Ring Circuit Cable Size.....

[newmodel]

Hi, I'm new to this and just learning so go easy on me....

My question here is probably silly but I was wondering why a ring final standard size cable is 2.5mm with a 32A MCB or 30A Fuse?

Standard 2.5mm PVC holds around 27A right, so how do I know how much current will be drawn through the cable. Because if the cable runs through more then 400mm of insulation then it halves the current rating of the cable, which means even less current can safely pass through?

Am I right in thinking that each leg of the ring can pull up to 16A, so both legs can pull 32A before the main MCB trips, so the cable only has to hold 16A under normal conditions, but this should be cut out before that by the Fuse in the socket?

I may be babbling but any help would be appreciated thanks.

P.S Don't get to technical, I'm learning remember

 

[EFLImpudence]

A ring circuit is a special dispensation which is really no longer needed.

To answer:

I was wondering why a ring final standard size cable is 2.5mm with a 32A MCB or 30A Fuse?

It just is is the real answer.
The ring conductors must have a minimum current carrying capacity of 20A (1.5mm² would do) but 2.5mm² is the minimum allowed.
This goes back to (BS3036) 30A fuses which by their nature had a derating factor of 0.725 - 20A/0.725=27A. This has not been altered to accommodate MCBs with no derating factor.

Standard 2.5mm PVC holds around 27A right, so how do I know how much current will be drawn through the cable. Because if the cable runs through more then 400mm of insulation then it halves the current rating of the cable, which means even less current can safely pass through?

Yes - bigger cable or don't do it.

Am I right in thinking that each leg of the ring can pull up to 16A, so both legs can pull 32A before the main MCB trips, so the cable only has to hold 16A under normal conditions, but this should be cut out before that by the Fuse in the socket?

Both legs must be able to carry 20A because the load is not all in the middle. Otherwise you are right.

 

[EFLImpudence]

Forgot this bit:

Standard 2.5mm PVC holds around 27A right, so how do I know how much current will be drawn through the cable.

The current is proportional to the load position on the ring.
At mid point - shared 50/50

A quarter way round 75% in short leg/25% in long leg.
Inversely proportionate to the resistance of each leg.

That's why 20A minimum and not 16A.

 

[newmodel]

Ok I think I get it thanks

On the question of insulation, if say for instance a you were wiring a bungalow, the cables would more than likely all run the in loft space, this will more than likely have lots of insulation in. The cables would be clipped to the top edge of the joists but as they enter and leave the loft space and through the room belows ceiling, as they go down to each socket in the rooms below, is the the total depth of the insulation taken as 1 amount eg 10mm or does it stack up for every instance it goes through it eg 4 x sockets so 40mm?

 

[EFLImpudence]

If a cable goes through insulation a number of times, just the longest single distance.

 

[ban-all-sheds]

http://everything.explained.today/Ring_circuit/

 

[ericmark]

The ring final is called a final as it is the final wiring before the appliance. It is NOT a ring main so don't try to google ring main as you may get the wrong thing. As explained it is a historical system which over time has used thinner cable than the original design, in other to stop overload there are some basic rules, for example any fixed appliance over 2kW should have a dedicated supply. So kettle is OK into the ring final as although 3kW it does not run for long, however the immersion heater will have a dedicated supply.

If used in a domestic wiring system by time we remove the non portable appliances like oven, dish washer, washing machine, tumble drier, and immersion heater there is very little left which is likely to put any great load on the ring final and as long as they are spread out then it is unlikely to cause an overload.

A kitchen with a kitchen ring final is often quite a distance from the consumer unit with all the sockets quite central so it is unlikely to be overloaded. Where the problem arises is where the consumer unit is close to the kitchen and the ring final feed the kitchen first then the rest of the ground floor so all the kitchen appliances are all on one end of the ring. However time also is important the immersion heater and the tumble drier can be one full power for an hour or more, but the rest only uses the heating element for a short time. So although ideal the ring final in a domestic dwelling works well.

However using the same system in industry one has to be careful not to allow an over load where portable heaters can be plugged in or other high power use items. The regulation ends by saying "and if, under the intended conditions of use, the load current in any part of the circuit is unlikely to exceed for long periods the current-carrying capacity (Iz) of the cable." so the designer must look at the likely load.

