Long Cable run to garden office

[Siyoung]

Hi all,

I am hoping to install a garden office in my Dad’s “garden” (more of a field), and wondering what the maximum distance is you can run a supply line from his domestic CU, as the office could be about 250m away. Am assuming some seriously heavy duty cable buried in a trench, but obviously concerned about voltage drop over such a long distance or if it’s even possible.

Thanks

Simon

 

[westward10]

Would need more information:
Design current.
Will it support main protective bonding for a TN-C-S earthing system.
Installation method.
How will ADS be achieved which will dictate your Zs.

 

[Siyoung]

Hey thanks for your reply, but not being an electrician, it may as well be in Chinese!! I have no idea what ADS, Zs, TN-C-S etc mean!!

But all I’m looking for is a supply to a garden office for two or three ceiling lights and maybe four or five double sockets to power laptops etc., with the heaviest load from maybe a kettle and fridge. It’s really just to know if it’s possible to run a feed that far into my dad’s field.

Thanks

Simon

 

[westward10]

Yes it is possible but it needs to be correctly designed however the size of cable required will be oversized to enable suitable connection in a consumer unit so it is not straightforward.
You will looking at £3.5 - 4k+ just for the cable.

 

[Siyoung]

Ok thanks! Any rough idea of cost of installation, not including digging the trench/laying cable (which I could easily do!), but allowing for suitable consumer unit/earthing/circuit breakers/ other safety devices.

 

[westward10]

You need an electrician to do a site survey.

 

[DefyG]

I just punched the details into a app I have on my phone, which has been quite reliable for a quick 'idea' when on site, (Although you should get an electrician to do a proper design) and 250m for a 3kw load came back as 25sq mm 2.79%VD.

But what are you doing for heating?
Is the field agricultural as trench depth is quite specific.

 

[Siyoung]

Hi and thanks for your reply, though again not sure what your figures mean…no idea what 2.79%VD means but guessing you’re implying a big arse thick cable! I just spotted Westward10’s previous reply saying it would need to be oversized and probably almost £4k just for the cable…ouch! But big thick cable explains the high cost!

The land was agricultural when he bought it 50 years ago, but he’s transformed it into an extension of his garden with an informal/non-commercial orchard where we plan to site the workshop.

As for heating, we’d possibly use a small room electric heater, but no more than 2kw…not planing on staying in it overnight, esp not in winter!

Thanks both!

 

[Siyoung]

Thinking about it, if the cable is that expensive, it might be a lot cheaper and easier to go solar for 12v LED lights and limited 240v for laptops etc and rely on gas for heating and brews?!

 

[westward10]

My guess of £4k was based on a 32A design current which would be in the region of a 50.0mm² cable. As I said we need to know the design current and the means of circuit protection.

 

[Siyoung]

Ok thanks Westward, though I’m starting to think solar might be easier and possibly cheaper all round…

 

[DefyG]

Thinking about it, if the cable is that expensive,

You really need to get a local electrician on board to design and price accordingly.

2.79%VD was the percentage voltage dropped over that distance for a particular load (ie 3kW).
So starting with a nominal 230v at the source, it would be 223.6 volts at the garden office. There are regulations regarding voltage drop!

As for the trench, I guess you'll be using a min digger or similar as I would recommend 600mm (or 1000mm if agricultural!)
Its not just about digging a trench and chucking the cable in, there are correct ways a cable should be laid with soft soil/sand and marker tape etc. but again a local sparks should know that.

 

[DefyG]

solar might be easier and possibly cheaper all round

Together with a battery to give you 24 hr power, you might be right!

 

[Siyoung]

Yeah have a feeling my Dad wouldn’t be too impressed with digging 600mm trenches with a digger through his back garden! Solar looking increasingly attractive but thanks for all the advice and explanations!

 

[John Gabriel]

Hi all,

I am hoping to install a garden office in my Dad’s “garden” (more of a field), and wondering what the maximum distance is you can run a supply line from his domestic CU, as the office could be about 250m away. Am assuming some seriously heavy duty cable buried in a trench, but obviously concerned about voltage drop over such a long distance or if it’s even possible.

Thanks

Simon

Hey Simon,
250m is a long run, but it's doable with the right setup. You’ll need a hefty SWA cable (maybe 25mm² or more depending on load) and definitely account for voltage drop. Best to get a spark to do proper calcs based on what you'll be running in the office. Trenching and cost won’t be small, though!

 

[Siyoung]

Hey Simon,
250m is a long run, but it's doable with the right setup. You’ll need a hefty SWA cable (maybe 25mm² or more depending on load) and definitely account for voltage drop. Best to get a spark to do proper calcs based on what you'll be running in the office. Trenching and cost won’t be small, though!

Hey John, thanks for the reply, but yeah, starting to think the cost and disruption of the trench would be too much for my dad, so looking into solar options. Thanks. Simon

 

[John Gabriel]

Hey John, thanks for the reply, but yeah, starting to think the cost and disruption of the trench would be too much for my dad, so looking into solar options. Thanks. Simon

You're welcome

 

[DefyG]

I read an article form National Grid that it is 5 x more expensive to go underground, so maybe a pylon option if your dad wont permit a trench

 

[Rockingit]

Given the very small current draw of some led lights and a few laptops that could easily be solved by a spv and battery combo.