requirements quite extensively. I have written out my conclusions which I will document here for comment. I am afraid it will be quite a long post.

BUT PLEASE NOTE THIS DISCLAIMER: I AM NOT A QUALIFIED ELECTRICIAN AND WHAT I DOCUMENT HERE IS MY INTERPRETATION OF WHAT I HAVE READ. I WILL BE SEEKING PROFESSIONAL ADVICE AS WELL BEFORE I PROCEED AND WOULD RECOMMEND THE SAME TO ANYONE ELSE.

I POST THIS SO THAT PEOPLE MAY ADD TO IT OR CORRECT IT FOR MY BENEFIT AND THE BENEFIT OF OTHERS PLANNING THIS TYPE OF WORK.

Running new Sub-Main Cable.
Wiring regulations were changed in 2008 and now any cable that is not suitably protected against mechanical damage must be protected by a Residual Current Device not exceeding 30mA.
Suitable protection may be provided by earthed metal sheathing (SWA, metal conduit or metal trunking) or by burying in a wall at a depth greater than 50mm.

So if you run your sub-main cable in SWA all the way from the existing domestic supply to a new CU in the outbuilding then the cable does not need to be RCD protected. Note that the armour must be earthed so the cable will need to be properly jointed with the appropriate glands.
If part of your sub-main is made up of T&E and this section is not protected with earthed metal conduit or trunking or buried to >50mm in a wall then the supply will need to be taken from a RCD protected side of the domestic CU with RCD protection not exceeding 30mA.

This seems to answer the question as to whether supply should be taken from a RCD protected way or from a non-protected way.

The PDF guide from here proved useful:
http://www.hager.co.uk/service/downloads/guides/17th-edition-consumer-unit-guide/5632.htm

Size of Cable and size of Circuit Breakers.

First it seems to be necessary to estimate the maximum power requirement. As an example in my case I am looking at a single user workshop so I assumed that only one tool would be in use at any one time but other ancillary devices would also be in use. I estimated the highest power consuming tool was likely to be a table saw and using the SIP 01332 as an example I took the power requirement for the saw to be 2.2 KW.
I estimated the total power requirement as follows:
Table Saw 2.2KW
Extraction 2.0 KW
Heating 1.0 KW
Lighting 0.5 KW
Total 5.7 KW which I rounded up to 6 KW as my estimate

Together with a known cable length, I plugged this 6 KW requirement into the calculator shown in jlawrence’s link above and got the following result:
Required Cable Size 4 mm
Voltage Drop 7.83 Volts.
Percentage Drop 3.4%
Current Load 26.0 Amps
Max Cable Load* 36.0 Amps

On this basis it seems I would be safe to install 4mm cable protected by a 32 amp MCB

However, to allow for any future unforeseen power demands I have decided to install 6mm cable.
I increased the power required incrementally in the Voltage Drop calculator until it showed me a requirement for 6mm cable, and I was then able to identify the max. allowable cable load for the 6 mm cable in my case of 44 amps.

From this I decided I would be safe to install 6 mm cable protected by a 40 amp MCB

Workshop Circuits
I decided to install a 32 amp ring main, a 6 amp lighting circuit and a 16amp radial circuit for the saw.
Prior to the changes in 2008 the lighting circuit would not have required RCD protection but since I intend to surface mount all cables and fittings then all the circuits will need RCD protection as they will not be protected against mechanical damage.
The PDF guide I provided a link to earlier indicates that protecting all the circuits from a single RCD would not comply with the current regulations so it seems I will need to provide split RCD protection. In my case I will probably opt for a consumer unit in the workshop with RCBOs on each circuit.

Termination of SWA Cable
The Steel armour has to be earthed to comply with the requirements and I found some useful information by Googling for "Terminating SWA Cable".

Is it okay to have RCD protection at both ends of the sub-main?
I have seen this discussed in a number of forums and from my research I have come to this conclusion:
The only concern over having a RCD at both ends of the system is how you determine which device will trip first. For example a fault on the 16 amp circuit could possibly trip the main consumer unit RCD rather than just tripping the 16amp RCBO and this would mean losing lighting as well as power, creating a potential hazard, and some inconvenience.
The regulations are written to cover much larger installations and this issue becomes a serious problem in certain circumstances. In a hospital for example you would not want the operating theatres to lose power through a trip caused by someone cutting the lawns. In a single use workshop it is a lower priority and safe protection of the sub-main cable takes priority over the inconvenience of total power loss.
Of course if your sub-main cable is properly protected against mechanical damage over it's full length then you can leave out the RCD at the supply end and eliminate the inconvenience.

