Why are unfused spurs allowed in the regs?

[piggoy]

Just a quick question that I had in my mind I hope someone can answer -

The regs permit the connection of one double socket to a 2.5mm ring by means of an unfused single 2.5mm cable. The maximum rating for 2.5mm t+e is 20A. However a double socket is capable of drawing 2x13A=26A if fully loaded. How is this allowed? My impression is that everything else in the regs seems so strict with regards to cable capacities, but this seems quite lax.

 

[EFLImpudence]

The maximum rating for 2.5mm² is 27A. So...

It is unwise to have two 13A appliances in one double socket but we don't want to start that discussion again, do we?

 

[piggoy]

The maximum rating for 2.5mm² is 27A. So...

It is unwise to have two 13A appliances in one double socket but we don't want to start that discussion again, do we?

Fair enough. But 2.5mm radial circuits need protection using a 20A MCB?

 

[EFLImpudence]

Fair enough. But 2.5mm radial circuits need protection using a 20A MCB?

Well, 25A MCB would be satisfactory.

That's why you are limited to one double socket.
The spur is protected by the fuses in the plugs - as you say, 26A.

 

[JohnW2]

The maximum rating for 2.5mm² is 27A. So...

True, with 'ideal' installation methods. However, the be fair to the OP, the reg which allows ring finals (and unfused spurs therefrom) only requires that the current-carrying-capacity of the cable (as installed) is at least 20A.

That being the case, one can but imagine that those who wrote the regs 'assumed' (rightly or wrongly) that a double socket would represent a maximum load of 20A.

Kind Regards, John

 

[EFLImpudence]

But 431.1.103 says that the minimum csa shall be 2.5mm² (unless MICC).

It then goes on to quote the 20A minimum IF the current is unlikely to exceed the CCC of thie cable for long periods.
Slightly contradictory as usual.

Obviously if installation methods are not ideal then derating applies but nothing to do with the question.


Not actually sure if the 20A minimum applies to the spurs.

 

[JohnW2]

But 431.1.103 says that the minimum csa shall be 2.5mm² (unless MICC).

True, but I don't think that is important, since CCC obvioulsy depends on installation method, and wht it goes on to say is all in terms of CCC.

It then goes on to quote the 20A minimum IF the current is unlikely to exceed the CCC of thie cable for long periods. ... Slightly contradictory as usual.

NO! It doesn't say just IF but, rather, AND IF - which makes a crucial difference and means that there is not really any contradiction. The requirement is that (a) the cable has a CCC not less than 20A AND (b) that the current is unlikely to exceed the CCC for long periods.

Obviously if installation methods are not ideal then derating applies but nothing to do with the question.

Well, if the de-rated CCC is less than 20A, it would seem that one cannot use the cable for a ring final (or unfused spur) per 433.1.103.

Not actually sure if the 20A minimum applies to the spurs.

I don't see why it shouldn't. The regs starts off talking about "ring final circuit, with or without unfused spurs" and nothing in the rest of the paragraph suggests that the unfused spurs are exempt from any of the stated requirements.

Kind Regards, John

 

[piggoy]

Cheers for the debate guys. I'm not an electrician but take a hobbyist interest in understanding the regs. As I suspected things are open to interpretation.

 

[JohnW2]

Cheers for the debate guys. I'm not an electrician but take a hobbyist interest in understanding the regs. As I suspected things are open to interpretation.

You are seeing a bit of a 'debate', but I personally don't think there is anything particularly ambiguous about the regulation. As I read it, it is clearly allowing an unfused spur to be fed with a cable with a CCC of only 20A provided that "it is not considered likely that the CCC will be exceeded for long periods".

Your question was therefore very valid, but the regs require that the designer is (somehow!) satisfied that it is unlikely that the cable's CCC will be exceeded 'for long periods' (whatever that means!).

Kind Regards, John

 

[securespark]

That is the bit that is open to interpretation...

 

[JohnW2]

That is the bit that is open to interpretation...

True, and it will always require a judgement on the part of the designer which cannot ever be more than a guess. Even if the regs were totally explicit as regards the meaning of 'long period' (e.g. "... not considered likely to exceed the CCC of the cable for more than 17 minutes"), there would still be a need for judgement/guessing as regards what is 'likely'/'unlikely' - given that they have no control over what gets plugged into sockets once they are installed.

This obviously also applies to ring finals as a whole, not just unfused spurs.

Kind Regards, John

 

[EFLImpudence]

But 431.1.103 says that the minimum csa shall be 2.5mm² (unless MICC).

True, but I don't think that is important, since CCC obvioulsy depends on installation method, and what it goes on to say is all in terms of CCC.

