Chapter G

Sizing and protection of conductors

Overcurrent protection principles

From Electrical Installation Guide

A protective device is provided at the origin of the circuit concerned (see Fig. G3 and Fig. G4 ).

  • Acting to cut-off the current in a time shorter than that given by the I2t characteristic of the circuit cabling
  • But allowing the maximum load current IB to flow indefinitely

The characteristics of insulated conductors when carrying short-circuit currents can, for periods up to 5 seconds following short-circuit initiation, be determined approximately by the formula:

I2t = k2 S2

which shows that the allowable heat generated is proportional to the squared cross-sectional-area of the condutor.


t = Duration of short-circuit current (seconds)
S = Cross sectional area of insulated conductor (mm2)
I = Short-circuit current (A r.m.s.)
k = Insulated conductor constant (values of k are given in Figure G52)

For a given insulated conductor, the maximum permissible current varies according to the environment. For instance, for a high ambient temperature (θa1 > θa2), Iz1 is less than Iz2 (see Fig. G5). θ means “temperature”.


ISC = 3-phase short-circuit current
ISCB = rated 3-ph. short-circuit breaking current of the circuit-breaker
Ir (or Irth)[1] = regulated “nominal” current level; e.g. a 50 A nominal circuit-breaker can be regulated to have a protective range, i.e. a conventional overcurrent tripping level (see Fig. G6) similar to that of a 30 A circuit-breaker.

Fig. G3 – Circuit protection by circuit breaker
Fig. G4 – Circuit protection by fuses
Fig. G5 – I2t characteristic of an insulated conductor at two different ambient temperatures


  1. ^ Both designations are commonly used in different standards.