Hopefully others will be able to confirm or correct my conclusions (if anyone has the patience to read it all )

 

[SemiSkilled]

Might just be me but I prefer SWA in workshop environments as it is tough enough to withstand knocks etc, T&E is a bit fragile

The other thing about SWA is it can carry a bit more and if you use the wire armour as the earth you get a really big cross sectional area and you can get the Zs right down and meet the specs for your breakers.

I 'spose I over spec jobs as I don't pay for the stuff :lol:

Just today at work someone decided to swap our compressor for one twice the size, now needs a bigger supply, £230 in bits and 3/4 of a day

Lee

 

[jlawrence]

I've read that bit about less than 50mm depth needing RCD protection.
My sparky said that they almost never put lighting circuits on the RCD side.
If the 50mm depth is completely correct (I'm not saying it isn't), then every circuit in a dwelling would need RCD protection. EVERY single circuit in a house is at somepoint not buried 50mm deep - take the point at which it connects to a socket or lighting point for example.

When I replace my connection to the workshop I won't be taking it from the RCD side. Like others I don't want the lights going out if (when) I trip something in the workshop - either that or I'm going to need to look at putting an emergency light in (which may not be a bad idea really).

 

[Hobbyshop]

I've read that bit about less than 50mm depth needing RCD protection.
My sparky said that they almost never put lighting circuits on the RCD side.
If the 50mm depth is completely correct (I'm not saying it isn't), then every circuit in a dwelling would need RCD protection. EVERY single circuit in a house is at somepoint not buried 50mm deep - take the point at which it connects to a socket or lighting point for example.

When I replace my connection to the workshop I won't be taking it from the RCD side. Like others I don't want the lights going out if (when) I trip something in the workshop - either that or I'm going to need to look at putting an emergency light in (which may not be a bad idea really).

Yes the document in the link I gave suggests that following the 2008 changes it is likely that all domestic lighting circuits will now need RCD protection, which was not the case previously.
In my case I will have to take supply from an RCD protected way on my consumer unit anyway, so I will probably add a couple of emergency light fittings too.

 

[DavidE]

There are probably a few points that could be picked up in this thread - if you are unsure it's best getting an electrician involved. That said...

When doing load calcs you need to consider voltage drop at startup not just the continuous demand.

If steel wire armoured cable is terminated into a junction box the sheath can be stripped back as others have said and the inner cable run to wherever.

If you get a steel wire armoured cable and connect a copper core to earth you must test the continuity of the armoured sheath to earth - otherwise you have a potentially dangerous installation. As SemiSkilled said you get an equivalent earth (and better depending on the cable size) in the steel wire armour providing it is properly terminated.

I've read that bit about less than 50mm depth needing RCD protection.

If the 50mm depth is completely correct (I'm not saying it isn't), then every circuit in a dwelling would need RCD protection. EVERY single circuit in a house is at somepoint not buried 50mm deep - take the point at which it connects to a socket or lighting point for example.

With the introduction of the 17th Edition regs (nothing to do with Part P) RCDs feature a lot so it is fair to say most circuits will require RCD protection. As an earlier poster said you in some cases you can mitigate it with other factors. However, there are other reasons they are required (not just mechanical protection). I've not got my copy of the regs with me so I'm not going to try to quote it (and it's best to get some professional help with your specific installation).

When designing circuuits with RCDs if you have two of the same trip current/time they will go together when there is a fault. You can get RCDs with higher trip currents/trip times to get proper discrimination. Of course the regs need to be checked to make sure you're doing it correctly.

As another thought to the RCD problem - if you want to keep it simple with an RCD and the lights are on it - stick an emergency light in the workshop (battery backup if the power goes off) they are quite cheap and will give you enough light to find your way out/get things back going. They'd also be good if you are in an area prone to power cuts!

Edit - Kevin you typed that at the same time as me!

I hope this helps
David

 

[matt]

I don't get why you'd NOT want to use RCD protection for a cable carrying a high current on a high AMP fuse in a potentially vulnerable route.

