Cable To Shed

[original smasher]

We're looking to run power from our house down to a shed outbuilding board that is 75m from the main fuseboard.

We had 2 electricians in and both have given different cable sizes.  Would be interested to hear what people's thoughts are on what the cable size should be before we pick one for the job.

The supply will be connected to a new consumer unit in the shed which will be running 21 x garden lights (5 on mains and 16 low volt), 4-5 double sockets to be used for computer etc and 240v power tools occasionally.  Cable will be buried at the correct depths and intend to use an armoured cable.  

Hope that's enough to go on but if any more info is needed please let me know. Any help much appreciated.

Thanks. 

 

[Zeespark]

Earthing type? have you accounted for bends/drops to CU in your cable length calculations?

Andy Guinness

 

[steptoe]

top of my head at least 10mm,

maybe 16 if I could be doing the calcs, but the bike racing is starting so Im in a rush,

yep, as Zee says, need to know the earth type as well.

 

[original smasher]

Thanks for responding so quickly.

Have accounted for bends etc in the cable length calculations and added a bit for tolerance.

Not sure what you mean by earthing type??

 

[alpha spark]

Both electricians who came to look at the job should have looked,

Can u post pics take a photo of your incoming electricity supply and post on here !

 

[SPECIAL LOCATION]

The actual maximum power you are going to draw is a key figure when calculating the actual cable size...

i.e.  just saying 21 lights and four or five double sockets does not actualy give the expected current & power requirements...

4-5 sockets for computer & power tools sounds like you could be working in there and be having heating on during cold weather???

 

[SPECIAL LOCATION]

Also...

How much further from the shed are the garden lights?

What is the actual max length from incoming supply to the furthest light position?

 

[ProDave]

What cable sizes did they come up with?

What questions did they ask about expected loads? did you give them both the same answers.

I expect since "computer" is mentioned, this is a posh Office shed. So what heating?  Electric most likely.  I'll bet the one who gave the larger cable size allowed for that, the one who gave the smaller size probably only allowed for light loads.

 

[batty]

Surely if you are employing somebody to do the job they will supply the correct cable for what is needed. They will give you a part p certificate that says the work complies to building regulations which hopefully means it will comply with BS 7671 if done correctly. I generally would install a 40 amp supply to outbuildings which usually will be 10mm or longer runs 16mm SWA. I would generally use 2 core and use armouring to protect cable and then TT outbuilding. Are you supplying the cable? Actually it sounds like you are going to do the job yourself reading between the lines did you just get sparks in to give you ideas how to do the job?

 

[Steve3948]

Well 16mm would give you 26A @ 75M according to cable calc

 

[original smasher]

Thanks for all the replies.

I can't seem to upload a photo for the incoming electric supply and couldn't add a link either as I'm a new member.

Regarding the overall draw we've just worked out what we think we'd be using, which as ProDave said probably explains why we've had 2 different specs of cable (one was 10mm 3 core and one 16mm 2 core).  We think the draw would be a maximum of 12kW (this is allowing for the lights, heaters, power tools etc with a fair amount of tolerance added on).   Questions on the usages that were asked weren't particularly in depth by either electrician.

Should also mention that we did discuss using the earth wire for internet with one of them, not really sure how this works.

Just want to state that we fully intend to use a qualified electrician for this job, we were just concerned why we had different specs and want to ensure we're getting the correct spec from the right person.

Thanks for all help so far.  

 

[batty]

He will need a bigger cable then actually calced it out for 40 amps he will need 25mm  its the 3% volt drop that brings it up.

12 kw that's 52 amps at 230 volts so you will be looking at a 60 amp supply. That is going to be a massive cable.

 

[original smasher]

Max distance to furthest light from the shed is 25m

 

[SPECIAL LOCATION]

 Wattage of lights?   big difference between 4w garden path light -vs- 1000w floodlight!

 

[original smasher]

In the overall 12kw allowance we put in 2000w for lights which should be more than we need:

2 x 200w flood lights

2 x 100w up/down lights

16 x 50w uplighters (run on 12v transformers)

 

[ProDave]

Thanks for all the replies.

I can't seem to upload a photo for the incoming electric supply and couldn't add a link either as I'm a new member.

Regarding the overall draw we've just worked out what we think we'd be using, which as ProDave said probably explains why we've had 2 different specs of cable (one was 10mm 3 core and one 16mm 2 core).  We think the draw would be a maximum of 12kW (this is allowing for the lights, heaters, power tools etc with a fair amount of tolerance added on).   Questions on the usages that were asked weren't particularly in depth by either electrician.

Should also mention that we did discuss using the earth wire for internet with one of them, not really sure how this works.

Just want to state that we fully intend to use a qualified electrician for this job, we were just concerned why we had different specs and want to ensure we're getting the correct spec from the right person.

Thanks for all help so far.  

12KW sounds excessive. Are you having a shower in there as well?

I would say 3KW for a heater, 3KW for a radial socket circuit, and a little for a lighting circuit.

Getting your loading right will make a big difference to the cable size.

One spark was planning to export the earth, and didn't trust the SWA, so chose a 3 core cable. the other one either trusts the SWA, or is planning a local TT earth.

"using the earth for internet" is tosh.

They are probably suggesting those horrible "home plug" things that send the network down a mains cable.  If you are going to all the trouble of digging a long trench, do it properly and bury a network cable and phone cable in a bit of ducting next to the mains cable.

EDIT: Just seen your lighting spec.

Save your self a lot of money in running costs and cable requirements by using all CCFL or LED lighting.  I really detest 50W Halogen downlighters (or uplighters)  We are supposed to be thinking of energy conservation these days, not warming up your garden.

 

[SPECIAL LOCATION]

So this furthest light 25m from shed + 75m from incoming supply... 

100m total from supply to load...

Is this a 50watt 12 light or 200w floodlight??

 

[original smasher]

Thanks ProDave - this is how we were working out the overall breakdown (which is probably excessive but more worse case scenario):-

Exterior Lighting  - max 2kW total:-

 - 2 x 200w flood lights

 - 2 x 100w up/down lights

 - 16 x 50w uplighters (run on 12v transformers)

In Outbuilding - max 10kW total:-

- Heater 2000w

- Kettle 3000w

- PC 150w

- Power tools x 2  3200w (1600w each)

- Lights inside shed - 150w

- other small electrical items - 200w

Think you're right about running a dedicated network cable down as we'll have a trench anyway!

Special Location: the lights furthest from the shed are only the 12v 50w, the flood lights will be on/near the shed.

 

[OnOff]

Well if you do happen to run the cable yourself   try and work with the natural "lay" of the cable as it comes off the reel. Pull it off of a cable trestle etc. Just saying as I got called to look at this this morning, somebody with no experience attempting to lay a cable to a shed:

 

[batty]

Is that right in the middle a new piece may be required.