Electrics to new garage

[glendragon]

Hi, I wish to run an electrical supply to my new garage/workshop at the bottom of my large garden, the installation work will be carried out in accordance with Part P ie a competant person, when I get one, however I wish to purchase the required items myself to avoid the inevitable markup. My question therefore is what size armoured cable would be required to run 8 double 13 amp sockets, 1X13amp water heater, 8x double tube lights, 1 x double garage door (runs off a single 13 amp plug), 2x 1.5Kw heaters, and spare capacity to run various garden items ie pond waterfall, garden lighting and security lighting.
I have tried to be as complete as possible to ensure I purchase the right cable the first time as it will be buried under 3ft of garden. many thanks

 

[festa]

Lets start with an exact length for the SWA cable,as length will determine volt drop amongst other things.
This is some hefty set up your planning and from just a quick glance over it the main supply would be no where near up to it.

 

[Chri5]

Just to back up Fiesta's post.

A 45m run of SWA of 10mm cores will take a 35A load, have a voltage drop of 7.4v or 3.2%

As mentioned that's quite a potential load taking all the items mentioned in to account.

Only you will know what useage level each of the sockets, heaters, light ccts will have.

I'd suggest you look here and consider the rules of demand and diversity

http://www.tlc-direct.co.uk/Book/6.2.1.htm

Then go through each of the 3 sections:-

Demand
Diversity
Applied Diversity

Overall you need to find the balance between load / use frequency / installation cost (materials and labour) and try to avoid any requirement to upgrade the house supply via the DNO (at silly cost).

Here's a vague detail on the method of burying cable, http://www.tlc-direct.co.uk/Book/7.13.3.htm.

For uncultivated land the cable should be 600mm down. Burying direct is OK due to being a SWA, some would cost in a black duct pipe to allow for cable replacement without a re-dig.

Other services such as alarm zone feed from house, Cat 5 for data, CCTV and phone might be required- these need to be in a seperate duct.

 

[glendragon]

Many thanks to festa and Chri5 for your prompt replies. festa, the cable run would be 30 metres.
Chri5 I would think that useage would depend on season, heating, lighting etc, however not all sockets would be in use at same time, but located for convenience, ie socket near garage door for pressure washer, over sink hand wash water heater - occasional use, garage door useage about 6 times per day, security lights activation occasionally- hopefully never !!.
I would certainally not wish to upgrade the house supply unless really neccesary. Thank you for the web links, I shall read through them.
once again thanks to you both.

 

[Chri5]

I'd go for 10mm core SWA with Xlpe thermosetting insulation giving an increased current carrying capacity.

Tables show a load availability of about 35a.

I would suggest the sub main to be on a 30a RCD (MK sentry ones can be used as an isolator)
with 20a Type B MCB for ring main, 6a for lights (subject to calculation- 16 x tubes would illuminate the London eye !!), 6a for garden lights and pond pump, heating on 16a.

Obviously I have no comprehension of the size and scale of this outbuilding, it sounds more like a warehouse.

Each double light fitting at 5ft length = 120w x 8 = 960w of lighting, is it a jewellers workshop?

Whatever you do I would strongly suggest you chat to the commissioned sparks about all the works.

Reason being:-

1- Gets the maths right, and they should be the solution designer- not you.

2- Certs will be required and building control will require work notice

3- Most sparks can buy at a price under yours. They buy materials all year and this allows them some good discounts, you may well find that, Sparks cost of materials + handling charge (ordering, collection and payment) will still be under your quote

4- Warranted works becomes an issue since you supplied the materials, why would / should a spark warrant your purchased materials?

If you need to save cost, then doing the trench, getting easy access between main building and garage, making cable routes for rings, lights and heaters in garage will save labour cost. Labour cost savings will be a better way forward than 'so called' material savings.

Best of luck, post further if you need.

Oh, and thanks for the thanks- manners cost nothing, but some people don't offer thanks- so I appreciate it

 

[ricicle]

I'd go for 10mm core SWA with Xlpe thermosetting insulation giving an increased current carrying capacity.

Tables show a load availability of about 35a.

I'd say 43A (4.7mV*43A*45m = 9.1V drop)

This depends on if the supply is from the mains or has extra voltage drop due to the internal cables supplying it.