UK DIY- advice on extending wiring into new shed

[swisstony]

Hi all, DIY here
First off let me just say it is advice I am after as I am fully aware that I will probably need an electrician to complete the job. My question is how much I can do myself up to a point.
So many years ago I built a shed at the top of the garden ( approx 30m away from garage). I needed power and lighting to the shed so purchased the armoured cable, fixings, outdoor sockets etc (on the advice of my sparky) and did the labour myself of digging the trench from the garage (where the feed would be ) laying the cable including tape, drainage etc. Then my sparky came in and did all the easy work Actually he was brilliant and connected up all the power, changed the fuse in the main board, added in lighting and fitted the exterior sockets for the garden. Tested and provided me with the certificate. All good.

So my next plan is to build even a bigger second shed behind the first one . During this lockdown project a man's got to have a project ! So I am building it myself over the course of the next few months. It will be positioned around 2 feet away from the first shed ( have attached a small rough diagram) . So the question at last : I would like to have power and lightning in the new shed (approx 10ftx8ft) pent roof style. So given that it will be so close to the old shed , how hard is to extend the power and lighting circuit into the new shed ? I will be using it as a small wood workshop so will need power sockets around the perimeter at worktable height. Probably about 4 double sockets. Lighting will be simple LED shop lights x 2 on the ceiling. nothing fancy.

How much could I do for the prep work ?
would it be an extension from inside the old shed across to the new shed or would it be a spur from the original feed ?
power tools are all 220V , I don't have any that are 110V

Thank you for any advice
cheers

 

[swisstony]

2.5 over 30m, 20A protective device exceeds the 3% volt drop for the lighting as it stands now.

I think I over estimated the distance. It is more like 21m so extension would be 22-23m, max 24m. not sure if that makes a difference
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The way the OP is written it sounds like it might be an extension off another circuit? But that doesnt really make sense with the breaker change etc?

consumer unit is in house which feeds the garage which is where the extension was carried out from. Hope that helps

 

[barninks]

consumer unit is in house which feeds the garage which is where the extension was carried out from. Hope that helps
[/QUOTE]
yes - ok that makes more sense - as i thought. In which case you need to think about the loading in the garage as well.

 

[westward10]

Volt drop is measured from the origin so you have to include the part from the consumer unit to the garage.

 

[barninks]

Was the armoured cable run in a duct at all? or just buried in the ground?

 

[swisstony]

Volt drop is measured from the origin so you have to include the part from the consumer unit to the garage.

That does make sense. I am getting the feeling here that given the below

consumer unit is in house which feeds the garage which is where the extension was carried out from. Hope that helps

yes - ok that makes more sense - as i thought. In which case you need to think about the loading in the garage as well.
[/QUOTE]

That does make sense. I am getting the feeling here that given the above comments a) the distance b) potential voltage drop and c) potential load in new shed, I may need to consider a separate unit in one of the sheds.

So is the consensus that once we start getting back to normal that as temp measure I could run a outdoor rated 13A extension lead through some ducting to the new shed. That way I could have some power, albeit temporary and just have a work light in there until my spark can return ?
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Was the armoured cable run in a duct at all? or just buried in the ground?

buried in the ground

 

[barninks]

I wouldn't necessarily go to all the effort of running a duct from one shed to the other if you end up taking another supply from the house/garage to the new shed, especially as the existing cable is buried and so you cannot easily draw another cable through.

You really need to first ascertain the volt. drop to the existing shed.

 

[pc1966]

@swisstony Is the shed fed from a dedicated breaker in your house?
As it it only feeds the shed and not anything else in your home. Just wondering what it was used for before the electrician changed from 32A to 20A.

Edited to add: Just realised it feeds your garage!
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Before going to the effort and expense of running another SWA cable you need to be realistic about what you plan on using in the sheds & garage to determine the load, and from there you can decide if the existing circuit and cable is acceptable or not.

Assuming the measured value of 0.21 is at the garage, and your cable is 22m long, then I estimate the resistance at around 0.63 ohms. That gives a 3% drop on 230V for around 11A.

The regs have 3% for lighting circuits due to the annoying flicker you see with filament bulbs which as quite voltage-sensitive, with LED lights you won't see as much effect. For other power circuits it is 5% drop, which would be 18A max.

So think about:

• What do you have in the garage using power?
• What do you plan for old shed?
• What do you plan for new shed?

Realistically if it is DIY style workshop with only you using stuff it will be one machine at a time, plus lighting, plus any electric heating (which might be the biggest load as 2kW is around 9A).

 

[swisstony]

@swisstony Is the shed fed from a dedicated breaker in your house?
As it it only feeds the shed and not anything else in your home. Just wondering what it was used for before the electrician changed from 32A to 20A.

