2.5mm Spurs - Why?

[Geflugenmitt]

Hi,

I have an electronics background so I understand the principles, and used to do my own house wiring before the regulations changed. I now employ an electrician, I'm renovating an old cottage, and we work side by side (I'll drill holes for him and he does the fitting etc). So all the wiring and CUs right back to the meter are all new and certified within the last 2 years.

I requested he run a spur for me from a ring main. Obviously better if I'd foreseen this as I had to rip out some decor, but it will be used to feed a heater I had not anticipated needing. This will be a 1.9kW clay core heater (hybrid storage/conventional heater). The heater supplier has specified this meets their regulations, in fact they often plug these heaters directly into a 13A socket. The ring main services two small rooms only (not the kitchen) and has bags of capacity.

My query is simply this: The electrician has used 2.5mm cable for the spur. The spur is only about 1m long, and I realise this is more than capable of feeding the 1.9kW load, but I expressed surprise because this won't be protected by the MCB which is 32A. It would have been difficult to jam 4.0mm cable plus 2 x 2.5mm into the source socket anyway. But he assured me this was acceptable. In fact on this website is says as much here
https://www.diynot.com/pages/el/el012.php#a0

As electronics professional looking in from outside, the regulations seem inconsistent to me. Is 2.5mm cable really OK? What is the reasoning?

 

[DetlefSchmitz]

At the end nearest the heater you will either have a 13A fused plug, or a 13A fused FCU, so limiting the current . The full story is way more complicated and there are potential issues, but that is the simple answer.

I just hope you haven't started a 20 page argument!

 

[AndyPRK]

Nah. Only be 15 pages

 

[PrenticeBoyofDerry]

Your heater is only going to draw about 8amp, 2.5mm twin and earth cable, can carry a maximum of 27amp. Providing there are not any extreme derating factors, that could bring the CCC down to less than 8amps (which is very unlikely). The cable will be very capable or safety carrying the load current.

 

[bernardgreen]

a 13A fused plug, or a 13A fused FCU, so limiting the current

It works when there is only one socket at the end of the spur. Even a double socket ( 2 time 13 amp fusing ) at the end of the spur is just within the current rating of 2.5mm² cable.

That said two 13 amp loads from a double socket is not a good idea as some double sockets are rated for 20 amps maximum load. ( the cable terminals being the "weakest" points )

 

[flameport]

because this won't be protected by the MCB which is 32A.

For the ring where there are 2 cables to each point, the MCB does both short circuit and overload protection.
For the spur, the MCB provides short circuit protection and the plug fuse (13A) provides overload protection.
Only one socket outlet is permitted, because with more than one, the total load could exceed the rating of the cable.
One double outlet is permitted because the standard for socket outlets doesn't rate them for 26A (it's more like 10A in one and 4A in the other) and it's exceptionally unlikely that two 13A loads would be connected there.

It's a similar situation where a 32A 4mm² or 6mm² cooker circuit can have the actual appliance connected at the end with 2.5mm² flex, the difference being with those there is no overload protection because a fixed heating load like a cooker cannot overload the circuit.

You could also have a socket outlet connected to a 4mm² or larger circuit using 2.5mm²

Or even a 32A MCB with a single piece of 2.5mm² cable going to one socket outlet (but not more than one).

 

[EFLImpudence]

I wasn't going to contribute to the twenty pages, however - I am sure the OP is wondering about that.

Only one socket outlet is permitted, because with more than one, the total load could exceed the rating of the cable.

Except with two.

One double outlet is permitted because the standard for socket outlets doesn't rate them for 26A (it's more like 10A in one and 4A in the other)

I feel confident in saying that not one single member of the public knows, would expect nor think that reasonable.

and it's exceptionally unlikely that two 13A loads would be connected there.

No more unlikely than two single sockets.

 

[Geflugenmitt]

Thanks for the replies, but I remain confused. I thought the MCB should be rated to protect the cable. Hence a radial using 2.5mm would have a 16A or 20A MCB and not a 32A. I don't see how a spur is conceptually different to a "radial off a ring". A fault condition (in the spur cable, not the appliance), however unlikely, could cause a current of 30A to flow. Agreed from that point onwards I assume the heater installers will be putting in a 13A fused connection unit, so we're covered (I've been asked to leave the final connection the the installers so I can only wait and see what they do).

 

[DetlefSchmitz]

Typically, the regulations consider two different things: overloads and faults. A fault would usually be a short-circuit and a very large current would flow which would trip any MCB, whatever its rating, within a very short time, so no problem. An overload is prevented by the downstream fuse (which you are happy about).
Something in between is not considered very likely.

 

[DetlefSchmitz]

However, as I hinted earlier, the regulations concerning ring final circuits are a pragmatic attempt to justify a circuit technique which doesn't really bear justification, as it leads to so many logical anomalies. The history is well-documented elsewhere and relates to a post-2nd-world-war situation, where in my opinion, logical engineering thought was not really demonstrated. However it has its value, mostly in reducing the number of MCBs required, compared to continental 'consumer units'. It has a major downside in that it absorbs many hours of forum time discussing the arcane detail.

 

[Geflugenmitt]

Sorry to set this off. I've read the posts again and I do actually get it now. I'm glad. I didn't want the heating engineers turning up and saying my electrician had done it wrong. I do assume they will be installing a FUSED connection unit. The fun will really start if they don't and then I, as a layman, have to either intervene or bite my lip. I've been in that situation a few times. It's better when I have plumbing installed. I know NOTHING about plumbing so I'm in blissful ignorance.

Thanks all for explaining, I'm clear now.

 

[winston1]

and it's exceptionally unlikely that two 13A loads would be connected there.

Hm. Seems to happen quite often with a washing machine and dryer connected.

 

[flameport]

washing machine

Only gets near 13A while the heater is on, which is a tiny fraction of the wash cycle.

 

[JohnD]

It has a major downside in that it absorbs many hours of forum time discussing the arcane detail.

So this is an upside, it keeps the theological fundamentalists occupied so they don't have time to do anything harmful.

 

[John D v2.0]

Hence a radial using 2.5mm would have a 16A or 20A MCB and not a 32A.

if there's only a double socket on the end, it could have a 32A mcb quite legally.