light to outside shed

[bigfoot2]

hi all
ive just erected a metal type shed for my dad in his back garden.
he wants to have an electric supply to it.
i have 2.5 armoured cable. can i tap into a socket from the kitchen by fitting an fcu as a spare, then run a 2.5 flat twin and earth to a to a metal junction box under the kitchen worktop, from there attach the t/e and the armoured cable using a cable connector. the distance from the house to shed is about 25 ft. im also not sure what to do in the shed side. do i fit another metal junction box and another fcu in the shed as another isolater. he wants a socket for his lawn mower and drill etc and a light fitting. also do i really need the 2.5 flat t/e cable?, can i just use the armoured cable if i cut back the armoured part?.
thank you.
regards bigfoot2

 

[dingbat]

Check out the response to the post by MerlinsDad, further down the page.

 

[breezer]

dingbat means this

 

[dingbat]

dingbat means this

I could do that, you know... if I wanted to. Honest. No worries... I was just a bit tired, you know?

Anyway, what he said....

 

[Big_Spark]

Bigfoot, in theory you can do what you state, however I would strongly advise against it.

Any building that is not part of the main structure, physically seperated from it, should have it's own sub-main supply. Taking the supply from an existing circuit is changing the characteristics of this circuit and it is possible you will not get the protective devices operating as required.

Domestic circuits have disconnections times of 5 seconds, that means the supply must be disconnected WITHIN 5 seconds should a fault occur. In reality it is a lot faster than that unless the circuit has not been installed correctly.

Outbuildings are a different animal, legally, and to comply with BS7671, the disconnection time must not exceed 0.4 seconds, and the circuit must be designed with this in mind.

To compound this you state that the shed is a metalk enclosure, this has all manner of ramifications which MUST be considered.


I would recommend you do this.

Install a 40A Type B MCB in your main consumer unit. Use this to supply a new enclosure, mounted somewhere convenient beside the consumer unit, into which you terminate the SWA feeding the shed. The cable from the 40A MCB to this enclosure should be 10mm Double Insulated Cable.

The smaller enclosure should be METAL not plastic.

Inside this enclosure mount a 32A 100mA Double Pole RCD. This should be DC sensing (It will have a small white cable attached to it which you connect to the Earth terminal)

This enclosure should then be connected to your main Earth point, or to the Earth bar in the main Consumer Unit, by a 6mm Earth Cable.

The SWA which you use to feed the shed should be 3 Core (Red, Yellow, Blue). The third core (Yellow) will be used to ensure a good earth connection (This should be sleeved in Green/Yellow sleeving).

Bury the cable at least 450mm deep into the ground, and over the top lay sand, about 100mm deep. On top of this lay some Electrical Warning Tape, then backfill the trench.

At the shed end, install a small four way enclosure. Ways 1&2 should be filled with a 25A 100mA double pole RCD to act as the main switch. This should, again, be DC sensing.

The lighting should then be protected by a 3A Type B MCB whilst the sockets a 16A 30mA RCBO.

Ensure that the metal of the shed is electrically connected to the Earth bar in the enclosure, and a small bonding wire should be fitted between the body of the shed and the door to ensure a permanent electrical connection.

 

[il78]

Is it me......or is it OTT with the RCD

100mA at the mains - 100mA in the shed

Does discrimination not come into it?

 

[Big_Spark]

Is it me......or is it OTT with the RCD

100mA at the mains - 100mA in the shed

Does discrimination not come into it?

No, it is not over the top, what you are doing is protecting the cable AND the circuits involved.

The RCD in the house will protect the cable from attack, then you need to concern yourself with the shed installation.

The 30mA RCBO in the shed will trip before the 100mA RCD acting as main switch, but you need that there in order to protect the lighting circuit, remember this is a METAL shed. If it were concrete or wooden then this particular RCD could be a 32A Type B MCB.

Discrimination has been taken into account as you can see, whilst still providing adequate protection for the buried cable.

 

[il78]

Granted you need the 30mA for protection to power equipment outdoors(didn't question that), but you still have 2x 100mA in line........

 

[Big_Spark]

Granted you need the 30mA for protection to power equipment outdoors(didn't question that), but you still have 2x 100mA in line........

Reread both my posts, then you will see why there are two there in this situation.

I noted that in another post you stated you intend to do the C&G2400 course, I think when you do that you will understand why I have put two in line.

 

[il78]

Cant wait for the course to start.....

If you said put a 100mA time delayed at the mains area, no problems I'll understand then, or even 100mA at mains, 30mA covering shed board (both circuits), fair enough if it were to trip the lights go off.....

Dont get me wrong, I aint arguing, if I'm not 101%, I ask......

 

[Big_Spark]

Cant wait for the course to start.....

If you said put a 100mA time delayed at the mains area, no problems I'll understand then, or even 100mA at mains, 30mA covering shed board (both circuits), fair enough if it were to trip the lights go off.....

Dont get me wrong, I aint arguing, if I'm not 101%, I ask......

No problem with you asking.

A couple of years ago I would have put a 300mA RCD at the mains end, but as these are as rare as rocking horse manure now there is no other option but to use the 100mA.

If you actually do the calculations, you will see that a fault on the lighting circuit will trip the shed end RCD before the Mains end RCD detects the fault, the real key is buying quality components.

 

[securespark]

Necessary to bury SWA that deep as it is already protected?

 

[Big_Spark]

Necessary to bury SWA that deep as it is already protected?

Yes, the only other option is to run it on catenary wire, which must be a minimum of 2.5 meters from the ground at it's lowest point of sag.

As other properties border his, clipping the cable along the fence or wall is not an option, he may not actually own the wall/fence in question for starters, but ther are legal requirements regarding this.

The shed is outside the Equipotential Zone, thus not legally part of the installation supplied by the supply company. The supply to it is a sub-main and thus the Electricity Supply Act comes into force, and this states the minimum depth for single phase cables to be 450mm. If you do not comply with this and there are problems, then the supply company have the legal right to disconnect the supply until the problem is corrected.

I actually witnessed the LEB (as it was then) do this many years ago.

 

[securespark]

Surely the protection afforded to the cable by the armouring does away with the need to bury the cable?

I may be veering away from the regs here, but in PRACTICAL terms, you could argue that an overhead cable is more at risk than a buried one, but a catenary is allowed. Do these regs make sense?

Now I'm starting to sound like all the 7671 doubters!!!!!

 

[Big_Spark]

Secure, I have been involved with the installation of a large number of temporary site installs, whilst we do not advertise the fact, a number of our clients gets us to do all their temporary work, and this is a good way to assess your question.

Firstly, rarely have I known an overhead SWA actually get damaged except by sheer negligance on the part of Crane, truck and Digger drivers, however normally this damage is to the sheath only, and this can be repaired easily with the right materials. Normally it is the support poles that take the brunt of the attack.

Cables on the surface are EXTREMELY susceptible to damage, so much so that all our Vans carry about half a dozen Cable joint Kits, three main sizes catered for. I would estimate that we get about 6 calls a week to cables damaged, accidently, that have been dropped off supports or walls and then damaged.

You would be very surprised to find out how easily a garden fork will penetrate an armoured cable below about 10mmsq in size. 1.5, 2.5, 4.0 and 6.0mm cables offer little protection in this situation due to the size of the tip of a fork prong.