Sizing and protection of conductors

Selection of cross-sectional-areas of cables is certainly one of the most important tasks of the design process of an electrical installation as this greatly influences:

• the selection of overcurrent protective devices,
• the voltage drop along these conductors
• and the estimation of the prospective short-circuit currents

This chapters details the different steps and methodologies for conductor sizing. And the specific recommendations for neutral and Protective Earth (PE) conductors sizing.

Chapter G - Contents

• Conductor sizing and protection
  • Conductor sizing: methodology and definition
  • Overcurrent protection principles
  • Practical values for a protective scheme
  • Location of protective devices
  • Conductors in parallel
• Practical method for determining the smallest allowable cross-sectional area of circuit conductors
  • General method for cable sizing
  • Recommended simplified approach for cable sizing
  • Sizing of busbar trunking systems (busways)
• Determination of voltage drop
  • Maximum voltage drop limit
  • Calculation of voltage drop in steady load conditions
• Short-circuit current
  • Short-circuit current at the secondary terminals of a MV/LV distribution transformer
  • 3-phase short-circuit current (Isc) at any point within a LV installation
  • Isc at the receiving end of a feeder as a function of the Isc at its sending end
  • Short-circuit current supplied by a generator or an inverter
• Particular cases of short-circuit current
  • Calculation of minimum levels of short-circuit current
  • Verification of the withstand capabilities of cables under short-circuit conditions
• Protective earthing conductor (PE)
  • Connection and choice for protective earthing conductor
  • Sizing of protective earthing conductor
  • Protective conductor between MV/LV transformer and the Main Low Voltage Switchboard (MLVS)
  • Equipotential conductor
• The neutral conductor
  • Sizing the neutral conductor
  • Protection of the neutral conductor
  • Breaking of the neutral conductor
  • Isolation of the neutral conductor
• Worked example of cable calculation

External links

• Electrical design software