Spur of a radial....

[martygturner]

Morning,

Spur of a radial, I believed that a radial circuit has no spur as per a traditional ring, ie a radial can be a star or tree shape to begin with and therefore a spur off a radial is just a nonsense.

The reason I ask is because I have just been quoted silly money to have two high level desk pedestal sockets ( at approx 1100mm above the floor) to serve a new built in desk workstation in a flat we own.

Please don't get me started on why you would rent office space for your employer, but what the market wants.

Existing is 5 sockets on a radial that is 3m from the CU that has a 20amp RCBO protecting it. Its 2.5mm t/e clipped direct and covered by dot and dab plasterboard with a total run from first socket of approx 12m round three walls at 450mm total space is 20 sqr m.

Apparently this now needs 2 x fused spurs and conduit in the walls , I wanted surface mount ( tastes change) and a direct take off from the low level socket. The reason given was 18th edition....I suspect that this is big sweaty round things but....not having a copy.

Are there any circumstances that this would hold true, Has a radial changed its definition in the 18th ????

 

[EFLImpudence]

Spur of a radial, I believed that a radial circuit has no spur as per a traditional ring, ie a radial can be a star or tree shape to begin with and therefore a spur off a radial is just a nonsense.

The definition of a spur is a branch on a ring or a radial circuit.

Apparently this now needs 2 x fused spurs and conduit in the walls

On a 20A radial, you obviously don't need Fused Connector Units - as you have pointed out the cable is the spur.
Conduit is not mandatory.

, I wanted surface mount ( tastes change) and a direct take off from the low level socket. The reason given was 18th edition....I suspect that this is big sweaty round things but....not having a copy.

You may have surface mount if you wish.

Are there any circumstances that this would hold true, Has a radial changed its definition in the 18th ????

I don't have a copy, but I wouldn't think so; how could it?

 

[EFLImpudence]

Has a radial changed its definition in the 18th ????

I assumed you meant the latest amendment; the 18th has been with us for a while now.

 

[ebee]

Yes you can have a spur off a radial. Easy example is a 4.0 T & E say with a 32A circuit protection device, you could spur off it in 2.5 T & E

 

[sparkwright]

If you have a 20 amp radial circuit for sockets in 2'5mm2 cable, as far as I am concerned, you can spur off it anywhere you like - and the only reason you may need fused spurs (FCUs) would only be for added fixed appliances, or lights, or something - NOT for extra sockets.

 

[JohnW2]

Yes you can have a spur off a radial. Easy example is a 4.0 T & E say with a 32A circuit protection device, you could spur off it in 2.5 T & E

There doesn't seem to be any general agreement (let alone official definitions) about this but,, for what it's worth, I tend to use the same terminology as you imply - In the context of a 4mm² radial, I call any bit of 4mm² cable a "branch" (or just 'part of the radial'), but would call any (fused or unfused) bit of 2.5mm² (or 1.5mm², or maybe now even 1.0mm² ) connected to the 4mm circuit as being a 'spur'.

Let's face it, if all is in 4mm, it is an arbitrary decision as to which cable is 'the radial' and which is the branch/spur - so neither word actually means anything!

Kind Regards, John

 

[flameport]

Has a radial changed its definition in the 18th ????

No. Same today as it's been for ever.

 

[martygturner]

Thank you all, it was just the way the quote was worded like something had changed in the regs.... One sparky down. Is work getting a bit tough out there..we are still doing renovations but the spec has definitely dropped off. Have not had anybody ask for porcelanosa stuff now for over a year.

 

[SUNRAY]

A couple of the popular youtube idiots electricians incorrectly insist branches or spurs are not permitted and one in particular boasts how the horrible mess he created running 2 SWA's along the back of an otherwise lovely looking house had to be that way to comply.

They are wrong as a radial can be any design/shape, branches/spurs can be added at any point as long as each element is compliant. I don't believe there is any issue with a 2.5mm² 20A radial, from memory I think that should be good for close to 30m providing there are no correction factors, such as insulation, heat etc.

 

[JohnW2]

A couple of the popular youtube idiots electricians incorrectly insist branches or spurs are not permitted [ion radial circuits]...

Indeed, and they are not alone - but I'm not sure why you 'crossed out' the first word you had typed

They are wrong as a radial can be any design/shape, branches/spurs can be added at any point as long as each element is compliant.

Indeed so.

I don't believe there is any issue with a 2.5mm² 20A radial, from memory I think that should be good for close to 30m providing there are no correction factors, such as insulation, heat etc.

Provided it is not installed in such an 'extreme' fashion that its CCC falls below 20A, a 2.5mm² T+E radial of any length will obviously be OK from the point-of-view of cable protection.
If one assumes a TN-C-S installation with a Ze of 0.35Ω, then, from the POV of fault protection (i.e. 'maximum Zs), a 20A 2.5mm² radial can be about 101 metres long.
If (as I assume you were doing) one wants to go with the VD recommendation of 5% max then, for an entire 20A load at the very end of a 20A 2.5mm² T+E radial, that would limit the length to just under 32 metres - but 20A worth of load all at the very end of a circuit would be a pretty extreme/unusual situation.

Having said that, as you probably know, I am personally not all that keen on 20A radials, since one can theoretically 'overload' such a circuit (draw more current that the rating of its OPD) with loads plugged into just two single sockets (or, give or take some arguments, one double socket!).

Kind Regards, John

 

[plugwash]

Spur of a radial, I believed that a radial circuit has no spur as per a traditional ring, ie a radial can be a star or tree shape to begin with and therefore a spur off a radial is just a nonsense.

That is true, as long as all the cable is sized to match the protective device.

The spur guidance in the regs, does however recognise the concept of a 20A/2.5mm rated spur from a 32A radial. Not relavent in your specific case though.

The reason I ask is because I have just been quoted silly money to have two high level desk pedestal sockets ( at approx 1100mm above the floor) to serve a new built in desk workstation in a flat we own.

The big question is what are these "desk pedestal sockets". If they are just ordinary wall sockets mounted on a pedestal then 2.5mm direct to the sockets should be fine. OTOH if they are more extension lead like they may well need to be fed from a FCU.

Is there a specific reason these pedestals are needed rather than just sockets mounted on the wall?

 

[EFLImpudence]

Having said that, as you probably know, I am personally not all that keen on 20A radials, since one can theoretically 'overload' such a circuit (draw more current that the rating of its OPD) with loads plugged into just two single sockets (or, give or take some arguments, one double socket!).

Do you feel the same regarding just three sockets on a traditional ring?

 

[EFLImpudence]

That the maximum length of a radial might be 30m. - or whatever - sounds rather restrictive but it could have multiple branches, each of which had a length of 30m. from the origin, such that a normal sized house could (not that anyone would) be covered by just one radial circuit.

 

[EFLImpudence]

The spur guidance in the regs, does however recognise the concept of a 20A/2.5mm rated spur from a 32A radial.

How would that be achieved?

 

[ebee]

a 2.5 T & E cable might be able to carry 27A therefore a twin 13A sockets (or even two single 13A sockets) would be OK. The problem was that , quite understandably, a single or two might in future be changed to a twin .
A twin socket itself is actually rated at 13A not 26A or more as some might think. However it is type tested to 13A on one side and simultaneously 7A on the other side and temperature rise monitored for a while (might be 4 hours, I`m not sure).
The only places I would be unhappy to put a twin socket is somewhere that has a likelihood of inducing a user to place two "heavy" loads simultaneously (e.g. washer, dryer, dishwasher, kettle, oven - although a kettle would only be on for a very short while. Otherwise I would almost always install twin sockets anyway.