Edited to add: Just realised it feeds your garage!
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Before going to the effort and expense of running another SWA cable you need to be realistic about what you plan on using in the sheds & garage to determine the load, and from there you can decide if the existing circuit and cable is acceptable or not.

Assuming the measured value of 0.21 is at the garage, and your cable is 22m long, then I estimate the resistance at around 0.63 ohms. That gives a 3% drop on 230V for around 11A.

The regs have 3% for lighting circuits due to the annoying flicker you see with filament bulbs which as quite voltage-sensitive, with LED lights you won't see as much effect. For other power circuits it is 5% drop, which would be 18A max.

So think about:

• What do you have in the garage using power?
• What do you plan for old shed?
• What do you plan for new shed?

Realistically if it is DIY style workshop with only you using stuff it will be one machine at a time, plus lighting, plus any electric heating (which might be the biggest load as 2kW is around 9A).

You called it . It will be 2 x LED shop lights on the ceiling with maybe a desk light on the bench for close up work. Power wise, yes it is going to be a small workshop where I do wood working. Eventually I will have a table saw, then everything else will be one at a time + lighting. Think the only time two units would be on at the same time is when I am using the shop vac (dust extraction) to any of the power tools. for example mitre saw, sander, table saw, circular saw etc.

Old shed will be converted into storage for wood. Garage currently has the tumble dryer in there ( not on all the time but if I let my wife have free rein it would !!! ) , two LED strip lights and occasionally an extension lead plugged in to work in garden or on car. oh and currently that is where the power tool battery chargers are , 3 of them.

 

[Mike Johnson]

If your proposed table saw has an induction motor that could change things, you will have to think about the surge current on start up to prevent nuisance tripping.

 

[pc1966]

If your proposed table saw has an induction motor that could change things, you will have to think about the surge current on start up to prevent nuisance tripping.

That is a good point. If needed he could have a C or even D curve 20A breaker for this and probably still meet the disconnection times (also has RCD protection so would meet it for L-E fault anyway). That might be enough for surge effects.

If the tumble dryer is used a lot along with electric heating in the shed and the tools then it might be too much, but that is for later consideration. Rather than replacing (or adding) the SWA cable he could have a separate circuit put in from main CU to the garage just for the SWA (assuming a spare location in the CU) to remove the tumble dryer from the equation.

Finally it raises a useful point, he might want to fit some sort of emergency lighting so if the power fails for any reason he is not in a dark and dangerous workshop trying to get out past various cutting tools!

Not too expensive and potentially a great feature, for example:

2W LED Non Maintained Emergency Bulkhead White

High quality polycarbonate construction with long life LED

www.cef.co.uk

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Just looked, a 20A C breaker has Zs max as 0.87 ohm which is probably met, but a 20A D is 0.44 ohms for 0.4s disconnection which is not likely to be met.

 

[swisstony]

If your proposed table saw has an induction motor that could change things, you will have to think about the surge current on start up to prevent nuisance tripping.

Dewalt 745 so I think that is induction motor with soft start
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That is a good point. If needed he could have a C or even D curve 20A breaker for this and probably still meet the disconnection times (also has RCD protection so would meet it for L-E fault anyway). That might be enough for surge effects.

If the tumble dryer is used a lot along with electric heating in the shed and the tools then it might be too much, but that is for later consideration. Rather than replacing (or adding) the SWA cable he could have a separate circuit put in from main CU to the garage just for the SWA (assuming a spare location in the CU) to remove the tumble dryer from the equation.

Finally it raises a useful point, he might want to fit some sort of emergency lighting so if the power fails for any reason he is not in a dark and dangerous workshop trying to get out past various cutting tools!

Not too expensive and potentially a great feature, for example:

2W LED Non Maintained Emergency Bulkhead White

High quality polycarbonate construction with long life LED

www.cef.co.uk

[automerge]1585909776[/automerge]
Just looked, a 20A C breaker has Zs max as 0.87 ohm which is probably met, but a 20A D is 0.44 ohms for 0.4s disconnection which is not likely to be met.

great comments and very useful. You have pointed out many things I hadn't considered
I will perhaps send over some of these to spark for consideration. In the meantime I have gleaned some useful tips on a temp measure .

 

[Mike Johnson]

Bit off subject, but do you really need a site saw? What are you going to be cutting?

 

[Baddegg]

Of course he does he a man!

 

[pc1966]

Not just a man, but a man with two sheds!
We all bow down humbly in awe...

 

[Mike Johnson]

The reason for asking is that my table saw gets very little use since buying a track saw.

I will see your two sheds and raise you four sheds, two barns and a piggery.