HUGE KITCHEN RE-WIRE & ISLAND (10 appliances) help!!!

[JohnW2]

Since 433.1.101 talks about "0.725 times the CCC of the cable", I have always presumed that they were not intending that the "CCC" had already been reduced by a factor of 0.725 if it were protected by a 3036 (otherwise that factor would get applied twice). I have therefore always assumed that 'CCC' took into account installation method and all 'cable de-rating factors', but not any adjustment for a 3036.

433.1.202 now. I'm not sure I understand why you think it is applied twice.

That makes total sense to me. The "current-carrying-capacity" of 2.5mm² Method C is obviously always going to be 27A, no matter what the type or rating of OPD.

No. With 3036s it has to be derated by 0.725

You really can't have it both ways. If you want to say that, when protected by a 3036, the "CCC" is 'de-rated' by 0.725, then that means that, in that situation, the "CCC" of Method C 2.5mm² would be about 19.6A. 433.1.101 (or 433.1.202 for those with a BYB) says that the In of the fuse must not exceed 0.725 times the "CCC" of the cable. If, as you above, you believe that the "CCC" is 19.6, then the reg would require that In of the fuse must not exceed 0.725 times 19.6A, namely about 14.2A (27A x 0.725 x 0.725) - which I doubt that even you believe is what is intended

The cable does not suddenly become able to safely carry more, or less, current just because of the type of OPD protecting it.

It has to be able to carry more overload when protected by 3036s.

That's just, IMO, a very confusing use of the word "overload". The maximum amount of current a cable can safely carry ("CCC") is obviously the same, regardless of the OPD. What differs between an MCB and a 3036 is what multiple of the OP's In that CCC represents.

All this is obviously based on the assumption that an MCB may allow 1.45 times its In to flow for up to an hour, and that a BS 3036 fuse may allow 2.0 times its In to flow for up to an hour. The tabulatated values of 'CCC' (Iz) assume an MCB - hence if the tabulated Iz is, say, 20A, that means that the cable is deemed to be able to safely carry 29A (20A x 1.45) for an hour. A 3036 which would allow 29A to flow for no longer than an hour would have an In of 14.5A (29A / 2.0). The 0.725 difference between those two figures (e.g. 14.5A/20A=0.725) arises simply because the 1.45In/2.0In=0.725.

The only difference relates to the OPD.

Isn't that contradicting what you just wrote?

Not at all. As above, a cable with a CCC of say, 20A, has to be able to safely carry 29A for up to an hour, regardless of the nature of the OPD. All that differs (between an MCB and a 3036) is what In of the OPD is necessary in order to limit the duration of that 29A to no more than an hour.

MCBs have a fusing factor of 1.45 - the standard - 3036s have a fusing factor of 2 so the cable must be derated (2x0.725 = 1.45)

That's exactly what I've written above, provided your'e talking about the 'fusing factor' to ensure operation within an hour.

You either have a 27A cable but must protect it with a 19.5A 3036 or you have a 27A 3036 but must use a 37.24A cable.

No-one is disagreeing with that, in the 'general' (non-ring) situation. It is the wording of 433.1.101 (or 433.1.204) for ring finals that is the issue.

433.1.101 states that the In of the 3036 shall not exceed 0.725 the Iz of the cable.

It does, and that is the point I am "going on about". Iz (aka 'CCC') is a characteristic of a cable given a certain installation method etc. and does not, as far as I can make out, include any adjustment because of the OPD by which it is protected (i.e. the 0.725 for a 3036) - which I presume is why 433.1.101 says that you have to multiply the "CCC" by 0.725 if it's protected by a 3036. If, as you seem to be suggesting, th "CCC" will already have been adjusted by one factor of 0.725 (if protected by a 3036), then applying 433.1.103 would apply a 0.725 factor for a second time - which is obviously not intended. I therefore conclude that "CCC" does not itself include an adjustment for 3036 protection (if that is what is protecting it!).

Kind Regards, John

 

[EFLImpudence]

Are you confusing the fact that a ring circuit has to have cables with a CCC of 20A (which is already roughly 27x0.725)?
I still do not understand why you think it is being, or has to be, derated twice.

Do you agree that a ring could be wired in 1.5mm² with a 32A MCB but not with a 30A 3036?
With a 30A 3036 the CCC (of 20A) has to be derated, i.e. 20/0.725 = 27A approximately, but that is only one derating.

