Sorry, This is the last I will post on this 
But this is a calculation using 2.4.0  (EIDG) equation first.
Then the equations from BS7671

gkenyon said:

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

I really believe there is a gap and a need. Believe me I look and I can't find many publications that go into the little details, and give a full explanation and worked examples. There is a lot of regurgitation and old content out there.
But I may of missed some so any suggestions gratefully accepted.

And thank again for the help

Hello,
I have not been following the discussion in its entirety, but I would like to clarify whether ( Iz ) is simply the value of ( It ) after applying the correction factors. I hope this is the case, as I have been in this profession for 30 years and this has been my understanding.

There is no term as far as I am aware for the calculated value, just our 'It' has to be ≥ than that value.
That has been a major part of my confusion. Though I think I have it now, thanks to the advice in this thread.
Be nice to give it a name

AMK said:

I have not been following the discussion in its entirety, but I would like to clarify whether ( Iz ) is simply the value of ( It ) after applying the correction factors.

NO.

The two are not directly related.

I_z is used in Part 4 of BS 7671.

I_t is introduced in Appendix 4, as a means of providing a "lookup value" that is at least I_z for the relevant conditions.

I_t is one of the values in the tables in Appendix 4. The minimum value of I_z, when you combine all of the requirements in Part 4, must, by definition, be less than, or equal to, I_t ... but since I_t relates to discrete values of cross-sectional area S, it's quite unusual (statistically improbable) for the two to be identical, if you are mathematically pedantic.

JohnE said:

There is no term as far as I am aware for the calculated value, just our 'It' has to be ≥ than that value.
That has been a major part of my confusion.

But ... that is the point. I responded a couple of minutes ago with the explanation that, I hope, makes sense:

gkenyon said:

NO.

The two are not directly related.

I_z is used in Part 4 of BS 7671.

I_t is introduced in Appendix 4, as a means of providing a "lookup value" that is at least I_z for the relevant conditions.

I_t is one of the values in the tables in Appendix 4. The minimum value of I_z, when you combine all of the requirements in Part 4, must, by definition, be less than, or equal to, I_t ... but since I_t relates to discrete values of cross-sectional area S, it's quite unusual (statistically improbable) for the two to be identical, if you are mathematically pedantic.

gkenyon said:

The two are not directly related.

I_z is used in Part 4 of BS 7671.

I_t is introduced in Appendix 4, as a means of providing a "lookup value" that is at least I_z for the relevant conditions.

I_t is one of the values in the tables in Appendix 4. The minimum value of I_z, when you combine all of the requirements in Part 4, must, by definition, be less than, or equal to, I_t ... but since I_t relates to discrete values of cross-sectional area S, it's quite unusual (statistically improbable) for the two to be identical, if you are mathematically pedantic.

HOWEVER ...

I will also throw this in.

Because of the definition of the symbol in Part 2, once you have selected the cable cross-sectional area S, I can see that, if you have followed the selection criteria in Part 4, the actual selected cable I_z exceeds the minimum value I_z that is gleaned from the requirements of Part 4 (which is, of course, the purpose of the design exercise).

BUT ...

We then need to consider voltage drop, adiabatic, etc., and any of these may lead to a larger S.

Which means that, in the general case, I_Z ≥ I_t, which is what EIDG advises ?

gkenyon said:

AMK said:

I have not been following the discussion in its entirety, but I would like to clarify whether ( Iz ) is simply the value of ( It ) after applying the correction factors.

NO.

The two are not directly related.

I'm not sure I agree.

Iz is It multiplied by all the relevant correction factors (which is what I think AMK was suggesting).

(unlike the usual calculation where we divide In or Ib by the correction factors - in effect converting our actual installation conditions to the conditions the tables are based on)

  - Andy.