There are rules to follow the cable must be at least 2.5mm² or 1.5mm² if using mineral insulated cable and installed in a manor where the current carrying is 20 amp or more. It also details how spurs are to be connected and even the length of a spur should really not exceed 3 meters. Not all the rules are always followed and some like the 3 meter rule are vague.

The "Reference Method 100# (above a plasterboard ceiling covered by thermal insulation not exceeding 100 mm in thickness)" and others have a set of pictures showing how when clipped to a wooden beam how the beam will assist removing heat and only when not clipped to something which can remove the heat is there a problem. So if you lay the cable on ceiling then cover with rockwool that's OK as the plaster board will remove the heat. Also if you lay the rookwool than put the cable on top no problem as free air will remove heat. The problem is when it is on top of the rookwool then more rockwool is added on top.

 

[EFLImpudence]

It also details how spurs are to be connected

Not really.

and even the length of a spur should really not exceed 3 meters. ... and some like the 3 meter rule are vague.

Not so.

 

[winston1]

The ring final is called a final as it is the final wiring before the appliance. It is NOT a ring main so don't try to google ring main as you may get the wrong thing. As explained it is a historical system which over time has used thinner cable than the original design, in other to stop overload there are some basic rules, for example any fixed appliance over 2kW should have a dedicated supply. So kettle is OK into the ring final as although 3kW it does not run for long, however the immersion heater will have a dedicated supply.

If used in a domestic wiring system by time we remove the non portable appliances like oven, dish washer, washing machine, tumble drier, and immersion heater there is very little left which is likely to put any great load on the ring final and as long as they are spread out then it is unlikely to cause an overload.

Not this again. There is no regulation saving fixed appliances over 2kW have to have a dedicated supply. What is more dish washers, washing machines, tumble dryers, and even some ovens come with mounded 13a plugs but no instructions saying don't plug them into a ring final.

 

[PBC_1966]

I've seen plenty of kitchens in which the consumer unit is in the kitchen itself and the kitchen sockets are right at one "end" of the ring, the first couple only a few feet from the fuse supplying the ring. If those happen to be the sockets which are used for dryer and washing machine, then it's quite likely that the short leg of the ring can be overloaded by the two running together, never mind once extra load in the form of a kettle etc. is added. In theory, the design of the ring should take into account such a possibility, but how often do we ever see that done in practice?

The ring final circuit in the form adopted was a kludge which was adopted in the immediate post-war years and which many feel should have been abandoned long ago. The bulk of the world doesn't use it.

 

[ericmark]

I would agree we often don't design as we should. The ring final has both advantages and disadvantages where RCBO protection is used the ring is cheaper to install then radials but the radials clearly don't have the overload problem when loading has not been considered correctly when installing the ring.

There are a number of items with the ring which are not defined in the regulations. It is not made clear if 433.2.2 applies to the ring, and the calculation of volt drop using 20 amp centre and remaining 12 shared equal so using 26 amp as the design current may have been voiced in IET lecturers, but there is nothing in the regulations to say you can do this. The 2kW rule is in the appendix as a method to make it unlikely to exceed for long periods the current-carrying capacity of the cable as referred to in 433.1.5, but again it allows other methods to be used.

Often the kitchen is supplied by two cables to a grid switch and radials from the grid switch supply sockets, and long as the pair of cables don't supply anything but the grid switch then in theroy they are not forming a ring final but are cables in parallel and could be supplied by a 40 amp MCB. Even when that is not the case the use of a dedicated ring for the kitchen or unity room clearly makes it unlikely to exceed for long periods the current-carrying capacity of the cable.

There is except where RCBO's are used nothing to stop one feeding a ring with a double 16A MCB it would still allow 32A but it would stop the problem of too much power going down one leg, at the time of the design we used fuses so these could not be linked but today we could use them if we wanted, in fact we could even use a 20A twin pole MCB giving 40A total and still ensure neither leg is overloaded. However we could not use RCBO's with this as they would trip. As to why we have never done this I don't know. It could be then a true ring which can be fed from either end or both and a central section could be isolated for maintenance as with supply rings. In other words if it was supplied with two MCB's should the ring be broken there would not be an overload problem. Why we use the old ring final design I don't know? But we do. I have found rings with two fuses but I am sure this was in error not by design. To use double MCB's for the ring would not really be a good idea as some one in the future would not understand what you had done and could modify and cause danger as a result. The double 20A MCB would be OK for the ring but it would not be OK for any radials or spurs from the ring. So although in theroy it could be used, in practice it can't be used.

