Why ring circuit on 32amp??

[Calebp43]

So the CCC of 2.5mm is 27 amps clipped direct. So a ring would be 54amps altogether, yet is put on a 32 amp breaker. I understand that the legs could be balanced.. bit if that's the case then why draw the line at 32, why not 40?
Let's say I'm at a job and there 32 amp breaker is overloaded on the ring circuit. There is no more ways in the board for me to split the load and put 2x 20amp breakers.. couldn't I put it on a 40amp?

 

[Pretty Mouth]

Minimum cable rating for a ring is 20A.

If all the load is placed at exactly the centre point of the ring, then yes, the current will divide equally between both legs and the circuit could happily take a 40A continuous load.

This of course never happens in the real world, a ring will never be perfectly balanced like that. One leg will be carrying more current than the other, and it has been decided that one leg is unlikely to take more than 20A, while the other has 12A. Hence the 32A breaker.

Back in the distant past, the minimum cable rating was to be 2/3rds of the fuse rating. Anecdotally, this was changed to minimum 20A so that 32A MCBs could be used with various installation reference methods that would have otherwise been prohibited under the old rule.

 

[Resu]

There's more to it than overcurrent protection, think about short circuit protection.

 

[Calebp43]

There's more to it than overcurrent protection, think about short circuit protection.

To my knowledge, short circuit is what makes the MCB trip if line and neutral touch? Same wether it's 32 amp or 40 amp or even 6 amp?

 

[Calebp43]

Minimum cable rating for a ring is 20A.

If all the load is placed at exactly the centre point of the ring, then yes, the current will divide equally between both legs and the circuit could happily take a 40A continuous load.

This of course never happens in the real world, a ring will never be perfectly balanced like that. One leg will be carrying more current than the other, and it has been decided that one leg is unlikely to take more than 20A, while the other has 12A. Hence the 32A breaker.

Back in the distant past, the minimum cable rating was to be 2/3rds of the fuse rating. Anecdotally, this was changed to minimum 20A so that 32A MCBs could be used with various installation reference methods that would have otherwise been prohibited under the old rule.

Okay this makes sense. My only thing would be that yeah sure it may be unlikely that one leg wouldnt have 21 or 30 amps but it may actually be the case. So that could cause the cable to overheat before the breaker tripps, causing a fire. I know that would be unlikely but not that unlikely?
Also I thought the current carrying capacity was 27amps for 2.5mm.
Sorry if I'm asking silly questions. I am an apprentice and been wondering this for a while.

 

[DPG]

Okay this makes sense. My only thing would be that yeah sure it may be unlikely that one leg wouldnt have 21 or 30 amps but it may actually be the case. So that could cause the cable to overheat before the breaker tripps, causing a fire. I know that would be unlikely but not that unlikely?
Also I thought the current carrying capacity was 27amps for 2.5mm.
Sorry if I'm asking silly questions. I am an apprentice and been wondering this for a while.

Always ask. Best way of learning.

 

[Pretty Mouth]

Okay this makes sense. My only thing would be that yeah sure it may be unlikely that one leg wouldnt have 21 or 30 amps but it may actually be the case. So that could cause the cable to overheat before the breaker tripps, causing a fire. I know that would be unlikely but not that unlikely?
Also I thought the current carrying capacity was 27amps for 2.5mm.
Sorry if I'm asking silly questions. I am an apprentice and been wondering this for a while.

2.5mm twin +earth is indeed rated to 27A , when installed clipped direct. But there are other ways the T+E may be installed that change the rating. If you install it in conduit, for example, then it's only rated to 20A. Install it in the middle of deep insulation, it drops to 13.5A. Same cable, different rating, depending on how it's installed.

The regulation for ring finals says min. 20A, so there is some flexibility as to how ring cables may be installed.

It is theoretically possible to overload one leg of a ring, but it would have to be a really, really badly designed ring for it to actually be dangerous. Following the design guide in app.15, it would never happen

 

[Calebp43]

2.5mm twin +earth is indeed rated to 27A , when installed clipped direct. But there are other ways the T+E may be installed that change the rating. If you install it in conduit, for example, then it's only rated to 20A. Install it in the middle of deep insulation, it drops to 13.5A. Same cable, different rating, depending on how it's installed.

