It and Iz

Im getting a bit confused with It and Iz, and which is out final value when calculating cable size

It  - the value of current tabulated in this appendix for the type of cable and installation method concerned, for a single circuit in the ambient temperature stated in the current-carrying capacity tables

Iz -  the current-carrying capacity of a cable for continuous service, under the particular installation conditions concerned.

In =32  Ca 0.87

so    It ≥  32 / 0.87

It ≥ 36.78A

Look up tables in BS 7671  clipped direct (c) one  2 core cable   

4mm=36A

6mm= 46A

So we go with 46A

So our It (Tabulated value we look up) is 46A and we will use a 6mm2 cable

So what is Iz?

Iz ≥ In ≥ Ib  understand the relationship

Now Iz = It x correction factors.
Iz = 46 x 0.87 =  40.02A  which is greater than In (32A) so thats OK

But we got a value of 36.78 A when applying correction factors to In

We also have this formula... 

It ≥ Iz / correction factors

40.02 / 0.87 = 46

We end up with the same number.

We have a calculated value of 36.78A  in the book the cable choice is 6mm which is rated at 46A 

I'm struggling to see what we would call our final cable choice? Its not Iz , but the description (continuous service) makes it sound like it is

What is Iz

Hope that make some sense

Its based around this IET article


electrical.theiet.org/.../appendix-4-of-bs-7671.pdf

Parents
  • Thanks for all your help with this.

    Don't want to keep bothering, but I think I am getting there..

    In the 3rd drawing I have 3 scenario's

    Using the 3 formula mentioned in EIDG 

    Calculating Iz 
    Calculating It  using In
    Calculating It using Ib



  • Not quite sure what is bothering you. Isn't the upper half of your image saying exactly what I did?

    In practical terms, you look up the tables and select cable size accordingly. It might be that on the north side of a house up north you could reduce the ambient temperature, but why bother?

    Even here on a hot day on the south coast, ambient temperature is not 30 ºC, though it could be higher if the sun is shining on the electrical equipment.

    Perhaps a bit of engineering judgement is required?

Reply
  • Not quite sure what is bothering you. Isn't the upper half of your image saying exactly what I did?

    In practical terms, you look up the tables and select cable size accordingly. It might be that on the north side of a house up north you could reduce the ambient temperature, but why bother?

    Even here on a hot day on the south coast, ambient temperature is not 30 ºC, though it could be higher if the sun is shining on the electrical equipment.

    Perhaps a bit of engineering judgement is required?

Children
  • Not quite sure what is bothering you

    I don't think that Iz is very clearly explained, in all of the books or articles I have read.

    I see lots of articles that seem to misuse the terms
    The fact that the calculated value, has no name just It≥  bugs me.  Because this can easily be mis thought of as Iz.

    I have seen no publications that give worked examples of  Iz  (2.4.0 formula  EIDG)

    A basic misunderstanding is that this is down rating the CCC of the cable, Not reducing the current, so a smaller cable could be used.


    The guidance notes are good but frustrating, because I find they tell you but don't teach you. I want deeper explanations.

    The actual basic cable calculation is really straight forward.


    But Iz is confusing until you get it, which I think was picked up here

    - and it's often not explained at well either.

    I can imagine that my basic questions, and misunderstanding can be a bit frustrating, but I like to ask on here because you guys know your stuff and are generous with your time to explain, without any condescension. Which I really appreciate.

  • I see lots of articles that seem to misuse the terms
    The fact that the calculated value, has no name just It≥  bugs me.

    I don't disagree with that statement.

    Because this can easily be mis thought of as Iz.

    At the end of the day, the normative requirements in Part 4 of BS 7671, state the requirements in terms of Iz.

    Appendix 4 explains quite clearly, that you are looking up a value It with a view to it meeting or exceeding the minimum acceptable value Iz for the installation conditions in question.


    The guidance notes are good but frustrating, because I find they tell you but don't teach you.

    That's interesting ... In my opinion, the guidance notes in general are not, in themselves supposed to be teaching materials, but provide information as either a 'refresher', 'cpd', or to inform those who are being taught using appropriate materials.

    I can provide this feedback, and perhaps some appropriate publications could be devised to fill the gap, if there is one?