Wiring the garden shed

[Declan]

During renovations I have access to run new cables from the CU to the back of the house. This access will soon be blocked. So, it's now or never for power in the wooden shed which is < 10m from the house.

I'd like something real basic, perhaps a single socket outside and a light inside the shed. I plan on using this for hand power tools outside the shed. Note I dont mind if the shed light goes off on an RCD trip (it's a small shed so I wont be doing much inside).

I was thinking of running 2.5mm T&E from the CU to the rear of the house and get it wired up later.

I imagine completion would be something like:

In the CU, connect the 2.5mm to a new 20A MCB on the RCD side (30ma). At back of house (inside) connect 2.5mm supply cable to a switch which feeds the SWA (underground) to the shed, connecting to the socket outside the shed which then feeds the light.

Would such a design comply with the regs or would I need a more complicated setup involving another CU and RCD ? Most importantly since I am happy to limit to 20A would 2.5mm T&E be suitable to run from the main CU to the back of the house ? Would an earth from the main CU to the shed with RCD protection on the CU be ok ?

[Declan]

I am renovating and temporarily have access to run new cables from my CU to the rear of the house. So, it's now or never for power in my garden wooden shed !

I'd like a light in the shed and a single socket outside for use with hand power tools and garden equipment. I've read several posts on this topic but wonder if the following would comply with the regs:

1. New MCB (20A or less) on RCD side of CU.
2. From there run 2.5mm T&E to the rear of the house (approx 15m cable).
3. At the rear connect the 2.5mm supply to a switch (indoor) which supplies a 3 core SWA underground to the shed (approx 10m).
4. At the shed connect the SWA to the external socket and continue to supply the light (via a 6A fuse).

Notes: As the socket is outside I think it's ok that the light goes out with an RCD trip. The shed is small and not suitable for work inside. With this in mind is it ok to rely on the main RCD for earth fault protection?

Most urgent is whether a 2.5mm T&E is sufficient from the CU to the rear of the house. I can figure out the rest later but the CU access will be blocked in a few days. The max load should be 60W light and 1kw power tool.

Any advice appreciated.

Cheers, Declan.

[htmark98]

First of all Part P which i recommend you read up on.

I'm thinking of doing this, what i'm doing it getting 6mm 30amp T&E from the CU, wired to a RCD in the shed from that run the 2.5mm T&E to parts around the garden and then 1.5mm for lighting.

[Tim S]

RCBO (MCB/RCD combined on the non RCD side would be better. You are more likely to get an RCD trip in the shed (damp, dodgey lawnmower etc) and you don't want that taking out most of the house. The regulations mention that a fault on one circuit should not affect other circuits (where practicable).

The other option (I'm not qualified BTW so it's upto you verify this yourself) is non RCD feed to the shed and local RCD for socket. This has teh benefit of not blacking out the shed when you have the 9" angle grinder in your hand spinnign away.

You could run bigger in case someone wants to upgrade the circuit later. How are you runnign the cable - will it be bunched with any other cables or buried in insulation - both lower the rating of the cable.

Have a look at this:

what type of earth have you got?

HTH

Tim

[John Rumm]

Fairy snuff... then again if running a cable is going to be a major PITA later it might be worth using a bigger cable now. If you don't use the capacity then you have only wasted a little money on the difference in cable cost. If you do however find you needs expand then you save major hassle later.

IIWM, I would go with a 6mm^2 cable through the house now.

I would tend to terminate it inside the shed first and then connect to your sockets. Remember you will need a fused connection to the lights.

However before you get too far into this there are some details you need to investigate and think through first. In particular you need to look at the type of earthing that your house has as this will influence how you do things at the shed end.

Have a look at:

Would such a design comply with the regs or would I need a more

Depends a bit on circumstances. I know you said you are not worried about losing shed lights on a RCD trip, but are you happy losing all the other circuits in the house that are protected by the same RCD? Or for that matter trekking into the house to reset it each time (when going to the shed might be simpler)?

Also outside circuits are more prone to causing spurious trip problems anyway (more likelyhood of damp getting into unwanted places), again if this should happen do you want be experiancing these on the house RCD?