If the Appendix 4 CCC tables had a column for 3036 fuses then the value for 2.5mm² would be 20A (19.5 + tweak) therefore acceptable for a ring circuit with a 30A 3036.
The same as 6mm² would be 34A and so acceptable for a 30A 3036 radial and 4mm² at 26.8A would not.

Do we agree that for radials it is quite clear?

 

[JohnW2]

Are you confusing the fact that a ring circuit has to have cables with a CCC of 20A (which is already roughly 27x0.725)? I still do not understand why you think it is being, or has to be, derated twice.

I'm finding this very frustrating because I have just one very simple point to make, but I'm clearly failing miserably in expressing it clearly enough!

Do you agree that a ring could be wired in 1.5mm² with a 32A MCB but not with a 30A 3036?

[do I take it that you mean 2.5mm² ? - we aren't allowed 1.5mm² rings, unless MICC]
In common sense terms I agree with that, but the very thing I am questioning is whether that is actually what 433.1.101 (433.1.204) actually says. I personally don't think it is.

It's entirely a question of what 433.1.101 (and other regs) mean when they talk about CCC/Iz. Given that 433.1.101 (433.1.202) says that the CCC has to be multiplied by 0.725 when using a 3036, I take that to mean that when they talk about CCC they are referring to something which has not (yet) been multiplied by 0.725, even though protected by a 3036. Is that such an unreasonable presumption?

If you did accept that the regs were using "CCC" to have that meaning, then it would mean that 2.5mm² Method A, as well as Method C, would be acceptable in a ring protected by a 3036, just as it would in a ring which was protected by an MCB - which is not what we would 'expect'.

Do we agree that for radials it is quite clear?

Sure. This whole discussion is entirely about the wording and meaning of 433.1.101 (433.1.204) in relation to ring finals.

Kind Regards, John

 

[EFLImpudence]

do I take it that you mean 2.5mm² ? - we aren't allowed 1.5mm² rings, unless MICC]

It was just an example to show the cable would comply electrically.

In common sense terms I agree with that, but the very thing I am questioning is whether that is actually what 433.1.101 (433.1.204) actually says. I personally don't think it is.

It may not but stating a minimum of 2.5mm² and 20A CCC must be considering derating factors.

It's entirely a question of what 433.1.101 (and other regs) mean when they talk about CCC/Iz. Given that 433.1.101 (433.1.202) says that the CCC has to be multiplied by 0.725 when using a 3036, I take that to mean that when they talk about CCC they are referring to something which has not (yet) been multiplied by 0.725, even though protected by a 3036. Is that such an unreasonable presumption?

I am obviously missing your point as it seems quite simple.
The maximum CCC of 2.5mm² is 27A so this cable must have a maximum 3036 protection of 19.575A (20A) FOR A RADIAL.

If you did accept that the regs were using "CCC" to have that meaning, then it would mean that 2.5mm² Method A, as well as Method C, would be acceptable in a ring protected by a 3036,

I presume you mean a 30A 3036.
Method A would be unacceptable as its CCC is already down to 20A so with a 3036 - only 14.5A.
Surely this is two forms of overheating which could happen together.

just as it would in a ring which was protected by an MCB - which is not what we would 'expect'.

Why. We discussed recently that with MCBs the rating could be increased to 40A but it is not allowed.

 

[ban-all-sheds]

If what you and BAS have suggested were a correct interpretation, it would mean that 2.5mm² could not be used in a ring final on a 30A 3036 if there were any de-rating factors to b applied to the cable. Is that your belief?

It is mine.

 

[JohnW2]

I am obviously missing your point as it seems quite simple. ... The maximum CCC of 2.5mm² is 27A so this cable must have a maximum 3036 protection of 19.575A (20A) FOR A RADIAL.

Let's try to take this in bite-sized steps ... Firstly, let me stress that I agree completely with you, electrically. The situation with radials is totally agreed. As for rings, if one wanted the situation with a 3036 to be consistent with the situation with an MCB, 2.5mm² cable on a 3036 could only b used if it were Method C and without any other de-rating factors. The question is simply whether that is what 433.1.103 (433.1.204) actually says - and that, in turn, depends crucially on what the regs (in particular 433) mean when they refer to "current-carrying capacity" (CCC) or Iz of a cable.

