Some advice ahead of running wiring to shed

[ac505]

Looking to run wiring to a shed/home office. The shed is around 12m from the consumer board in the house, I would expect to have 2 or 3 double sockets, lighting and would power a laptop, monitor, TV, Stereo, and an oil filled heater. I'm thinking either 4mm or 6mm armoured cable buried 450mm below ground ran from the house via an RCD to a small consumer unit in the shed, with MCB's for the ring and another for lighting. Have I missed anything, would 4mm be okay, or 6mm overkill? THanks

 

[JameZZ]

4mm would be ok, (give you about 20A supply, enough for the items you listed), 6mm would give a you a bit more power (not much though, you'd have to go bigger to make a difference really). What size mcb in the house consumer unit are you planning to use and is it RCD protected? You ever used swa before/familiar with electrics?

 

[ban-all-sheds]

I'm thinking either 4mm or 6mm armoured cable

10mm² minimum. Overall cost difference wil be negligible, and it will be future-proofing.

from the house via an RCD to a small consumer unit in the shed, with MCB's for the ring and another for lighting.

Have the RCD protection at the shed end. At the house come off a non-RCD breaker, or (better, IMO) avoid the CU altogether and have a switchfuse.

Have I missed anything

http://www.tlc-direct.co.uk/Book/8.1.1.htm

 

[ac505]

thanks for all the info.

I think my best option is to miss out the CU in the house all together and start afresh. Interesting you suggest a switchfuse, I would have thought an RCD would have been a safer option as it then protects the wiring run to the shed?

I've not had experience terminating SWA before so was going to get a friendly spark to terminate this for me and I'll do all the time consuming laying of cables etc. Can get testing done at the same time.
So I'll need an adaptable box for the feed from the meter to SWA, then either another adaptable box ahead of the CU or metal CU in the shed. Right?

Alternatively is there any merit going for T&E for the full run inside 32mm or 40mm conduit. The advantage here is that if I need to run network/telephone in the future there would be less digging up to do... Also no messing about terminating the SWA or having to use adaptable boxes?

 

[bernardgreen]

I would have thought an RCD would have been a safer option as it then protects the wiring run to the shed?

If you have an accident in the shed that trips the RCD in the house then at night you will be without lights in the shed until you get to the house to reset the RCD.

Alternatively is there any merit going for T&E for the full run inside 32mm or 40mm conduit.

No, not safe

The advantage here is that if I need to run network/telephone in the future there would be less digging up to do...

When youi dig the trench and lay the SWA lay in a conduit for your future cable requirements. [1] mains and telephone in the same conduit is not a good idea and [2] trying to get additional cables into a duct with a heavy gauge cable already in there can be very difficult and can damage the cables.

 

[ban-all-sheds]

I would have thought an RCD would have been a safer option as it then protects the wiring run to the shed?

If you have an accident in the shed that trips the RCD in the house then at night you will be without lights in the shed until you get to the house to reset the RCD.

Yup, and if as was originally planned that RCD was in the house CU all those circuits will be out as well, so any semi-permanent issues in the shed become a right PITA.

Interesting you suggest a switchfuse, I would have thought an RCD would have been a safer option as it then protects the wiring run to the shed?

1 - Not needed.
2 - An RCD on its own wouldn't be enough.

I've not had experience terminating SWA before so was going to get a friendly spark to terminate this for me and I'll do all the time consuming laying of cables etc. Can get testing done at the same time.
So I'll need an adaptable box for the feed from the meter to SWA, then either another adaptable box ahead of the CU or metal CU in the shed. Right?

If I were you I'd have the electrician do everything except the donkey-work of digging trenches etc. Which is an odd thing to say, because donkeys are carp at digging trenches.

if I need to run network/telephone in the future there would be less digging up to do

Install an empty duct (e.g. http://www.drainageonline.co.uk/Ducting.htm) for future use like that.

 

[ac505]

Unfortunately I am going to have to run about 20m, instead of 12m. So here's where I am at:

IP65 Adaptable Box 80 x 80 x 52mm

Volex Fully Insulated RCD Board Shower Unit 63A 30mA RCD or Switchfuse

Prysmian LV Armoured 3-Core Black 6mm² 25m Cable Reel or 10mm2 cable.