If you really only ever anticipate a small load being used at the shed, you could do an even simpler install by taking a spur from a fused (and switched if you want) connection point from a suitable point on an existing ring final circuit in the house, and feeding the shed from that. All you would need in the shed would be an incoming double pole switch, connections to your socket(s) and a FCU to feed your light.

See my comments above...

[Declan]

Based on the info so far I've modified my design to this:

1. Install a 20A MCB on the non-RCD side of the CU.
2. Run 6mm T&E from the CU to the back of the house. The cable will be bunched with other cables going through the ceiling joists. Can I connect this to a switch which connects to the underground SWA ? This is important now as I can run the cable to the back now and figure out the other details later.
3. Run the SWA underground from the switch to the shed (terminating inside the shed).
4. Fit a small CU inside the shed which has a 6A MCB for the light and

16A for the socket.

You've all convinced me that an RCD inside the shed would be more convenient and that I might regret not running 6mm.

I'm not sure what kind of earth I have. I live in an urban suburb in Ireland, I can only guess the earth goes back to the distribution supply. How can I check this?

Is there anything that would mean I can't run 6mm T&E from the CU to the back of the house? e.g. If there was an earthing requirement of running a continuous earth from the shed to the house CU then I'd be screwed as access to the CU will be gone.

Thanks for the help, Declan.

[Andy Wade]

An RCBO might be "better" but I suggest you're over-egging the pudding here, he only wants a light and socket for occasional use, and it sounds as if he already has a split-load CU with a 30 mA RCD side.

There will be an isolating switch in the house and most of the time the shed feed won't be live. Mostly Class 2 equipment will be used, so trips due to earth leakage are unlikely to occur, except in the case of a cut-flex mishap. Anyway, an RCD trip should not "take out most of the house," it should only kill the socket circuits (nothing else should be on the RCD side). Critically, it will not take out the house lighting.

A 16 A circuit will be fine here, 20 A is only necessary if two or more sockets are to installed. I'd suggest using a Type C MCB to handle tool starting currents (and tool transformer inrush current, if using hired

110 V tools).

The max overall circuit lengths from Table 7.1 of the OSG are as follows:

MCB C16, cable 2.5 (1.5 CPC) 29 m for TN-S earthing or 33 m for PME MCB C20, cable 2.5 (1.5 CPC) 17 m for TN-S earthing or 27 m for PME MCB C20, cable 4.0 (1.5 CPC) 19 m for TN-S earthing or 43 m for PME

[Tim S]

Possibly. But with a shed it's not beyond the bounds of possibility to get water ingress running into the electrics, or bugs depositing webs/crap inside fittings which then get damp. If I were doing it I would use a different RCD provision from that of the one covering the house sockets - cost difference is likely minimal anyway. It's true that he would have an isolator, but sod's law would be that he'd be on holiday, the shed roof would develop a leak, the isolator would be closed and the fridge/freezer would suffer assuming they are on the house RCD.

It's all debateable I know.

Cheers

Tim

[Declan]

Thanks Andy, I'm so easily influenced I'm thinking that your suggestion meets my requirements :-)

You're right in that it is for occasional use and I would keep it isolated when I'm not using it (probably use the light more than anything).

"and it sounds as if he already has a split-load CU with a 30 mA RCD side."

Thats correct, though I need to buy a bigger busbar as the RCD busbar is full. So sounds like using 2.5mm sharing the RCD with the house sockets would be ok regardless of what earth I have once the cable does not exceed

29m (dont think it will but I'll check).

Declan.

[Mungo]

Another thought: install FOUR core cable as a contingency so that (say) you can monitor the state of the shed PIR tripping from within the house?

My old man always suggested that an outside PIR sensor installed for security (deterrent) purposes ought to switch on a light WITHIN the house (to make it appear that the householder has heard a noise and is coming to investigate).

HTH

Mungo

[David Hansen]

On 9 Jan 2006 05:37:59 -0800 someone who may be "Declan" wrote this:-

Then your new cable may need to be de-rated, depending on the loads in the other circuits and how far they are bunched together.

Of course, or any other suitable cable. If this is SWA then it has a rather large gland that needs to be attached to a suitable hole in the enclosure with space to work around it. Metal boxes are often used to terminate SWA. You don't even need a switch at this point, but the enclosure needs to be suitable to terminate the "outdoor" cable.