I get the thing about the lights going out, but then so would everything else in the workshop so even a turning blade on a table saw, whilst still having some inertia, is no longer powered. But, on the other hand, damage to the cable or more likely a joint, or perhaps just a simple fault somewhere along the line, and you're gonna take quite a shock.

If light really is an issue then I'd go for emergency lighting every time.

FWIW - I have had RCD's at both ends of a circuit before and one RCD tripped the other (i.e. there was no fault - it was simply the presence of the RCD).

 

[DavidE]

I don't get why you'd NOT want to use RCD protection for a cable carrying a high current on a high AMP fuse in a potentially vulnerable route.

Just for background infomation on what RCDs are used for...

RCDs primarily provide shock protection for humans/livestock who come into contact with the live whilst earthed. There are also used to provide fault protection should an earth fault occur on a circuit where the fuse would not blow within the stated disconnection time. This is mainly for the sake of shock from the raised voltage on the earth of the circuit and the potential fire risk.

Therefore, the size of cable and current don't really come into it. If you touch a live wire whilst earthed no fuse is going to help you. The RCDs on sockets typically trip at 30mA very quickly. A fuse won't have even blinked with that current going through you - hence why the cable/fuse size doesn't come into it.

I hope this is useful - you can get breakers called RCBOs which incorporate overload protetcion too which leads to the confusion about the purpose of an RCD.
David

 

[matt]

I don't get why you'd NOT want to use RCD protection for a cable carrying a high current on a high AMP fuse in a potentially vulnerable route.

Just for background infomation on what RCDs are used for...

RCDs primarily provide shock protection for humans/livestock who come into contact with the live whilst earthed. There are also used to provide fault protection should an earth fault occur on a circuit where the fuse would not blow within the stated disconnection time. This is mainly for the sake of shock from the raised voltage on the earth of the circuit and the potential fire risk.

Therefore, the size of cable and current don't really come into it. If you touch a live wire whilst earthed no fuse is going to help you. The RCDs on sockets typically trip at 30mA very quickly. A fuse won't have even blinked with that current going through you - hence why the cable/fuse size doesn't come into it.

I hope this is useful - you can get breakers called RCBOs which incorporate overload protetcion too which leads to the confusion about the purpose of an RCD.
David

I'm not really sure I understand what you just said but if I had said:

I don't get why you'd NOT want to use RCD protection for a cable in a potentially vulnerable route.

Would that be a reasonable interpretation?

 

[SemiSkilled]

I used to tell apprentices ~

a fuse stops fires ~usually

an rcd stops it killing you ~ hopefully

The "Earth" makes the fuse work

After that I'd say, " look in the On Site Guide"


Lee.

 

[DavidE]

I'm not really sure I understand what you just said but if I had said:
I don't get why you'd NOT want to use RCD protection for a cable in a potentially vulnerable route.Would that be a reasonable interpretation?

Sorry I was't trying to cause confusion or anything else. My main point was the amount of current a cable is carrying or it's size has nothing to do with the provision of an RCD. What Lee has just posted I guess says it more clearly.

Going back to your quote above - yes that makes sense. Although... if it is buried with warning tape or tiles and care is taken where it goes in and out of the buildings to take a good route/add ducting or otherwise that will be the primary protection. With all that it is hopefully unlikely to get damaged and if someone does get it with a spade or otherwise the fuse would protect them as the sheath would earth whatever implement they were using and then the fuse would pop on contact with a live conductor.

For that last reason that is why you should be sure that the sheath of the cable is earthed if you use one of the internal copper conductors for earth too.

Cheers
David

 

[shortspark]

Just reading through this thread I have noticed part "P" has been mentioned but noboody mentioned that this only affects certain parts of the UK.

Just to throw another comment in, anytime I have installed an SWA cable underground to feed an outbuild etc... I always install a 3 core (for single phase) and 5 core (for 3 phase) supplies and use one of the cores as an earth bonded at one end.

Hope I'm not adding to the confusion :$

 

[garywayne]

Does anyone know how to calculate the startup current of a motor. Say a 2.2kW table saw?
Also, when giving cable distance for calculating what size cable is needed, do you give distance from house CU to shed CU, OR, from house CU to shed and back to house CU?