Another Garage to shed wiring question

[Monkeh]

I'd advise you get a couple of local electricians around and see what they think..

 

[mikhailfaradayski]

will it be cheaper to install yourself? There seems to be a lot of variation of cost for this. Expect to pay between £100-300, talk to your council BCO before going ahead, they may insist (legally or not) that you get an electrician to test it at your cost aswell. If they do that, then why bother, just get a spark?

I would still urge you to survey the amount of power that may foreseeably use in the shed/garage combined. I think you could be using a sledgehammer to crack a nut.

A ring final circuit in a shed? What do you intend to grow in there?

Also, as assumption is the mother of all fk-ups and you are involving the council (or care in the community as i like to call them) be 100% clear as to what they will want you to do.

Leave no stone unturned or you'll end up paying more

 

[ban-all-sheds]

I'd advise you get a couple of local electricians around and see what they think..

I second that, so I'm glad you thanked Monkeh for the advice, because given that you proposed to put 2.5mm cable on a 32A breaker, and create a 16A ring final (not what the Wiring Regulations recognise), I think you're way out of your depth.

What did you plan to tell your local Building Control department would be the way you'd comply with Part P?

 

[cymru001]

What did you plan to tell your local Building Control department would be the way you'd comply with Part P?

Essentially I was going to tell them exactly what I'd posted on here. I thought I could explain what I currently have and then what I propose to do.

I am thinking safety first which is the reason for the two CU's (Garage and shed). I felt this would give greater protection, even if slightly over the top. Additionally, I felt it wiser to use a 100a RCD in the garage as I intend, at some point, to work on my old camper which will involve some welding.

This idea has evolved over a couple of years and hence the stockpiling of equipment.

The logic behind using building control over a spark was because I felt the spark would still charge a fair amount of money even if I did all the digging, cable laying and other installation activities. Even if I terminated all the connections with him supervising, I would have to pay for him to test and certify. I thought, by using LBC, I would still be doing all the work and still get it passed. I have read on this forum somewhere that if the LBC then want the installation tested, it is something they have to do at their cost and cannot pass this on.

 

[cymru001]

I second that, so I'm glad you thanked Monkeh for the advice, because given that you proposed to put 2.5mm cable on a 32A breaker, and create a 16A ring final (not what the Wiring Regulations recognise)

The final ring (now planned to be radial) is not all I would be running from within the shed. As I've posted, I want to set up lighting for the garden.

 

[Steve]

You dont need 3 RCDs in a row like that. You end up with 3 RCDs protecting the shed. In the event of a fault, the quickest RCD will trip. Not nessecarily the one closest to you. Its not dangerous, but bad design.

Provided all your cables are surface-run and/or protected by an earthed layer (SWA), this is a better design:

Cable to garage not RCD protected (it doesnt need to be) - this may need re-think in the house CU.

In the garage CU, use a mainswitch (not rcd), use a 6A MCB for the lighting, and RCBO for sockets, and an MCB for the shed circuit.

in the shed, use an RCD mainswitch.

 

[cymru001]

You dont need 3 RCDs in a row like that. You end up with 3 RCDs protecting the shed. In the event of a fault, the quickest RCD will trip. Not nessecarily the one closest to you. Its not dangerous, but bad design.

I love my shed! I thought I was being safe!

Cable to garage not RCD protected (it doesnt need to be) - this may need re-think in the house CU.

What would I need to do in the house?

In the garage CU, use a mainswitch (not rcd), use a 6A MCB for the lighting, and RCBO for sockets, and an MCB for the shed circuit.

I could use the existing main switch in the garage. Would I go for a 20a RCBO? What size MCB would be good for the shed? Bear in mind it will have a pair of double sockets, an interior light plus 5 exterior lights

in the shed, use an RCD mainswitch.

Could I stick with the 63a RCD I originally planned?

 

[ban-all-sheds]

Essentially I was going to tell them exactly what I'd posted on here. I thought I could explain what I currently have and then what I propose to do.

They don't carry out design reviews, they want you to tell them how you will ensure that you will be making reasonable provision for safety etc.

And you're not, and shame on all the previous repliers for not picking up on this:

32amp MCB - 2.5 mm sq SWA to shed

No. 2.5mm² SWA cannot carry 32A, so you can't use a 32A breaker to protect it.

• //www.diynot.com/forums/viewtopic.php?p=81696#81696
  
  
• http://www.tlc-direct.co.uk/Book/4.3.1.htm
  
  
• http://web.archive.org/web/20080213151445/http://www.kevinboone.com/cableselection_web.pdf
  
  
• http://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html[/list]
  
  

  I thought, by using LBC, I would still be doing all the work and still get it passed.

  Not if it isn't properly done.
  
  
  

  I have read on this forum somewhere that if the LBC then want the installation tested, it is something they have to do at their cost and cannot pass this on.

  Yup.
  
  But have you also read elsewhere about the prevalence of LABCs refusing to do what they are supposed to do, and insisting that having paid them you also pay an electrician to certify your work?
  
  
  

  I feel confident enough to carry out the installation

  I fear your confidence is misplaced.
  • For a circuit to supply a <thing> (doesn't matter what), how would you go about deciding what cable and protective device to use?
    
    
  • What are the 3 different types of domestic single-phase supplies provided in this country, how would you recognise them, and what differences do each make to the requirements for the rest of the installation, particularly any outdoor supplies?
    
