Help please guys.

[scott1990a]

Hello guys, I have a question, a friend of mine has a shed attached to his house he has electrics already in there (done by previous owner) but the setup don't look right to me.. there is a 13a FCU on the kitchen ring that has a piece of 2.5mm cable going through the wall in to the garage CU in the shed, he then has 2 sockets coming from a 20amp breaker in the garage CU. The garage CU is RCD protected. Is this safe?

Cheers guys

 

[ericmark]

Sounds daft but not dangerous.

If supplied from a 13 amp FCU then no point in 20 amp overload, I assume that's what you mean by 20amp breaker? In the main we have a 13 amp fuse in the FCU in house, and 3 amp in switched FCU in garage for lights, and if no RCD in house then use a RCD FCU, however there is nothing wrong with fitting a second CU from a FCU likely done with the idea of latter supplying from a larger supply.

In the main will limit any large item load on the ring final to 2 kW or around 8 amp, as a ring final can be overloaded near the ends, not a problem near the centre, but saying that we seen washer/driers and driers on the ring final which typically draw 3 kW without it seems much of a problem.

 

[opps]

As Eric says... you won't be able to draw more than 13 amps at any given point.

 

[winston1]

, as a ring final can be overloaded near the ends, not a problem near the centre, but saying that we seen washer/driers and driers on the ring final which typically draw 3 kW without it seems much of a problem.

You'd have to try very hard to overload a ring final even near the ends. 2.5mm2 cable is rated up to 27 amps so a load near the end would need to exceed this.

 

[ericmark]

You'd have to try very hard to overload a ring final even near the ends. 2.5mm2 cable is rated up to 27 amps so a load near the end would need to exceed this.

The ring final rules only ask for cable able to handle 20 amp, 2.5 mm² cable has a range of current carrying capacities depending on method of install and the type of insulation, thermoplastic is lower than thermosetting, 7/0.029 was thicker than 2.5 mm² so when the ring final was designed the chance of over load was much reduced, and it was designed with the idea of electric heating, with bakelite plugs brown in colour which could dissipate heat from fuse better into the room and with no sleeves on the live pins also into the socket, the design today is not as good as when first introduced.

However the rules allow a spur to be taken from the consumer unit, as well as any socket on the ring final, so if close to the CU there is no need for the FCU to be connected to the ring. So we have

The load current in any part of the circuit should be unlikely to exceed for long periods the current-carrying capacity of the cable (Regulation 433.1.5 refers). This can generally be achieved by:
(i) locating socket-outlets to provide reasonable sharing of the load around the ring
(ii) not supplying immersion heaters, comprehensive electric space heating or loads of a similar profile frog the ring circuit
(iii) connecting cookers, ovens and hobs with a rated power exceeding 2 kW on their own dedicated radial circuit
(iv) taking account of the total floor area being served. (Historically, limit of 100 m² has been adopted.)

The testing for fig of 8 loops in the ring final was taught when I did my C&G 2391 although I see no rule saying we should not have a fig of 8, the main time when we find a fig of 8 is when a supply is taken to an outbuilding, I would think as you say the times when an overload is caused due to high loads at the ends are rare.

However one reason why I had my parents house rewired was I found where 2.5 mm² had been used to feed 7/0.029 the 2.5 mm² showed signs of melting, likely spur off spur, my dad took attitude if it works its OK. But always had 30 amp fuse or 32 amp MCB feeding the ring final, so clearly 32 amp can overload 2.5 mm² cable, in fact considering where fault was found it seems unlikely more than 20 amp could have been drawn, I would guess a 4 bar electric fire, and some table lamps, it was in living room, kitchen I could have understood, but not living room.

 

[JohnW2]

The ring final rules only ask for cable able to handle 20 amp, 2.5 mm² cable has a range of current carrying capacities depending on method of install and the type of insulation,

Very true, but I nevertheless have to agree with winston when he wrote ....

You'd have to try very hard to overload a ring final even near the ends.

