Can I join 2 radials to make a ring?

[Octopus]

Note: In my own house (which is 500 years old and made of wood) I did the opposite to what you're suggesting. I inherited only two 32A rings for the whole place. I started by splitting these rings, junking the 32A MCBs, and converting them to four radials, each with a 16A RCBO.

I'm guessing you didn't have much loading on them.................. 20A RCBO's may have been a better approach!

 

[Mark42]

I'm guessing you didn't have much loading on them.................. 20A RCBO's may have been a better approach!

Of course.
But whoever put the circuits in run the cables in and out of wattle and daub, up, down and all over the place, with tight bends: I just didn't trust the bloke, whoever it was.
I wanted to be sure that there was no chance, ever, of these hidden and buried wires running hot.
I added new 20A radial circuits for heavier loads, took the immersion heater off the old ring (aargh!) and installed all new kitchen circuits.

 

[Mark42]

Outdated?
If RFCs are outdated, what does that make Radials, which RFCs replaced?
It's not fashion week.

Indeed they did, to save copper.

But 2.5mm is now plentiful, cheap, and WWII is over

 

[kingeri]

Ring finals are not outdated, they are just another option for sparks to use where appropriate. These days I do install more radials than I used to, but rings are very versatile circuits, and where the building layout suits I still use rings when I need the 32A. Mainly kitchens and utilities these days. I don't like 32A 4mm^2 radials, the CCC is too close to the edge. The 32A 2.5mm^2 ring final has stood the test of time, and is far more flexible with installation methods. Fine, 20A radials for living rooms, bedrooms etc. are no problem, but for those areas which need higher current ratings, the ring final is still king for me.

 

[Mark42]

Ring finals are not outdated … rings are very versatile circuits, and where the building layout suits I still use rings when I need the 32A. Mainly kitchens and utilities these days. …

I’m not saying you’re wrong. It’s a possibly matter of personal preference.

Although I would argue that in kitchens and utility rooms, separate circuits for dishwasher, washing machine, tumble dryer, microwave, coffee machine and so on are preferable.

I concede that cost and time may be a consideration in commercial work, but that’s not the point I’m making.

… The 32A 2.5mm^2 ring final has stood the test of time, and is far more flexible with installation methods.

My main niggle with 32A rings is that a single point of failure (broken ring on one conductor - and surely every electrician has found this at some time) may result in a potentially dangerous cable overload which could sit undetected for years.

… I don't like 32A 4mm^2 radials, the CCC is too close to the edge.

I agree. Horrible, pointless things and awkward to connect accessories if the boxes aren’t deep enough. No argument there.

 

[Andy78]

My main niggle with 32A rings is that a single point of failure (broken ring on one conductor - and surely every electrician has found this at some time) may result in a potentially dangerous cable overload which could sit undetected for years.

I have found plenty of non-continuous ring conductors. I have never discovered any thermal damage to cable due to overload caused by this lack of continuity though.

It's a potential hazard that exists only in theory in my experience. It would be interesting to see a bench test however and see how 2.5mm cable performs in this overload situation.
Wonder if JW has ever done this ?

 

[Mark42]

I have found plenty of non-continuous ring conductors. I have never discovered any thermal damage to cable due to overload caused by this lack of continuity though.

It's a potential hazard that exists only in theory in my experience. It would be interesting to see a bench test however and see how 2.5mm cable performs in this overload situation.
Wonder if JW has ever done this ?

Hmm. I don't entirely disagree, but to extrapolate your argument would be to say it's OK to protect buried 2.5mm T&E at 32A. Would you ever do that?

It would indeed be interesting to run, say, two old-skool 3kw fires down a long bit of 2.5 T&E to see what actually happens. I do collect such things, but have enough on this week!

 

[Andy78]

Hmm. I don't entirely disagree, but to extrapolate your argument would be to say it's OK to protect buried 2.5mm T&E at 32A. Would you ever do that?

It would indeed be interesting to run, say, two old-skool 3kw fires down a long bit of 2.5 T&E to see what actually happens.

I would, and have installed 2.5mm cable protected by a 32A device. As a spur from a ring.

2 x 3kW fires on a bit of 2.5mm I would expect to be fine almost indefinitely with regards the cable. It is certainly just under the tabulated CCC of the cable.
If both were plugged into a double socket however, I would expect that to be the weak link as they are only rated at 20A.

 

[Mark42]

I would, and have installed 2.5mm cable protected by a 32A device. As a spur from a ring.
2 x 3kW fires on a bit of 2.5mm I would expect to be fine almost indefinitely with regards the cable. It is certainly just under the tabulated CCC of the cable.
If both were plugged into a double socket however, I would expect that to be the weak link as they are only rated at 20A.

Ha! Yes, correct of course, but you know that's not what I meant.

And sure, I once fused 0.5mm at 200A. For indicator lights in a main incomer switch panel. But I wouldn't use 0.5mm for 200A distribution, even if the budget was tight.

In the very specific case of a short spur or two, you are absolutely right in that the risk really is only theoretical, as no sensible person plugs two 3kW fires into the same double socket, or even in the same room. The radiated heat alone would set the curtains on fire before they needed to worry about the wiring.

My example was ill thought-out. How about a 3kW fire, a 1.5kW washing machine, a 1.5kW dishwasher, then a 1.5kW kettle on a broken long ring buried in insulation? Unlikely, but not impossible. And if it's said that such loads are very rare in practice, then why do we need 32A anyway?

