calble calculation Ib

[Terrycarl]

hello all
Im a trainee and have a question about Ib,
If im working out cable calculations for a 63amp sub main with a 6amp and a 32amp mcb then what would my Ib be? is there a way or do you need to know wattage such as a 3kw water heater for example? please help
thanks
Terry

 

[Terrycarl]

can i use power law say V x I = P (230 x 38amp (or 63amp or101amp) = p, getting confused with I, is it the 63amp for I or just the mcb or all combined, thanks

 

[telectrix]

if it's a 63A sub-main, then i would take my Ib for the cable supplying it as 63A. the existing 32A &6A circuits could be increased in the future.

 

[Terrycarl]

ok so In is 64amp? basically skip Ib? regards to discrimnation in the main cu what fuse would have to be installed to run this 63amp circuit? another 63 amp
thanks

 

[Guest55]

No !!
do not skip Ib
do not pass go
do not collect £200
;-)
in the most simplistic terms your max Ib will be the 32a mcb + 6a mcb = 38 A
then you can apply diversity to reduce this figure if you know what loads are to be connected.
never ignore this basic assessment , its the starting point for all cable calcs.

 

[telectrix]

first, Iz>In>Ib. i.e. if your design current (Ib) < fuse rating (In) < CCC of cable (Iz), so if you IB is 63A, then In >63 (e.g. 80A) so Iz needs to be >80A (e.g. 90A)

 

[telectrix]

No !!
do not skip Ib
do not pass go
do not collect £200
;-)
in the most simplistic terms your max Ib will be the 32a mcb + 6a mcb = 38 A
then you can apply diversity to reduce this figure if you know what loads are to be connected.
never ignore this basic assessment , its the starting point for all cable calcs.

i would allow for increased loading on the sub-main, unless it's specifically designed for a set purpose, in which case i agree with you.

 

[Badged01]

It all depends what you are designing the actual sub-main circuit for? But whatever you as the designer choose it has to meet the basic concept of Ib < In < Iz.

If you want a sub-main to take 63A then design it for Ib = 63A -- as Tel states.
If you want a sub-main to take 6A and 32A then design it for Ib = 38A -- as Biff states.

 

[lauriehurman]

Hi TerryCarl
I agree with Telectrix. It's not a question of what might happen later, it's a question of what fuse is protecting the cable in question from overheating?
63A ?
Ib < In < Iz is the first commandment.

Laurie

 

[Terrycarl]

thank you guys
so the main switch on the sub would be greater than the fuses combined in the sub and the fuse on the main board would be greater than the main on the sub!.... also, if burying the SWA how would i know the Ca factor, do i take it as 1 then add Cs and Cd, something Ive never been shown so not too sure what K.m/W means or how to apply it
Thanks

 

[Richard Burns]

thank you guys
also, if burying the SWA how would i know the Ca factor, do i take it as 1 then add Cs and Cd, something Ive never been shown so not too sure what K.m/W means or how to apply it
Thanks

Ca would generally be 1 because the the sub ground temperature is fairly stable and unless you have specific circumstances 20oC is fine.
kelvin meter per watt is is stating the temperature change over 1m for each watt of energy. Again if you have a standard soil type you can generally assume your soil is suitable since the regs give average values to make calculation easier.
When adjusting your In you divide In by the product of the factors you are applying to get the value you need to find in the appropriate table.

 

[bartkusal]

thank you guys
so the main switch on the sub would be greater than the fuses combined in the sub and the fuse on the main board would be greater than the main on the sub!.... also, if burying the SWA how would i know the Ca factor, do i take it as 1 then add Cs and Cd, something Ive never been shown so not too sure what K.m/W means or how to apply it
Thanks

Your protection device for sub main circuit in the main board does not need to be greater than the main switch in the sub main board, because the Main switch is just that SWITCH there is no fuse in it. The 63A label on your switch just saying that this switch can safely operate up to 63A, that’s all. Think about, it is like installing the 2 way garage consumer unit with main 63A RCD incomer and your 2 MCB’s, 32A and 6A, you surely not going to come from 80A MCB in the house are you? Imagine the size of the cable to garage, insane.
In your case I would normally go by MCB sizes adding up and applying diversity or the rule of thumb 0.4 factor to it if it is only 2 way board, But if it is more spare ways I would not apply the diversity and in that way it would leave me with provision in the future.