Re: cable size

Spir Georges GHALI

Spir Georges GHALI said:

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 “.



Dear ;


Sorry, I forgeten to mention a very importatnt point about ” Q1 : Section of Cable ” that after defining the section of cable as tI explained before, we should calculate the ” Drop Voltage ” for this circuit and add it to the drop voltage from the source to this circuit, if the percentage's result is suitable and equal or smaller than the standard percentage is OK, but oif not, we should over-size the section of cable.