My concern is that, from considerations of safety (that's surely what current carrying capacity of cables is all about?) one should be dividing, not multiplying, the cable's current carrying capacity by the factor (I2/In), whatever it may be (1.45, 2.0 etc.) for the device concerned (if one wanted to compare the cable's capacity with In) - or, equivalently, multiplying the device's In by that factor in order to determine the cable carrying capacity required.
As skenk says, the actual current that a cable can carry, for shortish-durations, is greater than the tabulated values. It is deemed, by the regulations, to be able to carry 1.45 x
Iz for the amount of time it will take the device to operate, and by implication currents >
Iz and < 1.45
Iz for longer periods than the fusing/tripping time.
Exactly, and that's why I would have assumed that, in the interests of safety, one should use a cable whose capacity is certainly adequate to carry the current that the device could pass 'for ever' and probably also the current which (in the case of an MCB) the device could allow to pass for up to 1 hour? This article is stating (in two different ways) that it is adequate for the cable's tabulated current carrying capacity to only be adeqaute for In, but not for I1 or I2. Whilst I realise that there is undoubtedly a 'safety margin' built into the tabulated 'current carrying capacities' of cables, such a practice would seem to make a mockery of having such tabulated values which we are expected to work to.
It doesn't make a mockery of it - the tabulated values in Appendix 4 allow for
I1 &
I2 being >
Iz (but
I2 being ≤ 1.45
Iz)
I don't think I've seen (nor would expect to see) anything to suggest that it's OK to exceed tabulated current carrying capacities by 13% indefinitely and 45% for up to an hour - but maybe I've missed that.
It's not OK for the design current
Ib and therefore the device rating
In to exceed the tabulated cable capacity
Iz because if it does you lose the margins which allow the cable to cope with the fact that
I2 >
I1 ≥
Iz.
since you are using the example factor (I2/In) of 1.45, then 1.45Iz>I2 is mathematically exactly the same as In<Iz. The same would be true if you were talking about a device for which the I2/In factor was something other than 1.45. If you call that factor f, then fIz>I2 is always going to be the same as In<Iz.
Yes, it would, but the requirement doesn't change as f changes, i.e. if f was 1.8 then the requirement for
Iz remains that 1.45
Iz ≥
I2 (or
I2 ≤ 1.45
Iz if you prefer).
It does not become 1.8
Iz ≥
I2.
multiplying by 0.725 is the same as assuming an I2/In ratio of 1.38,
No it's not - it's assuming an I
2/I
n ratio of 2 and adjusting
Iz so that
I2 remains ≤ 1.45
Iz.
However, if you are saying that the I2/In ratio for a rewirable fuse is 2.0, then I would have thought that one surely should be dividing the cable's tabulated Iz by 2 (rather than 1.38 ) if one is going to compare it with In to determine whether the cable is adequate to carry I2.
No, because the cable's
Iz value is chosen such that it will not be damaged by a current of 1.45
Iz flowing for a short-ish period.
However, from what you go on to say, it sounds as if the 2.0 ratio may be something more confusing/messy than simply I2/In for a rewirable fuse.
No, that's exactly what the 2.0 figure is.
As for "0.725 = 1.45/2, i.e. 1.45 ÷ the I2:Iz ratio", I find this even more confusing. As you've said, the 1.45 figure is specific to Type B MCBs - so are you saying that all other protective devices are referenced by a 'correction factor/ratio' to Type B MCBs. Whatever, there has surely got to be something wrong with dividing an adjustment factor of 1.45 (an increase of 45%) by 2 and thereby turing it into an adjustment factor of 0.725 (a decrease of 27.5%) would appear rather bizarre. There is surely no sense in the implication that a cable needs 'de-rating' (if one is going to size it in relation to In) in the case of a rewirable fuse (which makes sense) but can be 'up-rated' for an MCB (which does not make sense)!!
No, that's not what's going on, but I can see why you're confused.
Firstly and
many apologies
- with all the copying and pasting of all the various
I<whatever> variables I got one wrong, and I should have written
"0.725 = 1.45/2, i.e. 1.45 ÷ the
I2:
In ratio"
So that definitely didn't help, but I still think you are also getting confused about the 1.45 value.
