Shed Project

[moomaloo]

I'm looking to get a simple power supply into my shed. I live in Scotland so Part P doesn't apply (at the moment). However I'm not completely mad so I've enlisted the help of a qualified and competent electrician who lives near to me. He'll be doing most of the work and inspecting the installation. I've been tasked with gathering together the necessary bits and pieces to do this and, while I'm at it, I've been trying to learn more about domestic wiring installations because it's interesting and so that I don't look like a complete gimp when we actually get round to doing the work!

The idea so far is to take a spur off the existing ring main, run a new armoured 2.5mm twin and earth cable through the external wall of the house, bury it and properly sheath it for the very short distance between side wall of house and shed (approx 60cm) and then run it through the wall of the (wooden) shed and into a new garage consumer unit.

I have bought a Volex garage unit with RCD protection and two MCBs - one rated at 16amp to supply three 13amp wall sockets and the other rated at 6amp to supply three linked 60watt incansescent lamp fittings (linked together) with a two-way switch arrangement.

The power sockets will be used for a tumble dryer, a small freezer and one for general use with light power tools, strimmer etc.

We're planning on using 1.5mm twin and earth for the power socket circuit and 1.0mm twin and earth for the lighting circuit with 1.00mm three core and earth between the two-way switches (via a single junction box).

Question 1 - Do you think that a 16 amp MCB is sufficient to protect this circuit?

Question 2 - Should the power socket circuit be a new ring (connecting back into the garage consumer unit) or a radial circuit (terminating at the last socket)? - The installation instructions with the Volex are very poor, but my preference would be for the new ring.

I'm new to this but I'm enjoying trying to learn about the basics. I will NOT be attempting any of this work myself!

I know this is a very 'old chestnut' but any help or constructive comments would be much appreciated.

Many thanks.

 

[davy_owen_88]

The idea so far is to take a spur off the existing ring main

The power sockets will be used for a tumble dryer, a small freezer and one for general use with light power tools, strimmer etc.

Unless you only intend having one of those things on at a time, I'd reconsider the spur idea. You'll have to fit a FCU before the SWA which will limit all circuits in the shed to 13A - the tumble drier uses around 13A on it's own, coupled with the random on/off switching of the freezer which draws about 5A, it won't take long for you to overload the FCU when you start using tools at the same time.

Is the circuit you are spurring from RCD protected? If so an earth fault in the garage will knock off the house circuits and vice versa - not a good idea when you have a freezer full of food defrosting...

We're planning on using 1.5mm twin and earth for the power socket circuit and 1.0mm twin and earth for the lighting circuit with 1.00mm three core and earth between the two-way switches (via a single junction box).

I wouldn't use any less than 2.5mm² t&e for socket circuits.

Question 1 - Do you think that a 16 amp MCB is sufficient to protect this circuit?

Assuming you go ahead with wiring the sockets in 1.5mm² t&e you'll need to make sure that there are no derating factors that need to be applied before sticking it on a 16A MCB - hence using 2.5mm².

Question 2 - Should the power socket circuit be a new ring (connecting back into the garage consumer unit) or a radial circuit (terminating at the last socket)? - The installation instructions with the Volex are very poor, but my preference would be for the new ring.

If using 1.5mm² I'd say use a ring circuit but if using 2.5mm² I wouldn't bother.

 

[moomaloo]

Thanks for that Davy.

I hadn't bargained on fitting an FCU as none of the three sockets will be concealed and there is no requirement to permanently wire any appliance into the supply. Is this strictly necessary given the MCB protection from the new garage CU? - I guess, though, a FCU would allow me to isolate the shed spur from the house supply, though I could do this by switching off the shed supply from the CU (or will this knock out the house circuit too?).

The circuit I'm spuring from is the house ring main and doesn't have an RCD on the main CU so I think I should be OK.

It is possible that all three sockets will be in use at one time so, yes, this will overload a 13A FCU if I have to use one. Is there any alternative to this?

Any further comments / criticisms would be useful...

 

[electronicsuk]

It is possible that all three sockets will be in use at one time so, yes, this will overload a 13A FCU if I have to use one. Is there any alternative to this?