Volex Garage Unit 40A 30mA RCD with MCBs

Not sure on the diameter of the armoured cable so as suggested may spec up to 10mm given the longer cable run.

Can anybody see anything wrong with this?

 

[Luckyphil]

I do plenty of these subbying for a Garage/Shed supplier

You don`t need the RCD at both ends, You have two options with the supply or Shed Board , If you put the RCD board on the supply end and you have a fault when you are in the Shed you`ll need to traipse upto the House to reset it !

I`d go down this Route :-

Stand alone Board No RCD at Supply end
10mm Armoured buried as you state
Garage RCD Board with 6 & 16 OR 32AMP Mcb.
Either Radial or Ring Circuit for Sockets
Lighting Circuit

Lucky

PS - if you are digging the Trench , it would be a good idea to throw in a duct with a draw wire for future use if needed,

 

[ac505]

okay, getting round to placing my order for this lot now. Here's the list, any comments appreciated:

63A metalclad Switchfuse
10mm2 25m armoured cable
2 way metal garage unit, 63A RCD, 6 & 16 RCB

If there is a fault would I not run the rish of the fuse popping ahead of the RCD. Should I try and source a 40/45A RCD instead?

For future proofing I'll wire this up as a ring however keep the 16A RCB in for the moment, only 2 doubles so I doubt I'll draw near 16A anyway.

 

[slup]

RCD's don't trip on overcurrent. Your protective device is the switchfuse at the house end, and the MCBs or RCBO's in the shed.

Your solution still leaves you in a situation where an RCD trip will take out your lights.

In my opinion, in the shed consumer unit you should just have a standard main switch (non RCD) and two RCBO's.

 

[ban-all-sheds]

63A metalclad Switchfuse
10mm2 25m armoured cable

Have you confirmed that 10mm² cable will have an Iz ≥ 63A?

Should I try and source a 40/45A RCD instead?

Why are you doing design work when you don't know what the "63A" on an RCD means?

For future proofing I'll wire this up as a ring however keep the 16A RCB in for the moment

Why are you doing design work when you don't know what the Wiring Regulations say about ring final circuits?

And since you clearly have no intention of getting an electrician to do this job, what do you plan to do about testing?

 

[ac505]

ban - all - sheds, rash assumptions on your part, an electrician will be checking and testing the entire setup. This forum is for advice is it not?
Okay can downgrade switchfuse to 40/45A, God point re trip and lighting, will look for bare metal clad CU or metal clad CU with RCBO's

 

[ac505]

Assume I can use :Square D Twinbreak 40A SPN Switch Disconnector Screwfix, FuseProduct Code: 64648 as opposed to standard switchfuse?

 

[londonboy]

Do you have a TN-C-S supply?

 

[ban-all-sheds]

ban - all - sheds, rash assumptions on your part, an electrician will be checking and testing the entire setup.

Then why aren't you asking him these questions?

This forum is for advice is it not?

Yes.

My advice is to have an electrician do all of the design work, all of the materials specification, all of the installation work except trench-digging etc, as agreed up front with him, and as supervised and directed by him, and all of the testing.

That way you'll get an installation which is safe, complies with the regulations, and has an EIC.


OR - spend a lot more time learning things before deciding to tackle design work yourself. The thing is, installing new CUs, outside supplies, submains etc is not a trivial job, and I can assure you that it involves knowing far more than you think it does.

Asking questions here can be a useful part of a learning process, but they are not a substitute for proper structured studying. The key term there is "learning process" - you cannot learn all the things you need to know just by asking questions here. It isn't structured enough - it won't provide you with a way to progress where each step builds on what you learned before.

You can't carry out a job of this magnitude by asking whatever random questions happen to occur to you. You've already shown that you have some dodgy misconceptions - what if you get something wrong because you have no idea your knowledge is wrong? What if you miss something because you simply have no idea it even exists, and just don't realise you don't know it?

What happens when the electrican turns up to checking and test the entire setup and tells you that you've made mistakes?

• //www.diynot.com/wiki/electrics
  
  
• //www.diynot.com/wiki/electrics:books
  
  
• http://web.archive.org/web/20080213151445/http://www.kevinboone.com/electricity.html
  
  
• http://web.archive.org/web/20080213151445/http://www.kevinboone.com/domesticinstallations.html
  
  
• 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.