4mm ring

[russellsmiler]

Hi all
Currently doing up an old smallish barn at home, its a half height brick structure with a timber frame (old barn) and horizontal cladding, my good lady wishes to make this a photography studio (I have reservations about potential lingerie clad ladies roaming around the back of my property) I have the floor up at moment and are soon to replace with a 100mm thick polished concrete floor. I am currently installing 6 no double sockets at 450mm above FFL, because i want to keep the walls clean of wires i am burying the circuit underfloor (actually under the hardcore) I am using 3 core SWA, but have done this in 4mm (as this come from my own stock so to speak), an electrician says this need only be a radial not a ring as its 4mm, i have installed cabling to suit a ring....any issues????

 

[flameport]

Circuits can be designed in many ways. Ring, radial, or whatever else.

As to whether the design is suitable - only those that designed it and have all of the details of it can know that.

 

[pete01]

How will you gland the swa cables?
The glands will need to be accessible (I think)

 

[EFLImpudence]

an electrician says this need only be a radial not a ring as its 4mm, i have installed cabling to suit a ring....any issues????

Your electrician is correct.

The point of the UK ring circuit is that it uses a specific cable with a current rating lower than the overcurrent protection rating.

You cannot do that with 4mm² cable.

 

[Murdochcat]

Under the hard core ?

really?

 

[plugwash]

You cannot do that with 4mm² cable.

A UK ring circuit needs to be wired in cable that is at least 2.5mm² in size (1.5mm² if MICC) and at least 20A in current rating. What cable size is needed to achive a 20A rating will depend on the installation conditions.

In any case, there is nothing prohibiting use of over-sized cable, so if it has been wired as a ring may as well connect up both ends. Even if it's not strictly necessary.

 

[russellsmiler]

thankyou for all your comments!!
just a few comments to clarify
1. the 4 mm SWA is underbnearth the 150mm thick hardcore..sand blinded and running in parralells..its final depth will be 250mm underfloor, a full "as built" diagram will be part of the file I keep on the overall project of my renovation of this smallholding.
2. The cables are chased into the wall and have literally just now sunk in the metal backboxes, the cable will have the steel armour removed within 50mm of the back box, the insulator around the three cores will remain until inside the box, grommets have been fitted, this insulation will be stripped back when 2nd fixing.
3. Design, I havent really had a design done for this but feel I am following pretty standard guidelines...the barn is 7.8m long and 4.2 m wide, I feel that I can consider this a pretty smallish installation that would not need a overegg design, (but am open to ask here if my installation passess design criteria)

I know its easy to follow the advice that best fits the task I am trying to achieve, Plugwash you have given me confidence that there is nothing untoward about what I am doing, I hope others can agree.

 

[pete01]

How will you fit the glands? And if you're not how will you earth the steel armouring.

Why not use twin and earth? I'm confused.
Doesn't take much

 

[Murdochcat]

thankyou for all your comments!!
just a few comments to clarify
1. the 4 mm SWA is underbnearth the 150mm thick hardcore..sand blinded and running in parralells..its final depth will be 250mm underfloor, a full "as built" diagram will be part of the file I keep on the overall project of my renovation of this smallholding.
2. The cables are chased into the wall and have literally just now sunk in the metal backboxes, the cable will have the steel armour removed within 50mm of the back box, the insulator around the three cores will remain until inside the box, grommets have been fitted, this insulation will be stripped back when 2nd fixing.
3. Design, I havent really had a design done for this but feel I am following pretty standard guidelines...the barn is 7.8m long and 4.2 m wide, I feel that I can consider this a pretty smallish installation that would not need a overegg design, (but am open to ask here if my installation passess design criteria)

I know its easy to follow the advice that best fits the task I am trying to achieve, Plugwash you have given me confidence that there is nothing untoward about what I am doing, I hope others can agree.

if this is domestic , have you read the part p regs dated 2010 ?

 

[sparkwright]

Your electrician is correct.

The point of the UK ring circuit is that it uses a specific cable with a current rating lower than the overcurrent protection rating.

You cannot do that with 4mm² cable.

You can't use over-sized cable on a ring circuit?

 

[JohnW2]

A UK ring circuit needs to be wired in cable that is at least 2.5mm² in size (1.5mm² if MICC) and at least 20A in current rating. What cable size is needed to achive a 20A rating will depend on the installation conditions.

Indeed - but as EFLI has implied, there's usually no point in wiring it as a ring unless its CCC is appreciably less than 32A, since the only real point of the dispensation (for rings) in the regs it allows a cable with a CCC as low as 20A to be protected by a 32A MCB.

However, the regs only relate to a circuit protected by a 32A MCB (or 30A fuse). If they were logical, one might also expect them to allow a ring of cable with a CCC of, say, 32A to be protected by a 50A MCB - but they aren't that logical.

As things stand (i.e. as the regs actually are), the only reasons one might contemplate a 32A 4mm² ring are (a) if the installation mwthod of the 4mm² cable were such that its CCC was less than 32A or (b) if(pretty ulikely), voltage drop was going to be a problem with a 4mm² radial.

In any case, there is nothing prohibiting use of over-sized cable, so if it has been wired as a ring may as well connect up both ends. Even if it's not strictly necessary.

I can't disagree with that.

Kind Regards, John

 

[JohnW2]

if this is domestic , have you read the part p regs dated 2010 ?

I, for one, have read Part P, the entirety of which says:

“Reasonable provision shall be made in the design and installation of electrical installations in order to protect persons operating, maintaining or altering the installations from fire or injury.”

What bit of that do you believe it relevant to this discussion?

Kind Regards, John

 

[JohnW2]

You can't use over-sized cable on a ring circuit?

Of course you can - but EFLI's point (and mine) was that, if one does, one cannot protect it with an OPD with a higher rating than its CCC (whereas you can with 2.5mm² cable (or 1.5mm² MICC).

Kind Regards, John²

 

[pete01]

Dont think anyone said they was going to put it on a 40 or 50 amp protective device, just that they had it spare so they used it.

The only issue is how to gland which hasn't been answered

 

[sparkwright]

Of course you can - but EFLI's point (and mine) was that, if one does, one cannot protect it with an OPD with a higher rating than its CCC (whereas you can with 2.5mm² cable (or 1.5mm² MICC).

Kind Regards, John²

Yes, agreed, but there was no mention of using a higher OPD as I recall.