Conductor sizing: methodology and definition

Methodology

(see Figure G1)

Component parts of an electric circuit and its protection are determined such that all normal and abnormal operating conditions are satisfied

Following a preliminary analysis of the power requirements of the installation, as described in The consumer substation with LV metering, a study of cabling and its electrical protection is undertaken, starting at the origin of the installation, through the intermediate stages to the final circuits.

The cabling and its protection at each level must satisfy several conditions at the same time, in order to ensure a safe and reliable installation, e.g. it must:

• Carry the permanent full load current, and normal short-time overcurrents
• Not cause voltage drops likely to result in an inferior performance of certain loads, for example: an excessively long acceleration period when starting a motor, etc.

Moreover, the protective devices (circuit-breakers or fuses) must:

• Protect the cabling and busbars for all levels of overcurrent, up to and including short-circuit currents
• Ensure protection of persons against indirect contact hazards (fault protection), particularly in TN- and IT- earthed systems, where the length of circuits may limit the magnitude of short-circuit currents, thereby delaying automatic disconnection (it may be remembered that TT- earthed installations are necessarily protected at the origin by a RCD, generally rated at 300 mA).

The cross-sectional areas of conductors are determined by the general method described in Practical method for determining the smallest allowable cross-sectional area of circuit conductors of this Chapter. Apart from this method some national standards may prescribe a minimum cross-sectional area to be observed for reasons of mechanical endurance. Particular loads (as noted in Chapter Characteristics of particular sources and loads) require that the cable supplying them be oversized, and that the protection of the circuit be likewise modified.

[a] The type of overload protection (fuse or circuit breaker) has a direct impact on conductor sizing, as detailed in Practical values for a protective scheme.

Fig. G1 – Flow-chart for the selection of cable size and protective device rating for a given circuit

Definitions

Maximum load current: I_B

• At the final circuits level, this design current (according to IEV "International Electrotechnical Vocabulary" ref 826-11-10) corresponds to the rated kVA of the load. In the case of motor-starting, or other loads which take a high in-rush current, particularly where frequent starting is concerned (e.g. lift motors, resistance-type spot welding, and so on) the cumulative thermal effects of the overcurrents must be taken into account. Both cables and thermal type relays are affected.
• At all upstream circuit levels this current corresponds to the kVA to be supplied, which takes into account the diversity and utilization factors, ks and ku respectively, as shown in Figure G2.

Fig. G2 – Calculation of maximum load current I_B

Maximum permissible current: I_z

Current carrying capacity I_z is the maximum permissible that the cabling for the circuit can carry indefinitely, without reducing its normal life expectancy.

The current depends, for a given cross sectional area of conductors, on several parameters:

• Constitution of the cable and cable-way (Cu or Alu conductors; PVC or EPR etc. insulation; number of active conductors)
• Ambient temperature
• Method of installation
• Influence of neighbouring circuits

Overcurrents

An overcurrent occurs each time the value of current exceeds the maximum load current IB for the load concerned.

This current must be cut off with a rapidity that depends upon its magnitude, if permanent damage to the cabling (and appliance if the overcurrent is due to a defective load component) is to be avoided.

Overcurrents of relatively short duration can however, occur in normal operation; two types of overcurrent are distinguished:

• Overloads

These overcurrents can occur in healthy electric circuits, for example, due to a number of small short-duration loads which occasionally occur co-incidentally: motor starting loads, and so on. If either of these conditions persists however beyond a given period (depending on protective-relay settings or fuse ratings) the circuit will be automatically cut off.

• Short-circuit currents

These currents result from the failure of insulation between live conductors or/and between live conductors and earth (on systems having low-impedance-earthed neutrals) in any combination, viz:

• 3 phases short-circuited (and to neutral and/or earth, or not)
• 2 phases short-circuited (and to neutral and/or earth, or not)
• 1 phase short-circuited to neutral (and/or to earth)