I want to remove a 3 pin round lamp socket - how??

[PBC_1966]

"If you like"? It's not a whim of mine - that really is what it stands for.

Sorry, not worded as clearly as I could have done: I meant call 20A the design current if you like, but it still doesn't stop somebody putting a 26A load at the end of a cable which, according to the regs., need only be rated for that 20A design current but which is protected by a 30 or 32A device.

If you have a final circuit with 20 sockets on it there is simply no way to guarantee that somebody won't plug 20 13A loads into it at some point.

No, but in other cases the rating of the cable is equal to or higher than the rating of the protective device. Diversity of use with appropriate design of circuit layout means that we hope that the circuit rating will not normally be exceeded by such use, but if it does happen, the cable is protected.

I'm not a fan of ring finals

Neither am I.

But can you explain why a 32A ring can be overloaded but a 32A radial cannot?

As above. I'm not saying that a radial cannot be overloaded, but if it's wired per the regs. the cable is protected adequately and a fuse will blow or an MCB trip before it can become damaged. The exemption for both the ring itself and spurs to have the cable rating equal to or exceeding the fuse rating then goes completely against that basic principle.

On the diversity issue, the regs. have always seemed rather inconsistent right from the early days of rings. Why, for example, did they decide that a 30A ring could supply four dozen sockets all around a house (up to 1000 sq. ft.) while a 30A radial was restricted to six sockets?

 

[EFLImpudence]

The exemption for both the ring itself and spurs to have the cable rating equal to or exceeding the fuse rating then goes completely against that basic principle.

You seem to be forgetting that the actual CCC of a ring is 54A (when method C) and the spur is protected from overload by the fuses in the plugs, not the 32A MCB.

 

[ban-all-sheds]

Sorry, not worded as clearly as I could have done: I meant call 20A the design current if you like, but it still doesn't stop somebody putting a 26A load at the end of a cable which, according to the regs., need only be rated for that 20A design current but which is protected by a 30 or 32A device.

Firstly, the regulations do not explicitly say that spur cables need only be rated at 20A. Therefore they are bound by the normal rules.

Secondly the 30/32A device is providing fault protection, not overload.

Diversity of use with appropriate design of circuit layout means that we hope that the circuit rating will not normally be exceeded by such use, but if it does happen, the cable is protected.

The cable in a ring is likewise protected - you seem to be forgetting that there are two paths to each point of use on it.

I'm not saying that a radial cannot be overloaded, but if it's wired per the regs. the cable is protected adequately and a fuse will blow or an MCB trip before it can become damaged.

Exactly the same applies to a ring.

The exemption for both the ring itself and spurs to have the cable rating equal to or exceeding the fuse rating then goes completely against that basic principle.

So are you arguing against the basic principle of 433.2?

 

[PBC_1966]

You seem to be forgetting that the actual CCC of a ring is 54A (when method C)

Based on a 27A cable rating? It's only 54A for a load distributed in such a way as to result in no more than 27A at any point in the ring. But I thought even under the current rules the cables forming the ring are required (after correction factors) to be rated for no more than 20A?

and the spur is protected from overload by the fuses in the plugs, not the 32A MCB.

Only if the spur cable is rated for 26A or more.

Firstly, the regulations do not explicitly say that spur cables need only be rated at 20A. Therefore they are bound by the normal rules.

O.K., but you're saying that under the current regs. a double socket is considered to be only a 20A design load, even though two 13A loads could be connected to it?

Secondly the 30/32A device is providing fault protection, not overload.

Well yes, as far as the spur is concerned then obviously it cannot provide adequate overload protection for a cable rated 27A, or 20A, anything less than 30/32A.

The cable in a ring is likewise protected - you seem to be forgetting that there are two paths to each point of use on it.

Not at all, but that doesn't prevent a cable forming part of the ring rated for 20A from being overloaded if a substantial load is connected close to one "end" of the ring. To avoid that possibility, one would have to be very careful about the placement of sockets on the ring and the possible loads which could be connected to them.

I'm not saying that a radial cannot be overloaded, but if it's wired per the regs. the cable is protected adequately and a fuse will blow or an MCB trip before it can become damaged.

Exactly the same applies to a ring.

As above, if the ring cable is given an exemption from the normal rules and is required to be rated only for 20A, it's entirely possible for part of the ring to carry more than that while the total current is still within the 30/32A limit.

So are you arguing against the basic principle of 433.2?

Which specific rule is that?

 

[EFLImpudence]

You seem to be forgetting that the actual CCC of a ring is 54A (when method C)

Based on a 27A cable rating?

That's what it is with method C.

It's only 54A for a load distributed in such a way as to result in no more than 27A at any point in the ring.

Nevertheless.

But I thought even under the current rules the cables forming the ring are required (after correction factors) to be rated for no more than 20A?

No, a minimum of 20A.

and the spur is protected from overload by the fuses in the plugs, not the 32A MCB.

Only if the spur cable is rated for 26A or more.

It is.