The regulation for ring finals says min. 20A, so there is some flexibility as to how ring cables may be installed.

It is theoretically possible to overload one leg of a ring, but it would have to be a really, really badly designed ring for it to actually be dangerous. Following the design guide in app.15, it would never happen

I really appreciate the time you've taken to answer my question. You've made clear a lot of things I've found to be a bit fuzzy. Makes a lot more sense when you put it like that.
Thanks.

 

[Resu]

To my knowledge, short circuit is what makes the MCB trip if line and neutral touch? Same wether it's 32 amp or 40 amp or even 6 amp?

Yes but in upping the breaker size you'd be lowering the max permissible Zs which may put you the wrong side of 0.4s disconnection time.

 

[LastManOnline]

Minimum cable rating for a ring is 20A.

Curious regulation. How would a 16 amp mcb/fuse compromise circuit safety?

 

[Pretty Mouth]

Curious regulation. How would a 16 amp mcb/fuse compromise circuit safety?

Sorry bud, I should use the correct terminology: When I say cable rating, I mean current carrying capacity of the cable.

There would be no problem using a lower rated OCPD than the usual 30 or 32A from a safety point of view

 

[telectrix]

Sorry bud, I should use the correct terminology: When I say cable rating, I mean current carrying capacity of the cable.

There would be no problem using a lower rated OCPD than the usual 30 or 32A from a safety point of view

agreed, the onlyproblem could be tripping under overload for said 16A MCB/s. that's why 2" nails were invented.

 

[pc1966]

One aspect is the 32A RFC is a "standard" circuit, in the wiring regs and intended for the 13A sockets and/or FCU. Any variations of this start to get in to the territory of "Why did they do this? Why? WHY?"

Historically you would sometimes see the RFC on a 20A rewireable fuse in the supply Ze was too high to meet disconnection times on 30A, but with MCBs on B-curve that is easier to meet, and today the RCD aspect should guarantee it (though personally I would always design/test for OCPD disconnection in at all practical and keep the RCD very much as 'additional' protection).

Finally back to the OP's comment - if you are overloading a 32A RFC then something is not right! Either someone has put a shedload (perhaps literally) of loads on to the RFC that should have had their own circuit(s), or the RFC is covering a stupidly large area and plug-in electric heaters are being used.

 

[Lucien Nunes]

One aspect is the 32A RFC is a "standard" circuit, in the wiring regs and intended for the 13A sockets and/or FCU.

This is the key. In theory, an unevenly loaded 32A RFC can overload its cables, especially if it is long, the load is all at one end, and the cables are installed by a method that reduced their CCC. But, it has been tested in the laboratory, and by 74 years* of use in millions of installations that cable damage does not occur in practice with 32A OCPD.

That testing has not been done with 40A OCPD, so you would have to engage a qualified testing lab to prove that under all realistic operating conditions, the cable won't age prematurely etc. That could be a complicated and expensive R&D project! You could rely on the following assumption but again you would have to prove that it is correct:
If installing the cable so that Iz=20 A is OK with In=32A, then if the cable is installed with Iz=27A it should be OK with In=40A. (Subject also to meeting adiabatic for earth fault conditions with T+E CPC being only 1.5mm², or even 1.0mm² if early 1970s cable.)

Then you have to test all the accessories. Are the socket terminals OK with a 40A circuit? Your call, your R&D to prove the point, because the manufacturer won't sanction it. They will be designing to comply with BS1363 and not a smidgeon more.

And if you prove it's all OK, you still get a non-standard circuit that is a departure, as it neither obeys Iz>In nor is a defined standard circuit, and the next electrician who finds it will soon make mincemeat of your idea in front of the customer.

*The first 24 years of RFC's were installed with 7/.029 which is 3.0mm². But they were usually protected by coarse OCPD e.g. BS3036 with a fusing factor of nearer 2.0 rather than today's 1.45, which uses up some of that extra wiggle room in the larger conductor under overload conditions.