So, the first of my bite-sized questions ... 433.1.101 (433.1.202) says that if a cable is protected by a 3036, then the In of that fuse must not exceed 0.725 times the "current-carrying capacity" of the cable. Do you accept that this must mean that they consider "current-carrying capacity" to be the tabulated figure (adjusted for any other de-rating factors) before any adjustment has been made because of the 3036 - Yes or No?

Kind Regards, John

 

[JohnW2]

If what you and BAS have suggested were a correct interpretation, it would mean that 2.5mm² could not be used in a ring final on a 30A 3036 if there were any de-rating factors to b applied to the cable. Is that your belief?

It is mine.

In electrical terms, it would be my belief as well - but keep an eye on how my current exchange with EFLI evolves.

Kind Regards, John

 

[EFLImpudence]

As for rings, if one wanted the situation with a 3036 to be consistent with the situation with an MCB, 2.5mm² cable on a 3036 could only b used if it were Method C and without any other de-rating factors.

Yes. Isn't that what I have been saying?
Otherwise there would be no difference in circuit requirements for an MCB or 3036 but clearly there is.

The question is simply whether that is what 433.1.103 (433.1.204) actually says - and that, in turn, depends crucially on what the regs (in particular 433) mean when they refer to "current-carrying capacity" (CCC) or Iz of a cable.

Yes, but the CCC is dependent on derating factors.
Through thermal insulation the CCC of 2.5mm² is 13.5A.
The Appendix 4 tables are tables of CCC for different methods.
27A is the maximum CCC for 2.5mm².
So - method 103, 13.5x0.725 = 9.8A

So, the first of my bite-sized questions ... 433.1.101 (433.1.202) says that if a cable is protected by a 3036, then the In of that fuse must not exceed 0.725 times the "current-carrying capacity" of the cable.

Yes, the CCC of the cable is dependent on the installation method.
However, it would be more practical to state the reverse - that the CCC of the cable must exceed In/0.725 of the 3036.
So - 30A 3036, cable method 103, 30x2/0.725 = 82.76A

Do you accept that this must mean that they consider "current-carrying capacity" to be the tabulated figure (adjusted for any other de-rating factors) before any adjustment has been made because of the 3036 - Yes or No?

Yes, I do - but you seem not to have been accepting this, stating that the CCC of 2.5mm² is 27A and does not require derating for a 3036.



I agree with everything you have said in this post but it does seem to be in contradiction of your previous posts.

 

[JohnW2]

So, the first of my bite-sized questions ... 433.1.101 (433.1.202) says that if a cable is protected by a 3036, then the In of that fuse must not exceed 0.725 times the "current-carrying capacity" of the cable. Do you accept that this must mean that they consider "current-carrying capacity" to be the tabulated figure (adjusted for any other de-rating factors) before any adjustment has been made because of the 3036 - Yes or No?

Yes, I do -

Right. Next bite-sized question, then. Does the phrase "current-carrying capacity" in 433.1.103 (433.1.204) have he same meaning as the phrase "current-carrying capacity" in 433.1.101 (433.1.202) - Yes or no?

Kind Regards, John

 

[EFLImpudence]

Yes, it has the same meaning.
Did you really mean "Does it have the same value"?

.204 will have taken into account the requirement demanded by .202 as well as any other relevant regulations.
A resultant Iz of a cable is the product of many different regulations.

Iz - CCC of a cable for continuous service under the particular installation conditions concerned.

 

[JohnW2]

So, the first of my bite-sized questions ... 433.1.101 (433.1.202) says that if a cable is protected by a 3036, then the In of that fuse must not exceed 0.725 times the "current-carrying capacity" of the cable. Do you accept that this must mean that they consider "current-carrying capacity" to be the tabulated figure (adjusted for any other de-rating factors) before any adjustment has been made because of the 3036 - Yes or No?

Yes, I do -

Right. Next bite-sized question, then. Does the phrase "current-carrying capacity" in 433.1.103 (433.1.204) have he same meaning as the phrase "current-carrying capacity" in 433.1.101 (433.1.202) - Yes or no?

Yes, it has the same meaning.

OK - see below.

Did you really mean "Does it have the same value"?

No - I meant what I wrote/asked.

