Cable Calculations

→ British Standard 7671.

I beleive that 4mm SWA between the Office and the shed will be fine but my main concern is the size of the cable between the house and the shed.

Do you think 4mm SWA will be sufficent or would it be best to increase to maybe 6mm?

Regards,

Alun

We could take a guess but you really need to consult your electrical regulations applicable to your country (and state) to see what is legally required (especially when building-to-building wiring and water are involved) as requirements vary from country to country (and even state to state in some cases).

The electrical regulations will also indicate correct circuit protection and cable installation and mechanical protection (which is just as important as cable sizing in this application).

What country (and state if applicable) are you in?

Im in the UK

6mm cable will do considering in future a welder may need power from shed

→ British Standard 7671.

see the details at-

http://myelectrical.com/pages/welcome.aspx

open the site and enter at TOOL option and choose your option.

Hi Alun,

Your application appears to have quite a high diversity factor and therefore the feeder to the shed will not be of any great size. I've 'guessed' your load to be in the region of 6 Amps and by using one of the convenient 'on line' sizing services, doncastercables.com and selecting a 3% volt drop you will see that for an11 metre route that the selected cable is 1.5mm2. Volt drop is calculated as 2.2V.

You can play about with this selector to your hearts content!

Be aware that you must employ a qualified Electrical contractor for external installations of this type in England or contact your local Authority and get the system tested and submitted for their approval.

For your own protection the consumer unit in the shed should include a 30mA RCD, this will then protect all the connected equipment.

Hope this is of assistance.

Massey.

Alun,

I am trying to answer your questions and provide some advice. I am an MIEE but I am not a practising Electrician. I know, broadly,what I can do and what I am not allowed to do. UK Law. BUT SEE BELOW.

1 I assume that you have adeqaute provsion in your Main Consumer Unit to provide your external circuits. Preferably RCD protected. BUT SEE BELOW.

2 Your 3 outside cables must all carry the total load that you are providing. Theoretically this amounts to 1 Ring Main, 1 Light circuit, 1 Water pump - unspecified load, 1 light circuit,, office, 1 further ring main. A total of 2 Ring Main circuits, 2 Lioghting circuits and one unknown water pump supply.

Subject to the Water Puimp load we are talking about 2 13 A ring Main circuits nd 2 5 to 10A lighting circuits plus the water pump all to be RCD protecred the lights to be protected in case you decide to add an external light over each door!

How do you intend to keep warm in the office environment? Electrics, propane or??

3 In my book you need a Consumer unit with RCD protection for both buildings, the water pump to run from one of these consumer units. Your choice.

4 SWA outside should be 2.5mm, you can only use 1.5mm for light circuits. I am not sure why you chose to use 6mm for the water pump as this would be adequately sized for 30 plus Amps! You will need to size the fuse rating for this circuit to an adequate point .

5 The consumer unit here requires a 3 way connection, minimum, to cope with lights, power and water pump. It is always advisable, if possible to have a spare way on each consumer unit.

6 As someone else has said all external electrical connections may need to be checked by a competent engineer who can sign of Part P wiring - he will also measure all circuits. You need to check the regs. I have not put in outside circuits since the regs changed!

7 On the advice front, i would run all cables underground, if at all possible. U'ground cables ahould have a visual cover Black/Yellow tape to warn the itinerant digger! I try and cover such cables with a line of bricks - that is just me being cautious.

8 I would like to run cables to the 2 outhouses separately to avoid the problem of one issue affectimg both. That would mean 2 separate ways on your consumer unit one to driive each outhouse.

9 When running SWA between buildings I prefer to put the termination boxes inside the respective buildings and then take the internal connevction from there.

Finally I would always err on the side of the things that are yet to be thought of, I have over engineered my electrics but have lots of spare capacity - I hate using reel extension leads around the garden and have consequently fitted an external ring main for power tools, heaters, etc

I have lights round the house and am contemplating a circuit around the garden to put lights in bushes etc with a controller in one of the sheds. The garage and the two sheds are all run from the main consumer unit in the house 18 way, with a 12 way in the Garage and 3 ways in each shed.

One thing that I found out the hard way is that you should not fit RCDs in tandem as after some recent works - given to a practisibng electrician due to time constraints, I found that the system became unstable, he had taken the wiring to the garage from the RCD side of the House circuit, and the garage Consumer unit had a number of RCD circuits, moving the Garage feed to a non RCD circuit on the House Consumer Unit cured that!

Good Luck with your endeavours if you need any further advice I can supply you with direct ciontact details.

Sleepy