Hi Andy, yes all I was getting at was to take (It) from the regs, multiply by the correction factors to give you (Iz) So the cable could one carry the value of (It) but after applying derating factor s can now only carry a new value known as (Iz) 

gkenyon said:

Because of the definition of the symbol in Part 2, once you have selected the cable cross-sectional area S, I can see that, if you have followed the selection criteria in Part 4, the actual selected cable I_z exceeds the minimum value I_z that is gleaned from the requirements of Part 4 (which is, of course, the purpose of the design exercise).

BUT ...

We then need to consider voltage drop, adiabatic, etc., and any of these may lead to a larger S.

Which means that, in the general case, I_Z ≥ I_t, which is what EIDG advises ?

Thank you for your continued help, but, I got a bit lost again, with the last comment.
Don't want to be a pain, so no worries if you are done with this   

I can understand how Iz ≥ It when we  multiple by more than 1 , but generally its less than 1 so will be less.

Say--- If we have our calculated value ≥It   and it is 27A, which suggests  2.5mm2 cable clipped direct.

We do the volt drop and find because of the length of the circuit, the CSA of the 2.5 cable is too small, so we go up to 4mm2.
In doing so the tabulated value for 4mm2 cable will also be a higher value, so It would be still greater than Iz. ?

Sorry could you point out the section in EIDG that advises Iz≥It

I've tried to explain in the attached sheet.

Thank you.

Thank you for your continued help, but, I got a bit lost again, with the last comment.

I think Graham was just saying the current carrying capacity of a cable (Iz ≥ Ib) is just one of several points that have to be considered when designing a circuit - other factors such as achieving a certain loop impedance, or required maximum voltage drop, may well require a larger cable than CCC consideration alone would suggest.

Say for instance you had a 1A lighting load 1000m metres away - to achieve 3% v.d. (6.9V) you'd need a cable with a v.d. of 6.9mV/A/m or lower - which would probably mean 10mm² cable, even though 1mm² or even 0.75mm² would probably have been fine from just a current carrying capacity perspective.

  - Andy.

AJJewsbury said:

I think Graham was just saying the current carrying capacity of a cable (Iz ≥ Ib) is just one of several points that have to be considered when designing a circuit - other factors such as achieving a certain loop impedance, or required maximum voltage drop, may well require a larger cable than CCC consideration alone would suggest.

Well, that, plus the important fact that it's not as simple as looking up the current-carrying capacity from a table in Appendix 4, given the cross-sectional area and cable type, and bingo, the value you look up (I_t) is equal to I_z. because it may be subject to factors such as grouping etc. that are not accounted for in the Tables themselves.

Slightly confusingly though, it is NOT ALWAYS true that, when you go through the selection process in Appendix 4, at the point you choose a csa from the value of "minimum I_t" you have calculated considering the grouping factors, the value of I_t you choose from a table in Appendix 4, in order to be greater than or equal to the calculated minimum I_t, is I_z regardless of  have to increase S (as AJJewsbury says). This is because, when you apply correction factors, the tabulated value (you look up) drops down to I_Z.

More simply ... it's not a two-way process.

AJJewsbury said:

I think Graham was just saying the current carrying capacity of a cable (Iz ≥ Ib) is just one of several points that have to be considered when designing a circuit - other factors such as achieving a certain loop impedance, or required maximum voltage drop, may well require a larger cable than CCC consideration alone would suggest.

Well, that, plus the important fact that it's not as simple as looking up the current-carrying capacity from a table in Appendix 4, given the cross-sectional area and cable type, and bingo, the value you look up (I_t) is equal to I_z. because it may be subject to factors such as grouping etc. that are not accounted for in the Tables themselves.

Slightly confusingly though, it is NOT ALWAYS true that, when you go through the selection process in Appendix 4, at the point you choose a csa from the value of "minimum I_t" you have calculated considering the grouping factors, the value of I_t you choose from a table in Appendix 4, in order to be greater than or equal to the calculated minimum I_t, is I_z regardless of  have to increase S (as AJJewsbury says). This is because, when you apply correction factors, the tabulated value (you look up) drops down to I_Z.

More simply ... it's not a two-way process.