Then why does it mention a minimum csa as 1.5mm² is allowed for power and has a CCC of 20A (method C of course).

It then goes on to quote the 20A minimum IF the current is unlikely to exceed the CCC of thie cable for long periods. ... Slightly contradictory as usual.

NO! It doesn't say just IF but, rather, AND IF - which makes a crucial difference and means that there is not really any contradiction. The requirement is that (a) the cable has a CCC not less than 20A AND (b) that the current is unlikely to exceed the CCC for long periods.

If there is a crucial difference does that not emphasise my point even more?

Obviously if installation methods are not ideal then derating applies but nothing to do with the question.

Well, if the de-rated CCC is less than 20A, it would seem that one cannot use the cable for a ring final (or unfused spur) per 433.1.103.

Quite, so no point mentioning it.

Not actually sure if the 20A minimum applies to the spurs.

I don't see why it shouldn't. The regs starts off talking about "ring final circuit, with or without unfused spurs" and nothing in the rest of the paragraph suggests that the unfused spurs are exempt from any of the stated requirements.

I read it as it makes no difference to the ring whether or not there are spurs; not as relevant to the spur.

 

[EFLImpudence]

That is the bit that is open to interpretation...

True, and it will always require a judgement on the part of the designer which cannot ever be more than a guess. Even if the regs were totally explicit as regards the meaning of 'long period' (e.g. "... not considered likely to exceed the CCC of the cable for more than 17 minutes"), there would still be a need for judgement/guessing as regards what is 'likely'/'unlikely' - given that they have no control over what gets plugged into sockets once they are installed.
This obviously also applies to ring finals as a whole, not just unfused spurs.

With regard to unfused spurs, we know that people will plug in two 13A loads therefore the cable should always be rated for this.

 

[ericmark]

7/0.029 cable could take the current from a twin socket when we changed to 2.5mm² we kept to nearly the same rules.

There is also the finger protector on modern plug which reduces the amount of heat that can be transferred into the socket so a 13A plug can no longer take a prolonged 13A load.

As a result there has been a change and items which are fixed over 2kW now need their own dedicated supply.

 

[JohnW2]

But 431.1.103 says that the minimum csa shall be 2.5mm² (unless MICC).

True, but I don't think that is important, since CCC obvioulsy depends on installation method, and what it goes on to say is all in terms of CCC.

Then why does it mention a minimum csa as 1.5mm² is allowed for power and has a CCC of 20A (method C of course).

You tell me! - as far as I am concerned, a cable with a CCC of 20A is a cable with a CCC of 20A, no matter what its csa! Is the explanation perhaps that this reg was written before the 'tweaking/fiddling' of Table 4D5 (not the least to allow 2.5mm² Method A to be acceptable for a ring final) - since 4D2A did (and still does) give the CCC of 1.5mm² Method C as only 19.5A hence less than 20A)?

It then goes on to quote the 20A minimum IF the current is unlikely to exceed the CCC of thie cable for long periods. ... Slightly contradictory as usual.

NO! It doesn't say just IF but, rather, AND IF - which makes a crucial difference and means that there is not really any contradiction.

If there is a crucial difference does that not emphasise my point even more?

Maybe I misunderstood 'your point', then. You appeared to be suggesting that the minimum required CCC was 20A "IF" (i.e. 'only IF') it was considered unlikely that the CCC would be exceeded for long periods - which is not what the reg says (and would not really make any sense at all!).

Obviously if installation methods are not ideal then derating applies but nothing to do with the question.

Well, if the de-rated CCC is less than 20A, it would seem that one cannot use the cable for a ring final (or unfused spur) per 433.1.103.

Quite, so no point mentioning it.

I wouldn't say 'no point', in as much as the reg allows that the CCC of 2.5mm² cable in a ring final may be de-rated (below 'ideal') down as far as 20A.

Not actually sure if the 20A minimum applies to the spurs.

I don't see why it shouldn't. The regs starts off talking about "ring final circuit, with or without unfused spurs" and nothing in the rest of the paragraph suggests that the unfused spurs are exempt from any of the stated requirements.

I read it as it makes no difference to the ring whether or not there are spurs; not as relevant to the spur.

I read it as saying that the whole paragraph relates to "ring final circuits, with or without unfused spurs", and hence that the stated cable/CCC requirements within the paragraph relate to any unfused spurs as well as to the ring itself. If you are interpreting it differently (i.e. that the stated cable/CCC requirements apply only to ring, not to unfused spurs), does that mean that you think that a 1.5mm² (Method C) unfused spur would be compliant?

Kind Regards, John