 

[PBC_1966]

You seem to be forgetting that the actual CCC of a ring is 54A (when method C)

Based on a 27A cable rating?

That's what it is with method C.

But the rules for a ring circuit don't require that, so I'm not sure what point you're trying to make.

It's only 54A for a load distributed in such a way as to result in no more than 27A at any point in the ring.

Nevertheless.

Again, I'm not sure what relevance the theorectical maximum 54A load, if suitably connected, has to the issue.

No, a minimum of 20A.

Sorry, badly worded but that's what I meant.

It is.

So are you saying that under the current regs. a spur cable feeding a double socket must be rated for a minimum of 26A after correction factors are applied?

 

[EFLImpudence]

Please see here.

 

[ban-all-sheds]

O.K., but you're saying that under the current regs. a double socket is considered to be only a 20A design load, even though two 13A loads could be connected to it?

Well, it has contentious aspects, but basically yes. If the standard governing the construction of twin socket outlets does not require them to be able to cope with more than 20A, in what way would a designer be exercising reasonable skill and care if he decided to use them for loads using 26A? Yes, 2x13A loads could be connected to one, but anybody who does that risks overheating, damage, and possibly a fire.

Well yes, as far as the spur is concerned then obviously it cannot provide adequate overload protection for a cable rated 27A, or 20A, anything less than 30/32A.

Nor does it need to.

Not at all, but that doesn't prevent a cable forming part of the ring rated for 20A from being overloaded if a substantial load is connected close to one "end" of the ring. To avoid that possibility, one would have to be very careful about the placement of sockets on the ring and the possible loads which could be connected to them.

I would refer you to the calculations done by JohnW2 which show just how close to the origin of the ring a load would have to be to result in such an overload.

As above, if the ring cable is given an exemption from the normal rules and is required to be rated only for 20A, it's entirely possible for part of the ring to carry more than that while the total current is still within the 30/32A limit.

As above.

Which specific rule is that?

The one about devices for overload protection being installed along the run of the cable instead of at the origin. Which is what allows a 1.5mm² spur from a ring final.

 

[ban-all-sheds]

So are you saying that under the current regs. a spur cable feeding a double socket must be rated for a minimum of 26A after correction factors are applied?

1. What's the design current for that spur?
2. What is the overload protection?
3. What is the fault protection?
4. What does the adiabatic calculation tell you is needed given the fault protection?

 

[JohnW2]

Well, it has contentious aspects, but basically yes. If the standard governing the construction of twin socket outlets does not require them to be able to cope with more than 20A, in what way would a designer be exercising reasonable skill and care if he decided to use them for loads using 26A?

As often discussed, unless the designer has a crystal ball, (s)he cannot be sure that they won't be used to supply total currents of 26A. That being the case, it could be argued that the only safe/responsible course would be to assume that they might be used to supply 26A - in which case designing with a cable with CCC of only 20A would presumably not be acceptable.

Kind Regards, John

 

[ban-all-sheds]

I agree it would not be acceptable.

But neither would assuming that a 20A accessory would be misused to supply 26A.

 

[JohnW2]

I agree it would not be acceptable. ... But neither would assuming that a 20A accessory would be misused to supply 26A.

I'm a bit confused. If you regard that 'assumption' as unacceptable, what is it that you are agreeing with in the first part of that quote?

I would also add that, IMO, the responsible (and certainly 'acceptable') 'assumption' on should be that a double socket might be used to supply 26A. As often discussed, I strongly suspect that the proportion of the general public (i.e. users of BS1363 plugs and sockets) who have any reason to even think that they should not load a double socket with 2 x 13A is probably very small.

Kind Regards, John

 

[PBC_1966]

I agree it would not be acceptable.

So would you agree if I said that using a cable installed in such a way that it is rated only for 20A to feed a double socket would not be making "reasonable provision for safety" given that whatever the manufacturer of socket specifies as the intended maximum load, it could, nevertheless, have 26A of load applied?

But neither would assuming that a 20A accessory would be misused to supply 26A.

Does that absolve the installer from ensuring that he doesn't make the same mistake with the cable? Two wrongs don't make a right and all that.

 

[ban-all-sheds]

That being the case, it could be argued that the only safe/responsible course would be to assume that they might be used to supply 26A - in which case designing with a cable with CCC of only 20A would presumably not be acceptable.

I agree that if it is assumed that they might be used to supply 26A then designing with a cable with CCC of only 20A would not be acceptable.


And I also regard it as unacceptable to assume that they might be (mis)used to supply 26A.

 

[PBC_1966]

Whatever ideas the manufacturer of the socket might have about it, then as John has suggested, I think the average householder just sees "two 13-amp sockets" and would have no reason to believe that he could not plug two 13A loads into it. And that's assuming he even understands "13 amps." Many might just see "two sockets" into which two plugs on whatever appliances will fit.

"Ah, just what I need - I'll plug the dryer into this one, then when I'm out here working I'll put the big fan heater into the other one."