Adding to garage via fused spurs

[jim3010]

Hi guys, I'm trying to extend what lighting/sockets I have in my garage.
Currently I have a 2.5 t&e coming through the brick to what feeds 1 double socket and 3 tubelights. I'm not sure wether this is spurred off a ring main or is part of a radial circuit from the house. Anyway I want to add another 2 double sockets and 1 more tubelight. Here are some pictures. This is what I currently have

Current garage

• jim3010
• 28 Oct 2014
• 1

Current garage

Proposed garage

• jim3010
• 28 Oct 2014
• 1

Proposed garage

Would this work?

 

[deadshort]

Hi, yes is the short ansewer ! Do you have an RCD on your supply to the Garage ? If not you may want to fit an RCD FCU as your supply feed. NB to comply with regs - the lighting FCU must be fed from the load side of the non switched FCU !


Regards,

DS

 

[ban-all-sheds]

If not you may want to fit an RCD FCU as your supply feed.

If not, then doing that is not optional.

 

[jim3010]

Thanks for the replies, it is all dc'd protected at the CU.
There are 2 singles and 8 double sockets in total, including the socket in the garage on this circuit, + the 3 tube lights in the garage. All this is from a 20a mcb on the CU which I thought would point to it being a radial circuit? Or am I wrong?

It it is a radial would the above proposed garage plan still be viable?


Many thanks

 

[jim3010]

"It's all RCD protected at the CU" meant to type.

 

[deadshort]

Hi, yes 20a mcb should be radial but no reason why it could not be a ring on a 20a mcb. If it was a 32a on 2.5mm radial that would be a different matter.

And yes you can go with your proposed wiring, technically no requirement for the FCU on the feed from the house because you have a 20a mcb. I hope it's not supplying freezers !

DS

 

[JohnW2]

Hi, yes 20a mcb should be radial but actual reason why it could not be a ring on a 20a mcb

[I presume you meant to type "... no actual reason..."]
That's true - but, although there would be no actual reason why it could not be a ring, there would be no point in it being wired as a ring. The whole point of a ring final circuit is that the regs allow cable with a CCC of only 20A to be protected by a 30/32A fuse or MCB - if the OPD is only 20A, then there is therefore really nothing to be gained by wiring it as a ring.

Kind Regards, John

 

[jim3010]

I just read that 20 amp mcb ring circuits do exist where cabling has been run through insulation... I've got too much spare time today haha.

Erm there was a plan for 1 freezer.. This would be a problem?

 

[JohnW2]

I just read that 20 amp mcb ring circuits do exist where cabling has been run through insulation... I've got too much spare time today haha.

If, because the cable was run through insulation, its current-carrying capacity (CCC) were reduced to less than 20A, the dispensation in the regulations which allow ring circuits would not be applicable, so the MCB/fuse protecting the circuit would have to be reduced to no greater than the actual CCC of the cable (something less than 20A - so, in practice, probably 16A or 10A), whether it was wired as a ring or a radial. On the other hand, if the CCC were at least 20A, then it could be a 30/32A ring circuit.

So, as I said before, I really don't see what point there would ever be in having a 20A ring - but maybe I'm missing something!

Erm there was a plan for 1 freezer.. This would be a problem?

I presume that he was either thinking that a fault on any of the circuits protected by the same MCB and/or RCD would take out the freezer (maybe whilst you were on holiday!) AND/OR was wondering whether you have a freezer which will operate satisfactorily at the minimum temperatures which may be encountered in a garage (many stop working if the environment gets too cold).


Kind Regards, John

 

[jim3010]

Many thanks to you two guys, I think I've got a plan.

Wether it's a ring or radial, I'm going to need the 5 amp SFCU in place for the lighting circuit. As for the 13 amp FCU feeding the 3 proposed sockets and the 5 amp SFCU, I'll keep that also as if the circuit is a radial it isn't needed but won't do any harm. On the other hand if I never put this in and the circuit was indeed a ring, the wiring between the 3 sockets would be vulnerable...(think I've got this right..)

With regards to the freezer comment I was worried that they may have been power hungry or something and rinsed a large chunk of the 20 amp supply

Cheers

 

[JohnW2]

Many thanks to you two guys, I think I've got a plan. Wether it's a ring or radial, I'm going to need the 5 amp SFCU in place for the lighting circuit.

Indeed, although I think most people would probably use 3A - which should be more than adequate for your lighting load.

As for the 13 amp FCU feeding the 3 proposed sockets and the 5 amp SFCU, I'll keep that also as if the circuit is a radial it isn't needed but won't do any harm. On the other hand if I never put this in and the circuit was indeed a ring, the wiring between the 3 sockets would be vulnerable...(think I've got this right..)

If you're sure that the circuit is protected by a 20A MCB, the you don't really need that 13A FCU, regardless of whether it is a ring or radial circuit. Having it will not do any harm, other than theoretically limiting the total load (sockets plus lighting) in the garage to 13A, whereas it could otherwise be 20A.

Kind Regards, John

 

[ban-all-sheds]

With regards to the freezer comment I was worried that they may have been power hungry or something and rinsed a large chunk of the 20 amp supply

Well - if it's an unheated garage you now have something else to worry about.

 

[jim3010]

It would help if I knew how to copy/paste from an iPhone..

Ok John, so I'm taking that you have said no FCU would be needed if it was a ring circuit with a 20 amp breaker because 2.5mm can handle 20 amps? But if I had a 32amp breaker (which I dont) then I would need an FCU to run the 3 double sockets?

Sorry just trying to make sure I understand the idea rather than just doing what I'm told[/quote]

 

[ban-all-sheds]

... if it was a ring circuit with a 20 amp breaker

In practice the only ring final recognised by the Wiring Regulations is one with a 30A or 32A protective device. True the definition of Ring final circuit is "A final circuit arranged in the form of a ring and connected to a single point of supply", with no mention of protective device, but in reality nobody installs 20A rings.

What does happen is that if the ring gets damaged, people will replace the 30/32A device with a 20A as an alternative to finding and fixing the problem.

Sorry just trying to make sure I understand the idea rather than just doing what I'm told

Good man.

Do you fancy learning about testing, and getting test equipment?

 

[ban-all-sheds]

If, because the cable was run through insulation, its current-carrying capacity (CCC) were reduced to less than 20A, the dispensation in the regulations which allow ring circuits would not be applicable, so the MCB/fuse protecting the circuit would have to be reduced to no greater than the actual CCC of the cable (something less than 20A - so, in practice, probably 16A or 10A), whether it was wired as a ring or a radial. On the other hand, if the CCC were at least 20A, then it could be a 30/32A ring circuit.

So, as I said before, I really don't see what point there would ever be in having a 20A ring - but maybe I'm missing something!

Ah - well now - you are possibly missing the age of the installation. (Apologies if it's in the thread and I'm the one missing it... )

Prior to Amendment 1 2002 to the 16th edition, the old Regulation
433-02-04 required that the

“minimum current carrying capacity Iz of the cable be not less than 0.67 x the rated current setting In of the protective device.”

So it's possible that if installation method or other factors had reduced Iz to 14-16A there could have been a 20A RFC.