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Cable size determine

We all know that a cable is sized by:

1.- Current carrying capacity (with all installation adjusting factors)
2.- Voltage drop
3.- Starting voltage drop (in case of motors)
4.- Short circuit

If all this data is not available is hard to give a correct cable size, only a preliminary size can be estimated.

Now according IEC Practices, a cable must be sized in such a way that a fault between a live conductor and any exposed conductive part or a protective conductor in the circuit or equipment can be disconnected by a protective devices within a time which causes no harmful physiological effect to a person who is in simultaneous contact with those conductive parts.

Allowable disconnect time per network configuration (TN, IT) can be found in IEC 60364-4-41 part 413. A cable size vs. cable length must be selected in such a way that the fault current will trip a protective device within the specified disconnect time. Cases where the disconnect time can be extended but not exceed 5 seconds can be found in IEC 61200-413.

Formulas how to calculate the maximum allowable cable size can be found in IEC 61200-53 and IEC-60909-0.

Now here rise some questions:
  1. What kind of system are we talking about: TN, IT, etc?
  2. What correction factors are necessary to use (depends on installation)?
  3. What ambient temperature correction factors are to be used?
  4. Cable is copper or aluminium?
  5. What is the disconnect time for the protective device used?
  6. Is it necessary to consider the cable reactance or the simplified equations shown in IEC are enough?
  7. What is the prospective short circuit current in the circuit?
  8. What kind of fault must be used: line to line, line to earth, line to earth second fault, line to neutral fault?

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