So, just to be doubly sure about your combined answers to those two questions. Do you agree that you are saying that the meaning of the phrase "current-carrying capacity" in 433.1.103 (433.1.204) is "the tabulated figure (adjusted for any other de-rating factors) before any adjustment has been made because of the 3036" - Yes or No?

Kind Regards, John

 

[EFLImpudence]

Do you agree that you are saying that the meaning of the phrase "current-carrying capacity" in 433.1.103 (433.1.204) is "the tabulated figure (adjusted for any other de-rating factors) before any adjustment has been made because of the 3036" - Yes or No?

No

It is Iz, not It.

 

[JohnW2]

So, the first of my bite-sized questions ... 433.1.101 (433.1.202) says that if a cable is protected by a 3036, then the In of that fuse must not exceed 0.725 times the "current-carrying capacity" of the cable. Do you accept that this must mean that they consider "current-carrying capacity" to be the tabulated figure (adjusted for any other de-rating factors) before any adjustment has been made because of the 3036 - Yes or No?

Yes, I do -

Right. Next bite-sized question, then. Does the phrase "current-carrying capacity" in 433.1.103 (433.1.204) have he same meaning as the phrase "current-carrying capacity" in 433.1.101 (433.1.202) - Yes or no?

Yes, it has the same meaning.

So, just to be doubly sure about your combined answers to those two questions. Do you agree that you are saying that the meaning of the phrase "current-carrying capacity" in 433.1.103 (433.1.204) is "the tabulated figure (adjusted for any other de-rating factors) before any adjustment has been made because of the 3036" - Yes or No?

No. It is Iz, not It.

In that case, do you wish to withdraw one of your first two 'Yes' responses above - since Yes to the first two surely must mean Yes to third (which is merely a combining of the first two)?

In fact, if you agree with me that the first answer is (as you said) 'Yes', then it's not either Iz or It but, rather, something in between ('almost Iz') - in as much that, as you agreed, it's the It to which all de-rating/adjustment factors have been applied other than an 'adjustment' for the 3036.

Kind Regards, John

 

[EFLImpudence]

In that case, do you wish to withdraw one of your first two 'Yes' responses above

I don't think so.

.202 obviously requires the CCC of a cable to be derated if it is to be protected by a 3036 resulting in a new reduced CCC(Iz).
It would, as I said earlier, be better if it did not say the In of 3036 must not exceed 0.725 of Iz, because it will be the Iz which will be increased rather than the In reduced.
Does that, technically, mean then that the first Iz was not really Iz?

.204 refers to the final CCC(Iz) of the cable taking into account whatever derating factors have been required including that for 3036s when used.


It doesn't matter in which order the derating calculations are done.

 

[JohnW2]

In that case, do you wish to withdraw one of your first two 'Yes' responses above

I don't think so.

Well, as I said, it is logically impossible that the answers to the first two questions is Yes, yet the answer to the third is No - since the third is merely a statement of what those two Yes's means. Don't forget, I was merely asking you what was meant by the phrase "current-carrying capacity" in the two regs (and whether it meant the same thing in the two regs) - I was not asking any 'electrical' questions.

.202 obviously requires the CCC of a cable to be derated if it is to be protected by a 3036 resulting in a new reduced CCC(Iz). It would, as I said earlier, be better if it did not say the In of 3036 must not exceed 0.725 of Iz, because it will be the Iz which will be increased rather than the In reduced.

This is where I disagree with you, and think that you are probably 'going a bit wrong'. Installation method and all the usual de-rating factors affect CCC/Iz (i.e. the maximum current the cable can safely carry - or, at least, a function thereof**), and that CCC/Iz is the same, regardless of the nature of the OPD.

[ ** If the tabulations we used were literally the maximum current the cable could safely draw for an hour, then we would have to apply 'adjustments' for any OPD - about 0.69 (1/1.45) for an MCB or 0.500 for a 3036. Given that MCBs have become far more ubiquitous, they actually tabulate the figure for MCBs, so we only have to make an adjustment (of 0.50/0.69 = 0.725) if a 3036 were being used. ]

However, I personally don't believe that the OPD does, or should, alter CCC/Iz (which is literally the 'current-carrying capacity' of the cable) - and for that reason I think the wording of .202 is correct and appropriate. Rather than saying that the OPD affects the CCC (which, electrically speaking, would really be nonsense) they talk in terms of what maximum In the OPD can have and still give adequate protection to a cable with a given CCC - which makes sense.

Kind Regards, John