I don't disagree that with quality modern cable, it would probably be OK for years. But I still say that any design which may need to take advantage of over-cautious specs and luck, is bad design!

 

[Andy78]

Ha! Yes, correct of course, but you know that's not what I meant.

And sure, I once fused 0.5mm at 200A. For indicator lights in a main incomer switch panel. But I wouldn't use 0.5mm for 200A distribution, even if the budget was tight.

In the very specific case of a short spur or two, you are absolutely right in that the risk really is only theoretical, as no sensible person plugs two 3kW fires into the same double socket, or even in the same room. The radiated heat alone would set the curtains on fire before they needed to worry about the wiring.

My example was ill thought-out. How about a 3kW fire, a 1.5kW washing machine, a 1.5kW dishwasher, then a 1.5kW kettle on a broken long ring buried in insulation? Unlikely, but not impossible. And if it's said that such loads are very rare in practice, then why do we need 32A anyway?

I don't disagree that with quality modern cable, it would probably be OK for years. But I still say that any design which may need to take advantage of over-cautious specs and luck, is bad design!

The new example given is not really relevant to highlighting the shortcomings of a ring final circuit either.
A 2.5mm twin and earth cable buried in insulation would likely not have been installed to comply with the regulations as this installation method would likely see the CCC below the required 20A at the end of the cable calculation.

I do get your point though and it's very refreshing that you are debating this issue with a bit of thought. Most ring haters really don't know why they do so.

As I say I'd like to see prolonged bench tests of 2.5mm cable under various overload conditions with arrangements simulating non-continuous ring final arrangements of various permutations before I was convinced there is an inherent problem with the standard arrangement.

 

[Sparkingmad]

Just to throw another curve ball 2.5mm t&e will carry 27A, how many sparks can honestly say they have checked at what overload current their protective devices operate at?
If you haven’t I suggest you try it for a giggle when you have time and inclination. 16A mcb’s operating after extended time with loads reaching 24+Amps. 32A mcb taking almost 60A before operating on overload.

 

[Richard Burns]

Just to throw another curve ball 2.5mm t&e will carry 27A, how many sparks can honestly say they have checked at what overload current their protective devices operate at?
If you haven’t I suggest you try it for a giggle when you have time and inclination. 16A mcb’s operating after extended time with loads reaching 24+Amps. 32A mcb taking almost 60A before operating on overload.

All this is accounted for in the cable rating values as it is a design characteristic of the MCB.
The bad situation is loading a cable at less than 1.45xIn for extended periods (small overload of long duration).
The 27A cable on a 32A MCB would have to be carefully overloaded to have a short term problem but the cable would have a reduced life in the long term as it would be running closer to its maximum temperature.
All the loading examples above will have little effect as they are short term loads, less than an hour. Having 7kW of lighting running 24 hours a day would be the killer. Heaters all have thermostats and switch off after a while, so the cable would warm then cool; possibly bad for thermal cycling but not too bad overall.

 

[Sparkingmad]

All this is accounted for in the cable rating values as it is a design characteristic of the MCB.
The bad situation is loading a cable at less than 1.45xIn for extended periods (small overload of long duration).
The 27A cable on a 32A MCB would have to be carefully overloaded to have a short term problem but the cable would have a reduced life in the long term as it would be running closer to its maximum temperature.
All the loading examples above will have little effect as they are short term loads, less than an hour. Having 7kW of lighting running 24 hours a day would be the killer. Heaters all have thermostats and switch off after a while, so the cable would warm then cool; possibly bad for thermal cycling but not too bad overall.

I agree Richard but the point I was making is that even though we may do the calculations for Design current and taking into account correction factors to determine our protective device rating to protect our cables and the circuit, the protective device can still allow continued overloading for more than just a short duration. The example I gave above would allow that circuit to operate at currents higher than it was intended to for longer periods than it should be possible too. It would not be unreasonable IMO to expect that mcb to allow that circuit to be overloaded by an extra 10A for quite a long time before it operated and continued overloading like this is possible until the mcb gets tired or weakened what is the unseen damage to the cable and detrimental effects could or would happen?

 

[Richard Burns]

I agree Richard but the point I was making is that even though we may do the calculations for Design current and taking into account correction factors to determine our protective device rating to protect our cables and the circuit, the protective device can still allow continued overloading for more than just a short duration. The example I gave above would allow that circuit to operate at currents higher than it was intended to for longer periods than it should be possible too. It would not be unreasonable IMO to expect that mcb to allow that circuit to be overloaded by an extra 10A for quite a long time before it operated and continued overloading like this is possible until the mcb gets tired or weakened what is the unseen damage to the cable and detrimental effects could or would happen?

This problem is most likely to occur on socket circuits where the load cannot be determined in advance and where say a standard 32A ring is installed in a commercial kitchen, a situation I have seen, then the circuit breaker could be routinely overloaded, if the load is over 46A then the circuit breaker will trip in less than 1 hour but during that hour the cable will be getting warm, but because it is only for one hour it should not suffer much (remembering that the cable can take its rated current 24hrs a day 365 days per year without serious degradation), it is the constant resetting of overloaded breakers that will take its toll on the cables and breaker and start to degrade them through thermal damage.