Spur of a radial....

[martygturner]

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 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?

They are just angled bench sockets, we are trying not to cut the wall out as I said tastes change. and mounting them on a workbench that is fixed to a wall.

https://www.floorbox.co.uk/collections/pedestal-boxes/products/pb002-double-pedestal-box is the type of box and I speced metal round industrial trunking you know the 1950's style as thats the type already in the flat

 

[JohnW2]

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

It recognises a 2.5mm² unfused spur supplying a double socket, but what exactly do you men by a "20A rated spur"? Are you just assuming that a double socket represents a maximum load of 20A?

Kind Regards, John

 

[JohnW2]

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

Not to the same extent. It's obviously a matter of degree/judgement, since any multi-sockets circuit can only be 'designed' on the basis of a degree of guesswork - nearly all real-world sockets circuits (of any type) have enough outlets to theoretically allow a total of loads which exceeds the rating of the OPD many times over! The designer therefore has to make a judgement as to what is the "likely" use of the circuit.

I particularly question the wisdom of having (seemingly not all that unusual these days) 20A radials supplying a kitchen

Kind Regards, John

 

[JohnW2]

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 .

As often discussed, there is a limit to the extent which one can (should?) attempt to anticipate incorrect things that might be done in the future. An FCU could be changed to a switch or JB (or a socket with a spur). An MCB could be changed to one of much higher rating ... etc. etc.

A twin socket itself is actually rated at 13A not 26A or more as some might think.

We've been through all this before, countless times, and no-one seems to really know the answer. According to the "Technical Data Sheet", MK double sockets are 'rated' at "13A per socket outlet", and when I asked their Technical Support people whether that meant that a double socket was 'rated' at 1 x 13A or 2 x 13A, he said he didn't know "because the Technical Data Sheets are written by the Marketing Department"!!

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).

It's actually 14A + 6A, but that is the minimum requirement to satisfy BS1363. There is nothing to say that a manufacturer cannot produce a product which exceeds that minimum standard and 'rate' it accordingly.

Kind Regards, John

 

[JohnW2]

That the maximum length of a radial might be 30m. - or whatever - sounds rather restrictive ....

Even if it were, as I said, a 'maximum' of that order only exists if one wants to follow the VD 'guidance', and follow that guidance even if the entire 20A load is applied at the very end of a 20A radial. As I also said, from the point of view of Zs (which is what really matters0, a 20A radial in a TN-C-S installation can probably be about 100m long.

.... 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.

Indeed so. Despite what I say above (i.e. even if one feels obliged to follow the 'VD guidance' in the most extreme of loading scenarios) , I don't think that even 30m would be particularly 'restrictive', since, in the average house, I doubt that any sockets would usually be more than 30m (even 'as the cable flies') from the origin of the circuit.

So, I think the bottom line is probably that people worry far too much about such issues.

Kind Regards, John

 

[martygturner]

Not to the same extent. It's obviously a matter of degree/judgement, since any multi-sockets circuit can only be 'designed' on the basis of a degree of guesswork - nearly all real-world sockets circuits (of any type) have enough outlets to theoretically allow a total of loads which exceeds the rating of the OPD many times over! The designer therefore has to make a judgement as to what is the "likely" use of the circuit.

I particularly question the wisdom of having (seemingly not all that unusual these days) 20A radials supplying a kitchen

Kind Regards, John

Can I ask why they use of 20 amp radials in a kitchen is not a good idea ? Its just that is exactly what tends to go in our renovations, one down one wall, one down another and one down the middle for the b loddy island... 20amps for say 5 appliances seems a lot in these days of a rated appliances.

 

[ericmark]

There seems to be some odd words or phrases all to do with being a final circuit.

To my mind a spur is where you branch off a final circuit with less current carrying capacity than the main part of the final circuit can carry.

But where you have an overload disconnection device to further protect the branch, then it would not be a spur with most installation methods, it would form a new circuit, however a final circuit clearly can't have any further circuits from it, as it is the final circuit, so we seem to call it a fused spur, which does some how not seem correct, as it is forming a new circuit, so one could call it a radial, except can't be a new circuit as it comes from the final circuit.

More down to English than electrics, there are some things we should never say, like the very end, the end is the end, no need for "very". We have most, but can't have moster, or mostest.

So if we have a ring final circuit, there can't be a further circuit formed after it, however we all know that is not really true, we can have fused connection units which can then supply a series of sockets, which we can't call a circuit as they come from a final circuit. When I was an apprentice we did call the fused spur a radial, as to if this was incorrect or if the names changed not sure?

Same way we refereed to live and neutral, but now it's line and neutral as both are considered live.

It is not easy to work out how one has brown, blue, and green/yellow cables with site 110 volt supplies, or markings on the plugs and sockets L and N when we don't have a neutral, should be brown and black and L1 and L2 but it's not.

I think calling is a fused spur is confusing when we had rules like can't take a spur from a spur, as you can take a spur from a fused spur, but I don't select the names. We have many confusing names, electronic transformer is at least different from transformer, but with ballast we give same name be it wire wound or electronic.