In addition, if you are reducing the cable size at this point you need to consider how you will protect this reduction in cable size and how you will pass on this point to others.

Does the shed have a water tap in it, or other extraneous conductive part? Unless it does you do not need to do more on earthing than use the protective conductor of the supply.

[Declan]

Just one question before I run the cable tonight. I figured I'd go with

6mm in order to future proof the wiring at the rear of the house. Based on the info Andy provided below it seems that the thicker the cable the shorter the max length (for earthing). So with 6mm I am more likely to need an earth rod in the garden shed than with 2.5mm, is this correct ?

MCB C16, cable 2.5 (1.5 CPC) 29 m for TN-S earthing or 33 m for PME MCB C20, cable 2.5 (1.5 CPC) 17 m for TN-S earthing or 27 m for PME MCB C20, cable 4.0 (1.5 CPC) 19 m for TN-S earthing or 43 m for PME

[Andy Wade]

I don't follow what you're saying. All other things being equal, a larger cable will always allow you a longer run. There's not really any need to go to a separately earthed TT installation here, given the very simple nature of the installation and the likely use.

[Chip]

On Wed, 11 Jan 2006 23:13:48 +0000,it is alleged that Andy Wade spake thusly in uk.d-i-y:

I am just seeing '4.0 with a 1.5mm cpc' there and thinking 'yes, the length would have to be bloody short'. I suspect for conduit wiring or SWA with equal or oversized earth conductors, the lengths would be more in tune with intuition.

[Declan]

I was looking more at the TN-S side and got the wrong impression. The extra 4A drops the length from 29m to 17m and the 4.0mm cable only increases the length by 2m. I find this surprising but I'm sure there's a good reason for it. I need to read up on the different earthing types. I tried to find table 7.1 online but didnt have any luck. Can you post a link to it please, or send the info for 6mm ?

Thanks again for the help, Chris.

[Andy Wade]

I doubt you'll find it on-line, you'll have to buy the OSG, which is well worth having if you're going to do any amount of electrical work. The TN-C-S/PME figures are voltage-drop limited, whereas in this case the TN-S ones are limited by earth fault loop impedance (due to the use of a Type C MCB). Yes, I know there is an RCD on the supply side which in practice will take care of earth faults, but in TN systems it's recommended that you still design on the basis of earth fault protection being provided by the fuse/MCB and rely on the RCD only for supplementary protection against direct contact.

It doesn't cover circuits using 6mm^2 cable with only 16 or 20 A devices, so won't help you anyway. For TN-C-S/PME you can just scale the lengths in proportion to the conductor size, for TN-S we'd have to do a calculation, but it would be substantially more than 19 m because the CPC has increased from 1.5 to 2.5 mm^2. In any case you have a mixed T&E and SWA run, so the detailed calculation is a bit more complicated. The full set of information required is:

- supply type: TN-S or TN-C-S/PME? - MCB type and rating - length of the 6 mm^2 T&E section - length & size of the SWA section, and whether 2-core (armour as earth) or 3-core.

[Declan]

Interesting..... any tips on how I find out what my supply type is?

I've run the 6mm to the back of the house so I can complete the wiring at my leisure. I'm also considering putting the switch on the outside of the house to avoid the SWA requirement. This is if I can find a good solar powered light for the shed that will survive the winter (min temp -10oC).

[John Rumm]

I've run the 6mm to the back of the house so I can complete the wiring > at my leisure.

Not sure I follow that...?

[Declan]

I meant as an alternative to running power to the shed I could just put an external socket on the rear wall of the house. My main requirement is power in the garden and a light in the shed. I'm thinking it might be cheaper and a lot less hassle if I can get a good solar light for the shed, then I dont need to run power to it. My worry about solar powered light is that the equipment might not last through the winter and I might not get enough sunlight to get good light in the shed - even for say 30 mins a day.

[John Rumm]

Ah, I see what you mean... yup that would be easy enough...

Must admit I have not been that impressed with any solar lighting setups I have seen so far. Still I suppose it depends on what you are prepared to pay to make it work.