Query on cable calcs

[Paul505]

Just a quick one....
When working out cable calcs I have always used the Ib<equal to In<equal to Iz.
Unless overload protection is not required for the cable Iz can be taken from Ib.
(It)...i believe is the tabulated values in the tables in appendix 4.
Iz is the current-carrying capacity of the cable after other factors have been taken into account such as grouping, ambient temperature, thermal insuation and protective device etc
Then go to the appendix 4 tables relating to the cable in question and pick the size cable which has a higher value than your Iz value.

A friend of mine is doing a home study and its shown as an example where It is calculated from In over the factors, then Iz is taken from the tables. I could be missing the point somewhere, but I'm sure it's wrong.

Any replies would be appreciated.
Paul

 

[GLENNSPARK]

In is the rating of the OPD (overcurrent protective device)
I delta n is for RCDs
Iz is the CCC of a conductor
Ib is the design current

 

[GLENNSPARK]

In has to be equal or greater than Ib
Iz has to be equal or greater that In

 

[Guest55]

It is the tabulated ccc of a cable
Iz is the in service ccc of a cable.
;-)

 

[Paul505]

It is the tabulated ccc of a cable
Iz is the in service ccc of a cable.
;-)

That's what I thought but in my friends home study it's the other way round... (It) is the worked out value from In over factors rather than (Iz)worked out from In over factors. Then Iz was the value taken from the tables to get the cable size.

 

[Paul505]

In has to be equal or greater than Ib
Iz has to be equal or greater that In

I understand that bit....cheers anyway

 

[Guest55]

all your doing is substituting the Iz prefix for It , its a minor detail that has no effect on the outcome of the equation if carried out in the right steps.
to be honest , i do it the same way as your mate and use It , the end figure stays the same though so makes no odds.

 

[Paul505]

all your doing is substituting the Iz prefix for It , its a minor detail that has no effect on the outcome of the equation if carried out in the right steps.
to be honest , i do it the same way as your mate and use It , the end figure stays the same though so makes no odds.

Your right it's only minor detail...
(It) was the worked out value and (Iz) was the value taken from the tables in appendix 4 to get the cable size, which I thought was the other way around.
Cheers anyway Biff

 

[kingeri]

Just use 16mm2 for everything then you can't get it wrong!