Addition DSOs on a radial - good practice?

[matt94]

Hi, I was going to post in the DIY forum but don't seem to have permission to at the moment. If i could ask a quick question - are you able to offer your opinion on my plan below:


I have a b16 circuit breaker in the CU for a radial circuit out to my garage. In the garage the cable feeds into a 13amp switch (for the light) and then spurs/continues on to a double-socket outlet.


I wish to extend the circuit and add two more double-socket outlets.


The plan is to continue the circuit from the existing DSO, and extend the radial to the two new DSOs.


Question 1: Is this acceptable practice?


Question 2: If I accidentally overload the circuit, eg by drawing 4000w from an outlet (4000w / 240v = 16.6a) the worst case scenario is that the 13amp fused switch will blow - is that correct?


Many thanks

 

[telectrix]

the switched fused spur should be for the light, fused at 5A. the sockets should come direct from the 16A MCB. any socket overload would trip the 16A MCB, which is there to protect the cable it feeds. is the cable to the garage 2.5mm?

 

[telectrix]

the switched fused spur should be for the light, fused at 5A. the sockets should come direct from the 16A MCB. any socket overload would trip the 16A MCB, which is there to protect the cable it feeds. is the cable to the garage 2.5mm? i hope you have the equipment and the knowledge to ensure adequate earthing, bonding if required, and that disconnection times are met.

 

[Darkwood]

If you wish to extend the circuit you will be required to ensure the alterations comply to current BS7671 thus rcd protection will be required, you have not mentioned this so a question to you is it protected with a 30mA rcd?

Secondly plugging a 4kw load through a plug top will take out the plug top fuse after a duration specified on the time/current curve charts set out in the BS7671, if this is your intention then a larger circuit is required and TBH i would be asking the input of a competent Electrician who can make a onsite visit and see any potential issues problems with your set-up.

This isn't just about adding to existing as you can't guarantee the integrity and safety of the original without knowledge and testing the circuit and any other factors will need to be taken into account including the type of supply system which could have knock on effects to the earthing arrangements in the garage.

Has the garage got a Gas/Water supply fed in metal piping?
Has the garage got structural steel or metal construction?

 

[Darkwood]

Tel - so passionate you posted it twice + edit lol

 

[matt94]

Ah, thank you both. I will check the arrangement of the switched fused spur, I may have miunderstood how this was configured.


Yes the cable is 2.5mm. The garage is brick construction and has no gas or water supply.


Good point about earthing, I will research more on this.

 

[matt94]

Apologies you are indeed correct, the switch is fused at 3amp and for the lights only, the socket does come direct from the 16a MCB


The circuit has a 30mA RCD, it comes from the main CU.


The 4kw load isnt my intention, it was just an example in case of accidental overload. Eg a 2kw fan heater on one DSO, a 1kw drill on another and someone plugging in something else at 1kw. I honestly don't foresee this load on the circuit, I was purely thinking in a hypothetical 'what if...' scenario.


The cable coming into the garage is three core, whilst the DSOs are fed via twin core. Presumably this makes no difference, and its just there for potential future additions?

 

[telectrix]

if it's SWA, maybe 3rd core is E.

 

[SirKit Breaker]

Am i missing something here or am i just plain stupid. What is a "DSO"?:confused5:

 

[GMES]

Double socket outlet!

 

[HandySparks]

...whilst the DSOs are fed via twin core.

With a third (earth) conductor? If not, stop using the socket and get this sorted.

 

[matt94]

With a third (earth) conductor? If not, stop using the socket and get this sorted.

Sorry, yes DSOs are twin and earth, main cable into garage is three core and earth. The third core terminates in a choc box within the fused switch and is not connected to anything.

 

[SirKit Breaker]

Double socket outlet!

Thanks. Is this some new terminology then.

 

[HandySparks]

... main cable into garage is three core and earth. The third core terminates in a choc box within the fused switch and is not connected to anything.

If this is three core and (uninsulated) earth, it's unlikely to be 2.5mm². More likely to be 1.5mm² or 1.0mm². It may be adequate, or it may not, depending on installation method.

You might be best to get this looked at.

 

[Engineer54]

Think we really need to see some photo's here, both of the house CU and the fused spur in the Garage. Ideally with the covers off!!

That, or better still, get a qualified electrician in to advise and correct as well as installing the additional socket outlets. It's going to be notifiable work anyway, so you won't be able to conduct any installation work yourself unless you notify the LABC a have them inspect and certify. All of which, will cost you around £250. So could well be cheaper, and more importantly give peace of mind knowing the installation is safe and meets with the 17th ed electrical installation regulations!!

 

[HandySparks]

.... It's going to be notifiable work anyway...

Why?..

 

[matt94]

As suggested, please see attached pictures (if this works...)

I didn't think i needed to notify the LABC because I'm only adding DSOs to an existing radial circuit. I see this as almost the equivalent as plugging in a 4-gang socket extension lead.

hopefully the photos will make it clear what I'm trying to do...

 

[HandySparks]

Ah. I wouldn't describe the SWA as "three core and earth" it's four core (one of which is used as an earth).

"Twin and earth" or "3 core and earth" is described as such because the earth conductor is a reduced size compared to the other cores, and un-insulated, and therefore unsuitable for any other function.