The 1.45 in "0.725 = 1.45/2" is not the ratio of
I2:
In for a Type B breaker it is the 1.45 in 1.45
Iz ≥
I2.
The 2 in "0.725 = 1.45/2" is the
I2:
In for a 3036 fuse.
I was also wrong, in practice, when I said that "for any other device you should do a second check, in addition to
Ib ≤
In ≤
Iz for 1.45
Iz≥
I2 when choosing a cable".
In theory you should, I guess, but in practice the regs say that for all other protective devices you can ignore the fusing factor (
I2:
In) and simply ensure that
Iz ≥
In.
Were it not for that rule of thumb then I think it would all be a lot clearer.
The table below shows the fusing factor for various devices, and in the last column is the value by which the cable capacity would have to be multiplied by if one wanted a strict non-rule-of-thumb adherence to 1.45
Iz≥
I2:
[code:1] Type I2:I1 1.45 ÷ I2:I1
BS 88 fuse 1.6 0.906
BS 1361 fuse 1.5 0.967
BS 1362 fuse 1.9 0.763
Type 1/2/3/4 MCB up to 10A 1.5 0.967
Type 1/2/3/4 MCB over 10A 1.35 1.074 **
Type B/C/D MCB 1.45 1.0
BS 3036 fuse 2.0 0.725
[/code:1]
** An interesting value - in reality if you were doing this you'd have to limit it to 1 to ensure
In ≤
Iz
As you can see they are all close enough to 1 for it to make sense that only for 3036 and 1362 fuses do you need to bother adjusting
Iz so that
I2 remains ≤ 1.45
Iz.
This really does seem all wrong to me. I would have expected that the regulations would be simple, merely requiring that cables should be sized so as to have an adequate current carrying capacity to carry that current which the protective device colud allow to flow for appreciable periods of time (i.e. certainly I1 and probably I2)
They are and they do - the tabulated values for current carrying capacity allow for this:
As far as I can make out (although I'm very open to correction), and somewhat to my surprise, they seem to be saying that no allowance needs to be made for I2/In for most types of protective device - i.e. it is acceptable for the cable to be sized simply on the basis of the 'rating' (i.e. In) of the protective device (which is what most 'lay' people would probably assume).
That's because for most types of protective device the value of 1.45 ÷ the device's fusing factor, I
2/I
n is close enough to 1 for no allowance to be needed.
If my interpretation is correct, then maybe the wiki should say just that - i.e. that Iz should be no less than In, without potentially confusing some people by all the other discussion, interesting though it is, about I1 and I2 etc.
I think it's worthwhile to keep the 1.45
Iz ≥
I2 in there, but to add more information on what
I2 is, and how
I2:
In is close enough to 1.45 for most protective devices for it to be disregarded, and only for BS 3036 and 1362 fuses does it need to be considered.
I should probably also add an explanation of just what the purpose of the adjustment is to make sure people have got it the right way round, i.e. if you've got a 30A rewirable fuse it's not that you're looking for a cable with a capacity of 21.75A, you're looking for a cable with a capacity which when multiplied by 0.725 is no less than 30A.
Per the regs, the one exception seems to be if the protective device is a semi-enlclosed (i.e.rewirable) fuse, in which case one has to apply a single correction factor of 0.725; it looks to me as if it a total co-incidence that this figure happens to be half the I2/In ratio for Type B MCBs, but I may be wrong.
What do you think?
TBH I'm not sure how much of a coincidence it is.
It's half of something because the fusing current for a 3036 is 2 x its nominal rating.
It's half of a value which is the
I2:
In for a type B and which is also the required maximum ratio of
I2:
Iz.
The Q is is it a coincidence that the maximum ratio is the same as the fusing factor for a Type B, or was the value chosen to be the fusing factor?
Or, was that value of 1.45 already established as the maximum safe value and BS EN 60898 written to specify that the fusing factor for devices was to be no more than 1.45?