Yes, wire directly back to your consumer unit and have the shed on its own circuit and breaker.

 

[davy_owen_88]

I hadn't bargained on fitting an FCU as none of the three sockets will be concealed and there is no requirement to permanently wire any appliance into the supply. Is this strictly necessary given the MCB protection from the new garage CU? - I guess, though, a FCU would allow me to isolate the shed spur from the house supply, though I could do this by switching off the shed supply from the CU (or will this knock out the house circuit too?).

The rule is that a maximum of one single or one double socket can be run from an unfused spur from the ring circuit... I'm pretty sure that feeding a whole submain would require the 'spur' to be fused.

FCU protection has nothing to do with sockets being concealed nor anything to do with appliances... it is there to provide overload protection to downstream cables.

 

[moomaloo]

I'm obviously not getting it but I thought the overload protection to downstream cables would be provided by the MCBs in the new garage CU...?

 

[davy_owen_88]

I'm obviously not getting it but I thought the overload protection to downstream cables would be provided by the MCBs in the new garage CU...?

But what protects the cable between the socket and the new CU...?

 

[ricicle]

I've enlisted the help of a qualified and competent electrician who lives near to me

 

[moomaloo]

Hmmm, good point! This isn't something I had considered. I had thought we would 'just' wire the new 2.5mm t&e into the back of an existing socket in the house nearest to the planned point of entry into the shed.

I take it this isn't a possibility then? When I first spoke to my electrician neighbour, I got the impression that this is how he would do it. No mention of any FCU so I didn't put one on my shopping list.

Is it possible to install an FCU with a higher rating to accommodate the likely current drain from the three new shed sockets? Or might the MCB in the house consumer unit 'cover it' (I note that there are actually 2 B32 amp MCBs in the main board covering the existing sockets. This seems odd to my novice mind as it's a bungalow. Why two separate rings for the socket circuits I wonder...?)

 

[davy_owen_88]

You might have a separate ring circuit for the kitchen. Anyway, if I were you, I'd call your spark over for a cup of tea and run through the whole thing again, because it seems he's not been very clear on what he intends to do...

Oh, is your electrician/neighbour a member of a self certification scheme as this work is notifiable.

 

[ricicle]

Oh, is your electrician/neighbour a member of a self certification scheme as this work is notifiable.

In Argyll ............

 

[davy_owen_88]

Pfft, I'm not a map Ricicle, give me a break

I'd be in favour of having the option of selecting Wales, England, Scotland or Ireland instead of a county and UK just to make it easier for idiots like me.

 

[moomaloo]

Yes, not notifiable in Argyll. They don't care if we die here. We're just an embarassment to the Union...

Yes, I'll consult with my expert. Just trying to increase my understanding of these things (plus perhaps checking up on him just a little as he's retired and may be pretending he was a sparky...)

 

[moomaloo]

I guess one of the (many) things I don't understand about all of this is that I've currently got a bog-standard 10A extension reel plugged into the socket in the house, trailing out through a window and under the shed door. It runs the dryer and a power tool very nicely!

Now this is very far from ideal but surely if that works with no apparent issues, why can't a simple spur (which is, in effect, a 'hard-wired' version of the same thing with much better protection care of the new C.U.) safely serve more than one socket?

 

[Steve]

I guess one of the (many) things I don't understand about all of this is that I've currently got a bog-standard 10A extension reel plugged into the socket in the house, trailing out through a window and under the shed door. It runs the dryer and a power tool very nicely!

Now this is very far from ideal but surely if that works with no apparent issues, why can't a simple spur (which is, in effect, a 'hard-wired' version of the same thing with much better protection care of the new C.U.) safely serve more than one socket?

How long has your setup been in existance?

A spur cannot serve more than one double socket because 2.5mm² is rated at 27 amps, at best. thats approx 2 x 13 amp plugs.

If you fuse it at its source with a 13 amp FCU, then you can feed as many sockets as you like, as the total load on the cable cannot then exceed 13 amps. A fused spur is just like a 13 amp extension lead if you like.