I would imagine that it is is extremely unusual for a ring final in a domestic installation to be anything like fully loaded (i.e. 32A total) "for long periods of time" and when the total load is appreciably less than 32A, the entirety of that load would have to be pretty close to one end of the ring for even a cable with 20A CCC to be "overloaded for a long period of time" (which, as you have quoted, is all that the regs require).

Kind Regards, John

 

[SUNRAY]

I nevertheless have to agree with winston when he wrote ....

You'd have to try very hard to overload a ring final even near the ends. 2.5mm2 cable is rated up to 27 amps so a load near the end would need to exceed this.

I have to agree too.
AFAIA the only one suffering in that way was due to a very wrong 32A socket for a welder within a very short distance from the CU (I looked at a 32A socket for a welder and initially struggled with the reason one of the 2.5 t&e's was showing signs of overheating but the other was not. This was in a domestic property with a ring covering the whole house [apart from the kitchen] including the garage which was being used as a workshop for metal fabrication. This 32A socket [yellow of course] was less than 2m from the CU. The house was running at around 16A or so when I clamped and the welder around 40A with peaks over 200A [limit of meter], most of the welder current appeared in the short leg of the ring at around 45A. MCB had been changed to D32 to prevent 'false tripping'. https://www.diynot.com/diy/threads/...s-is-this-good-pic.483672/page-9#post-3927729 ).

In my experience ring finals suffering heat damage have generally been due to something wrong, such as: faulty termination or simply running at significantly higher loads than designed for. Or of course within high ambiant temperature.

 

[JohnW2]

I have to agree too. ... AFAIA the only one suffering in that way was due to a very wrong 32A socket for a welder within a very short distance from the CU

Well, that was obviously ridiculous, and one cannot take that to mean anything in terms of a properly designed/.installed (and 'compliant'!) ring final!!

.... In my experience ring finals suffering heat damage have generally been due to something wrong, such as: faulty termination or simply running at significantly higher loads than designed for. Or of course within high ambiant temperature.

Quite so. For the reasons I gave, I think it extremely unlikely that any part of the cable of a domestic ring final would be "overloaded for a long period of time", or come to any harm, even if relatively large loads were applied fairly near one end of the ring.

Kind Regards, John

 

[SUNRAY]

Well, that was obviously ridiculous, and one cannot take that to mean anything in terms of a properly designed/.installed (and 'compliant'!) ring final!!

Kind Regards, John

Totally so but it did give an excellent demonstration of overloadig one end of a ring circit. There was simply nothing correct about it, I think it way even a yellow socket.

 

[JohnW2]

Totally so but it did give an excellent demonstration of overloadig one end of a ring circit. There was simply nothing correct about it, I think it way even a yellow socket.

Fair enough, but that's not really got much to do with overloading a ring by having fairly large loads applied close to one end of the ring - a welder drawing at least 45A (and up to 200A+at times) would probably overload a 32A ring even if it were the only load and were connected right at the middle of the ring - and would similarly overload a 32A radial, no matter where it were plugged in to that circuit! Indeed, as you observed, your 200A+ peaks ought to have caused (and apparently did!) instant magnetic tripping of a B32!

Kind Regards, John

 

[bernardgreen]

You'd have to try very hard to overload a ring final even near the ends. 2.5mm2 cable is rated up to 27 amps so a load near the end would need to exceed this.

As well as the load near the end of the ring there could be a significant load in the middle of the ring,

 

[winston1]

As well as the load near the end of the ring there could be a significant load in the middle of the ring,

There could be. Let’s say 16 amps in the middle so 8 amps in each leg. So another 16 amps to fully load the ring. Even if this went all in one leg that would only be 24 amps so not going to overload 2.5mm2 cable.

 

[JohnW2]

As well as the load near the end of the ring there could be a significant load in the middle of the ring,

As winston has pointed out by illustration, provided that the total load on the ring in within the design limit (i.e. ≤32A), then the worst case is for all of that total (32A in the very worst case) to be applied close to one end of the ring.

If the ring itself is overloaded, then all bets are obviously off - but that's true for any circuit.

Kind Regards, John