If that's true, it does nothing to increase my respect for the Wiring Regulations
Why?
since they are undermining their own concept of tabulated current carrying capacities - particularly in relation to the 13% 'for ever' potential scenario. In other contexts, I'm sure that those who wrote the regs wouldn't be happy with my using an undersized cable on the basis that the maximum load was 'no more than 13%' (or even 'no more than 45%) above the officially 'permissible' level!
No, they wouldn't be happy, because that's not what they are doing.
The tabulated values for current capacity are not what the cable can carry before becoming damaged. They are the values which, after correction for things like installation method, grouping, ambient temperature, type of protective device etc, give you an
Iz which is safe to use when what's going on is as described pictorially in the diagram above.
Indeed, I think that's what makes the wiki a bit confusing, because it also discusses all of the above in relation to cable sizing (determination of required Iz), even though those figures (1.45 aand 2) are not actually relevant to selecting required cables size.
Actually they are - the problem with the Wiki is not that it contains too much information it's that it doesn't have enough. If (when) it's expanded to cover fusing factors then the relevance of 1.45I
z should become clear.
If my understanding of the regulations is correct, the required Iz (as tabulated) appears to be simply In in most cases (e.g. where the protective device in an MCB), or 0.725In in the case of a rewirable fuse
No!
The
required I
z is not 0.725I
n in the case of a rewirable fuse, you've got that the wrong way around.
If you have a rewirable fuse then the cable's I
z value has to be multiplied by 0.725. i.e. if you are trying to determine what the I
z of the cable
is, you start with its tabulated value, you apply any de-rating factors for grouping, ambient temperature, thermal insulation etc and the de-rating factor (i.e. x) of 0.725. If the value which falls out the end is < I
n you're hosed.
If you're trying to find out what the
required value of I
z is when using a rewirable, you divide the fuse rating by 0.725. e.g. for a 30A fuse you need your cable, after you've applied all the other de-rating factors to it as normal to have a rating of 41.4A.
but the wiki makes it sound much more complicated and confusing than that.
Hmm.
I didn't think so, but after this....
The wiki also obvioulsy somewhat adds to a reader's confusion by pointing out that the 0.725 'de-rating' factor for a semi-enclosed fuse is half of the 'fuse factor' for an MCB - even though this seems to be nothing but co-incidence!
Oh be fair - it doesn't mention any of that at all.
I don't have immediate access to the Appendices, but I have no reason to doubt what you are saying. Do I take it that this is explicitly stated in the Appendix 4?
No, it isn't, but it doesn't need to be as for protective devices it doesn't talk about I
1 or I
2, only I
n.
However, particularly given that In is what 'is written on the tin' I remain unimpressed by any document which writes:
I2 <= 1.45 Iz, where I2=1.45In
... rather than simply In <= Iz !!
As explained above, those two 1.45s are not the same thing. They are two different things which happen to have the same value, and the simplification you seek only works when they do. It's possible for one of them to change (e.g. to 2.0 for 3036 fuses), but the other one remains at 1.45.
Even if the 1.45s are the same because one was chosen to be what the other one
is (i.e. the fusing factor for Type Bs) or because the fusing factor for Type Bs was written into the spec to be what was known to be the safe value for cables (i.e. 1.45) that does not alter the fact that they are not the same thing, they just have the same value.
(or, even more logical and helpful, since we're given In and are interested in Iz):
Iz > In
But to calculate I
z you have to allow for the fact that I
2 is
not always 1.45I
n, but must always be ≤ 1.45I
z....
Furthermore, if the Iz figures in Appendix 4 actually allows for currents >Iz for up to I hour does this mean more generally that a design could be compliant with the regulations if it was based on an assumption that current might exceed a cable's Iz by up to 45%, provided one knew that this was never going to persist for more than 1 hour. I would suspect not, but where then would be the consistency?
Well, ignoring the provisions for cyclic loads and thermally equivalent constants you suspect right, and the consistency you seek lies in the diagram above - all the cable ratings are based on the assumption that I
z lies between I
n & I
1 and that I
2 ≤ 1.45I
z....