 

[JohnW2]

I've seen plenty of kitchens in which the consumer unit is in the kitchen itself and the kitchen sockets are right at one "end" of the ring, the first couple only a few feet from the fuse supplying the ring. If those happen to be the sockets which are used for dryer and washing machine, then it's quite likely that the short leg of the ring can be overloaded by the two running together, never mind once extra load in the form of a kettle etc. is added. In theory, the design of the ring should take into account such a possibility, but how often do we ever see that done in practice? ... The ring final circuit in the form adopted was a kludge which was adopted in the immediate post-war years and which many feel should have been abandoned long ago. The bulk of the world doesn't use it.

All completely true. However, evidence that any 'harm' has ever resulted from the use of ring finals is very lacking. That's probably due primarily to two things - (a) that long-term high loads are rare, even in kitchens and (b) that there are enormous safety margins built into the design figures we work with.

Even a dryer (easily the worst culprit) will usually only draw high current for an hour or so, and (assuming that ovens aren't connected to the ring) any other high-current kitchen load (even a WM or DW) will usually draw high current only for very brief periods (30 mins or less). Averaged over, say, an hour or so, I doubt whether the total load in a kitchen (excluding ovens) will hardly ever be much over 20A. In any event, per my (b), I feel sure that 2.5mm² cable would come to no significant harm (and certainly would not 'burst into flames'!!) carrying 32A for a moderate period of time (and probably the same with 40A or much more!).

We (including myself) all talk about the theoretical possibility of the cable of the short leg of a ring being 'overloaded' by several high loads plugged in close to one end of the ring - but I would be very very surprised if any electrician has ever seen a case of 'harm' of any sort resulting from this, even if it does happen occasionally!

Kind Regards, John

 

[JohnW2]

Often the kitchen is supplied by two cables to a grid switch and radials from the grid switch supply sockets, and long as the pair of cables don't supply anything but the grid switch then in theroy they are not forming a ring final but are cables in parallel and could be supplied by a 40 amp MCB.

Indeed. In fact, if (as commonly) the two cables were 2.5mm² 'clipped direct', one could have a 50A MCB. Some people would, of course, presumably have even more to say about a "20A" grid switch on a 40A or 50A circuit than they do in relation to a 32A circuit!!

There is .... nothing to stop one feeding a ring with a double 16A MCB it would still allow 32A but it would stop the problem of too much power going down one leg, at the time of the design we used fuses so these could not be linked but today we could use them if we wanted, in fact we could even use a 20A twin pole MCB giving 40A total and still ensure neither leg is overloaded. ... As to why we have never done this I don't know.

Yes, one could do that with 'linked' MCBs. The reason we don't do it is probably that one could cut that ring in the middle, making it into two conventional radials (maybe even 25A ones if the cable were 'clipped direct'), without, in practice, any appreciable downsides.

The double 20A MCB would be OK for the ring but it would not be OK for any radials or spurs from the ring. So although in theroy it could be used, in practice it can't be used.

I don't really think that would be a problem. A spur would still be protected with by a 13A fuse or (at worst) 2 x 13A downstream fuses (seemingly deemed to be a maximum load of 20A!) in a double socket.

Kind Regards, John

 

[ban-all-sheds]

I've seen plenty of kitchens in which the consumer unit is in the kitchen itself and the kitchen sockets are right at one "end" of the ring,

And there's a very large number of houses with the CU under the stairs and the nearest room is the kitchen.

 

[JohnW2]

I've seen plenty of kitchens in which the consumer unit is in the kitchen itself and the kitchen sockets are right at one "end" of the ring,

And there's a very large number of houses with the CU under the stairs and the nearest room is the kitchen.

Indeed so - and, as I recently wrote, despite that, any evidence that the use of ring finals has actually resulted in any significant 'harm' is very lacking.

I'm not defending the existence, nor necessarily even the continued use, of ring finals, but I do think that the arguments against them are primarily theoretical/hypothetical, with little/no evidence that they result in significant problems in practice.

Kind Regards, John