But is seems we follow what the international standards organisation says, only the Americans seem to ignore what they say, and not call 120 volt low voltage, or 12 volt extra low voltage.

 

[JohnW2]

Can I ask why they use of 20 amp radials in a kitchen is not a good idea ? Its just that is exactly what tends to go in our renovations, one down one wall, one down another and one down the middle for the b loddy island... 20amps for say 5 appliances seems a lot in these days of a rated appliances.

Well, three 20A radials in a kitchen is obviously a lot better than one (or maybe two), which is not all that uncommon.

A while back I looked at a friend's electrical installation because he had been experiencing trips of an MCB (or maybe it was an RCBO) one one of the two 20A radials supplying his kitchen. It transpired that, given the way he had organised appliances and work spaces in his kitchen, most of the 'large' loads (including an oven and tumble tryer, and probably DW/WM and, occasionally, toaster/kettle etc.) were plugged into one of the radials.

I have no particular problem with a kitchen being supplied by two or more 20A radials, provided that the user is aware of which sockets are on which circuit and spreads the large loads across circuits accordingly. If you are doing a 'renovation' and designing/installing the kitchen, as well as the kitchen electrics, that can probably be largely achieved - but it will rarely be the case when a house owner does their own 'arranging of the kitchen' and using the existing electrics.

However, as I implied, there seem to be a good few kitchens supplied by just one 20A radial, and that would seem to be "pushing one's luck" - if there is only one circuit, I would expect it to be a 32A one.

Kind Regards, John

 

[plugwash]

It recognises a 2.5mm² unfused spur supplying a double socket, but what exactly do you men by a "20A rated spur"? Are you just assuming that a double socket represents a maximum load of 20A?

I'm assuming the cable is expected to be rated at at least 20A as installed. The same as the cable on a ring is required to be rated at 20A, but maybe that is just an assumption and not actually stated anywhere.

 

[EFLImpudence]

I'm assuming the cable is expected to be rated at at least 20A as installed.

No, the requirements for the ring do not apply to the spurs.

The same as the cable on a ring is required to be rated at 20A,

Except that it isn't. 20A T&E (1.5mm²) is not allowed.

but maybe that is just an assumption and not actually stated anywhere.

It is not stated anywhere - and is apparent for spurs to lights, fans and other similar low current appliances.

 

[plugwash]

Can I ask why they use of 20 amp radials in a kitchen is not a good idea ? Its just that is exactly what tends to go in our renovations, one down one wall, one down another and one down the middle for the b loddy island... 20amps for say 5 appliances seems a lot

Without being overloaded, a 20A radial can only support one full size appliances plus smalls. Kettles are sometimes a full 13A, as are some washing machines and dishwashers. I don't think it's unreasonable to expext that two of these appliances may be drawing full power at the same time. Three is much less likely.

This line of thinking would lead one to conclude that if using 20A circuits the washing machine, diswasher, tumble drier and countertop sockets all need to be on separate circuits.

That said the kitchen in my parents house was all on a 15A BS3036 for years and it very rarely blew. Granted they didn't have a tumble drier at the time.

in these days of a rated appliances.

The wheeze with washing machines and dishwashers is that the energy rating only reflects the most economical cycle, which may not be the cycle people actually use.

 

[JohnW2]

I'm assuming the cable is expected to be rated at at least 20A as installed. The same as the cable on a ring is required to be rated at 20A, but maybe that is just an assumption and not actually stated anywhere.

Yes, that's what we tend to assume - but, as you say, it's only an 'assumption'.

It could be that you're right, that ine intention is that the dispensation in 433.1.204 (essentially allowing violation of 433.1.1 in the case of a ring final) applies to the cable of spurs as well as of the ring itself. If not, to have a length of cable with a CCC or 20A protected upstream by a 32A OPD and downstream by 2 x 13A fuses (=26A)) would clearly not be compliant with 433.1.1. However, I don't think it's a foregone conclusion that such was the intention, since although no part of cable in the ring will ever have to carry the full load current, the cable of a spur does carry all of the current being provided by that spur.

Kind Regards, John

 

[JohnW2]

No, the requirements for the ring do not apply to the spurs.

Well, it's now you that is making an 'assumption'. As both plugwash and I have said, we just don't know, because the reg is not clear enough.

For what it's worth, 433.1.204 says ..

Accessories to BS 1363 may be supplied through a ring final circuit, with or without unfused spurs, protected by a 30 A or 32 A protective device ........... are deemed to meet the requirements of Regulation 433.1.1 if the current-carrying capacity (Iz) of the cable is not less than 20 A and...

... and I wouldn't think i would be too difficult to argue that the intention of that was that the 20A minimum CCC applies to spurs from ring finals as well as the cable of the ring itself, is it?

Kind Regards, John

 

[EFLImpudence]

Well, it's now you that is making an 'assumption'.

No. I think it is obvious because you may use cable with a CCC of less than 20A.

 

[RandomGrinch]

They are just angled bench sockets, we are trying not to cut the wall out as I said tastes change. and mounting them on a workbench that is fixed to a wall.

For simplicity, I would make them as extension leads, 1.5mm flex to a single plug. We have literally 100's of lab benches made in the same way!