    
  • 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?
    
    
  • What are the rules for cables run outdoors, buried in the ground or overhead?
    
    
  • Do you know the requirements for main and supplementary bonding of extraneous conductive parts?
    
    
  • Which circuits must be RCD protected?
    
    
  • Do you know what tests you would carry out on the installation - what sequence you'd do them in and at what point you would energise the installation, and for each test 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?
  You're asking all sorts of random questions as and when they occur to you, which is not a good way to learn. What if you fail to ask about something you don't know because you have no idea you don't know it, and no idea it even exists?
  • //www.diynot.com/wiki/electrics
    
    
  • //www.diynot.com/forums/viewtopic.php?p=75416#75416
    
    
  • http://web.archive.org/web/20080213151445/http://www.kevinboone.com/electricity.html
    
    
  • http://web.archive.org/web/20080213151445/http://www.kevinboone.com/domesticinstallations.html
    
    
  • //www.diynot.com/forums/viewtopic.php?t=7553
    
    
  • http://www.tlc-direct.co.uk/Book/1.1.htm[/list]I suggest you get stuck into the last link right away - it won't give you design ideas, and unfortunately it doesn't refer to the current edition of the Wiring Regulations, but it's free, and will still give you a good grounding which you can augment with more up to date publications.

 

[Monkeh]

and shame on all the previous repliers for not picking up on this:

32amp MCB - 2.5 mm sq SWA to shed

No. 2.5mm² SWA cannot carry 32A, so you can't use a 32A breaker to protect it.

I did! I just forgot to mention it. :{

 

[SUNRAY]

No. 2.5mm² SWA cannot carry 32A, so you can't use a 32A breaker to protect it.

Oh yes you can, if its a spur on a ring....

Please note I don't agree with that reg

 

[ban-all-sheds]

In that case overload protection is provided downstream, either by an FCU or by imposing a limit of 1 socket outlet.

In this case there is no such overload protection - the 2.5mm² would be supplying a CU.

 

[1john]

To be fair, a FCU installed onto the existing socket circuit in the garage could feed the shed socket(s) and a seperate FCU in the shed for all the additional lighting.

This also gets rid of the 3 RCD's in a row nonsense which will be more greif and a pain in the a$$ when things go wrong as the quickest rcd will trip (prob the one in your house as it will have alot more leakage from appliances in your property too)

 

[mikhailfaradayski]

And you're not, and shame on all the previous repliers for not picking up on this:

32amp MCB - 2.5 mm sq SWA to shed

I did notice that, but rather than picking up on every little fault, I mentioned a few and i suggested the OP review the possible load and work from there.

 

[SUNRAY]

In that case overload protection is provided downstream, either by an FCU or by imposing a limit of 1 socket outlet.

In this case there is no such overload protection - the 2.5mm² would be supplying a CU.

Its totally permissable to fit a dsso on a 2.5 spur on a ring final and as I have mentioned before its common practice to make this provision for a washing machine and dishwasher for example. Much of this sort of appliance coming in from abroad with CE markings and moulded 13A plugs is rated at 3KW or even 3.2KW at 220V. Doing the sums 3200/220=14.55A, So thats wrong. 220/14.55=15.2ohms and 240V/15.2=15.7A or 3.7KW. and there is 2 of those plugged into a DSSO so thats wrong too as the 2.5mm is running at peaks of 31A on a cable rated at 28A tops and yet it still complies with the regs and works ok till the cable gets too hot and fails.

I know I am making reference to unrelated regs but that does not change the fact that in some circumstances it is permissible to inadequately protect a 2.5mm cable with a 32A device, knowing that the following protection is inadequate too.

 

[ban-all-sheds]

FGS - am I the only one who read the OP?

His proposed design was 2.5mm² SWA on a 32A breaker supplying a CU with 1 x 16A and 6 x 6A breakers in it.

Its totally permissable to fit a dsso on a 2.5 spur on a ring final and as I have mentioned before its common practice to make this provision for a washing machine and dishwasher for example.

Spurring off a ring final does not absolve you from considering the current carrying capacity of the 2.5mm² cable as installed. Method C is 27A, which is less than 2 x 13A.

You cannot blindly shove in a 2.5mm² spur and rely on the downstream protection of plug fuses to prevent overload if the CCC of your cable as installed is less than 26A.

Much of this sort of appliance coming in from abroad with CE markings and moulded 13A plugs is rated at 3KW or even 3.2KW at 220V. Doing the sums 3200/220=14.55A, So thats wrong.

So stop there.

It's wrong.

Don't do it.

It contravenes the wiring regulations and therefore cannot be used as a justification for other, related, designs which also contravene them.

220/14.55=15.2ohms and 240V/15.2=15.7A or 3.7KW. and there is 2 of those plugged into a DSSO so thats wrong too

Ditto.

as the 2.5mm is running at peaks of 31A on a cable rated at 28A tops and yet it still complies with the regs

No it does not.

and works ok till the cable gets too hot and fails.

Or until the socket overheats and fails, because 31A is in excess of the current that BS 1363 sockets are rated at.

I know I am making reference to unrelated regs but that does not change the fact that in some circumstances it is permissible to inadequately protect a 2.5mm cable with a 32A device, knowing that the following protection is inadequate too.

No it is not.

It may be done in practice by the ignorant and incompetent, and it may even be fairly widespread and not responsible for many problems but that does not make it permissible.