# 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

Parents
• OK, I understand the confusion. What's done is a little trick to make the procedure easier, but it doesn't help understanding - and it's often not explained at well either.

In principle what you should do is take It of all the various cable sizes, apply all the correction factors to each It as multipliers, to get the true Iz for each, then select the cable based on Ib (or In) against those values.

But that requires a lot of working out - you might have to go through the whole calculation umpteen times over, once for each candidate cable size, which especially in the days before electronic calculation (when most of these processes were first invented), was a bit of a pain in the butt to say the least.

So rather the textbook procedure is to take Ib (or In) and divide by the correction factors to get an It equivalent needed for your design current - then look down the table to pick a cable size large enough. Which saves a lot of calculation.

It doesn't directly give you a value for Iz though - you need to take It of your chosen cable size and multiply by the correction factors to get that. But mostly you don't need to know the exact value of Iz, since you've picked the correct cable size already ... which is the main point of the process.

- Andy.

• OK, I understand the confusion. What's done is a little trick to make the procedure easier, but it doesn't help understanding - and it's often not explained at well either.

In principle what you should do is take It of all the various cable sizes, apply all the correction factors to each It as multipliers, to get the true Iz for each, then select the cable based on Ib (or In) against those values.

But that requires a lot of working out - you might have to go through the whole calculation umpteen times over, once for each candidate cable size, which especially in the days before electronic calculation (when most of these processes were first invented), was a bit of a pain in the butt to say the least.

So rather the textbook procedure is to take Ib (or In) and divide by the correction factors to get an It equivalent needed for your design current - then look down the table to pick a cable size large enough. Which saves a lot of calculation.

It doesn't directly give you a value for Iz though - you need to take It of your chosen cable size and multiply by the correction factors to get that. But mostly you don't need to know the exact value of Iz, since you've picked the correct cable size already ... which is the main point of the process.

- Andy.

Children
No Data