Re: cable size

Spir Georges GHALI

latafat junaid said:

Dear Sir,

Q1: during making a single line diagram,when i calculated the the total load,then which approperciate cable size i will select,means formula

Q2: For example: f the load s 40 A,Then how i will select MCCB, means i can use the 60A MCCB 3-Phase,but s there s any special arrangement to calculate for MCCB.

Q3: Cable size let say ” 3C,185 mm2″ how capacity of current it will take,any possible formulas r tables for all cable sizes and required MCCB?

A good /satisfaction answers & prompt response will be highly appreciated.



Dear ;

Q1 :

To do the correct choice of cable, we do as following :

– We calculate the load's current that we called ” Ib “.

– We define the ” Kind ” and the ” Ibtallation Methode ” of cable, then define the correction factors, and multipy them to have the total factor correction ” k “

– We calculate the carrying of current for cable in standard conditions that is ” Iz = Ib / k “.

– Then by the tables of ” IEC 60364-5-52 ” we can do the coorect choice of cable's section.


Q2 :

To de define the specification of Circuit Breaker that shpould be installed, we should do our choice of the main specifications of CB that are : ” Nomber of Poles  ;  Thermal Protection – Ir  ;  Magnetic Protection – Isd or Im  ;  The breaking Capacity “, so, we do as following :

– The nomber of poles will be defined accordongly to the ” kind of circuit  &  Earthing System used “.

– After doing the above methode to define the Cable's Section, we can define de value of ” Ir ” accordingly to ”  Ib  ≥  Ir  ≥  Iz . k “.

– To define the value of ” Isd  or  Im “, we should calculate the ” minimum short circuit current – Isc1min ” between ” phase & neutral ” at the end of cable, and calculate the ” Earth Fault – If ” also at the end of cable, then the value if ” Im ” should be msaller than the smalest calculated value.

– To define the ” Breaking Capacity “, we should calculate the ” maximum short circuit current – Isc3max ” at the begining of cable, then the value is ” Breaking Capacity  ≥  Isc3max “.


Q3 :

We can know the capacity of current or the carrying of current of cable from the tables of ” IEC 60364-5-52 ” after define the kind of cable that means : Single  or  Multi   ;   Cu  or  Alu   ;   PVC  or  XLPE ” and also the ” Methode of Installation ” . Noting that value is in Standard Conditions that means the total correction factors is ” 1 “.