Shed Power from Spur for Frige and Lights Ask questions and find answers to many subjects relating to electrics and electrical work

[Vike]

Hi everyone. New user here.

I'm in the process of installing power to our new shed. It needs to supply a couple of 2-gang sockets and two LED or fluorescent light fittings. The idea is to have a permanent fridge- freezer in there and occasionally run a garden vacuum/blower and power the odd drill or charge power tool batteries.

I know, ideally, I would run a new ring from the consumer unit but it's a terraced house with the CU at the front and this would be impractical. The CU has all MCBs and no RCD

My plan is to take a spur (2.5mm 2c+e) from a kitchen socket through an outside wall into a wiska box. From there I'll run a 3 core 2.5mm SWA armoured cable round the garden into the shed and directly into a 13 amp switched fcu. From there using 2.5mm 2c+e again I'll supply one or two 2-gang sockets and (1.5mm 2c+e through a light switch to 2 LCD or fluorescent light fittings.

I know that I can have any number of sockets and lights on a fused spur as long as the load is no more than 2.9kw and if it ever gets overloaded the 13 amp fuse in the fcu should blow.

Specifically, Is it OK to run the SWA directly into a Fused Spur Switch with a 13 amp fuse in the shed rather than have the SWA start from an fcu in the kitchen? And do I need another fcu switch to control the lights with a 3 amp fuse or can I just use a normal unfused switch?

I also plan to plug in stand-alone RCD adaptors between any appliances/tools and their sockets. Is all this correct, do-able and safe?

Any advice gratefully received and many thanks.

[Mr White]

Is all this correct, do-able and safe?

No, No, and No

Your plan has too many flaws to be safe. Also, how will you test it? (Just because it did not go bang does not mean it is safe)
I suggest you get an electrician in to design it all for you, and install and test it, but you could offer to do the donkey work.

[ericmark]

"The load current in any part of the circuit should be unlikely to exceed for long periods the current-carrying capacity of the cable (Regulation 433.1.5 refers). This can generally be achieved by:
(i) locating socket-outlets to provide reasonable sharing of the load around the ring
(ii) not supplying immersion heaters, comprehensive electric space heating or loads of a similar profile frog the ring circuit
(iii) connecting cookers, ovens and hobs with a rated power exceeding 2 kW on their own dedicated radial circuit
(iv) taking account of the total floor area being served. (Historically, limit of 100 m² has been adopted.)"
Taken from BS7671:2008 so a bit out of date, but the 2 kW limit means using a blower is likely going to cause problems.

For any single phase refrigeration motor volt drop is really important, it needs to reach running speed before the pressure raises or it will stall, and the overload will operate, the overloads are not designed to operate regular and if they do then they fail.

To work out volt drop, and to ensure the overload device will operate within the required time, needs the prospective short circuit current or the loop impedance to be measured, since looking at 230 volt, ohms law means the two are linked together, most meters allow either to be read by pressing one button.

However the meters are not cheap, the same goes for the RCD tester, I can't measure 40 mS with a stop watch, so no option, need to use an expensive tester.

To work out what cable can be added to a circuit, step on is measure the volt drop of the existing circuit, I can then calculate the volt drop after adding the new circuit.

For a ring at mid point I am looking for 0.94Ω line to neutral, the cheap plug in testers will not measure below 1.9Ω so are no good. To pay out £500 for test equipment is not what most DIY people want to do, so in real terms not a DIY job.

[Vike]

Thank you for your reply, Mr White. When you say so many flaws, please can you let me know what they are? My design was taken from my son's installation by an electrician which was essentially identical. Spur from a socket through outside wall, wiska box to SWA round the garden into a brick shed to switched fcu supplying a double socket and a fluorescent light fitting, controlled by the switch in on the fcu. He also has a fridge freezer in there. Is it the design itself that is wrong or because it will then need testing?

[ericmark]

This is why I did a detailed reply, as an electrician I visibly weigh up what is likely to be required, so a small house is unlikely to exceed the loop impedance limit of 1.37Ω so I do the job and then test, but a large house may exceed the limit, so I test first.

We can't see your house, so we rely on you telling us things like the earth loop impedance. But electrics is not a case of everything forward and trust to the lord. We use experience to speed up the design, but things still need designing, not just thrown together.

One would not use mini wheels and brakes on a Rolls Royce, but they are OK for a mini. What you are asking is can I fit mini wheels without telling us the size of the car.

[Vike]

Hi erickmark. Thank you for your replies. I'm sorry I didn't reply to your first one. My house is a small two bedroom terraced cottage. In fact we added the shed because of the lack of storage space, especially in the kitchen. The kitchen ring has 3 double sockets off a 32 amp MCB with a separate 40 amp cooker circuit with another single socket integrated in the cooker switch. There is no RCD protection in the CU only MCBs. The boiler is also fed from the kitchen ring. As mentioned, my son's supply to his shed is also taken as a spur from a socket on an outside wall. That socket is in a living room that has 4 double sockets on that ring but he has 2 RCDs in the CU. The other thing to mention is that the socket that I was planning to take the spur from already supplies a washing machine and tumble dryer. I don't know if this is helpful in telling me what the flaws to my plan are. I did get a quote from a electrician but that was to the the wall of the house but he couldn't take the cable through the kitchen to the CU and that was down to me or he could bring somebody else in to do that part of the job at additional cost. He was obviously going to connect up to the CU. Then just as he was leaving he asked if there was a socket on my outside kitchen wall and when I said there was it's where the washing machine and tumble dryer plug into, he said there's your answer. My original post was to confirm if a) it was a good idea and b) if it was something I could handle myself. I thinking I could but would need to call the electrician back to test and certify the work. I should speak to him before connecting anything up and ask if he could install an RCD in the CU at least on the kitchen ring in question. What say you, please? Am I making sense now?