Continuous Current-Carrying Capacity

Continuous current-carrying capacity (Iz) is the maximum value of electric current which can be carried continuously by a conductor, a device or an apparatus, under specified conditions without its steady-state temperature exceeding a specified value [this term is defined in the IEC 60050-826-2022]. In the United States, the term “ampacity” is used instead of “continuous current-carrying capacity”.

Annex B of IEC 60364-1 explains the term “(continuous) current-carrying capacity” as follows: “This current is denoted Iz

The British Standard BS 7671 defined the term “current-carrying capacity of a conductor” in the same way as in IEC 60050-826:1982:

The maximum current which can be carried by a electrical conductor under specified conditions without its steady-state temperature exceeding a specified value.

BS 7671:2018+A2:2022

Note – For conductors, the rated current is considered as equal to the current-carrying capacity.

Thus, the term “rated current of a conductor” like the term “continuous current-carrying capacity” identifies the greatest electrical current a conductor is capable of carrying on a continuous basis without its steady-state temperature exceeding a certain value. The rated current of a conductor must be greater than or equal to the electric current flowing through it. Otherwise, the conductor circuit should be broken by a circuit breaker or fuse to protect it from overcurrent.

Features

In international regulations, the term “continuous current-carrying capacity” is generally used as a characteristic of conductors by which the maximum electrical current that a conductor can carry for a continuous period (weeks, months, years) without overheating is defined. The continuous current-carrying capacity of a conductor is actually its rated current.

The cross-section of conductors used in the electrical installations of buildings must always be chosen taking into account the electric currents that may flow through them under normal conditions. The electric current flowing through any conductor must not exceed its permissible continuous current. If this condition is met, the steady-state temperature of the conductor will not exceed the maximum allowable temperature given by the regulations.

Otherwise, if the electric current flowing in a conductor exceeds its allowable continuous current-carrying capacity, the conductor will overheat. Its insulation will be subject to accelerated aging. At very high electric currents, a conductor heated to several hundred degrees may cause a fire. To prevent overheating of conductors in the electrical installations of buildings, special protection, called overcurrent protection, is used to reduce to a safe value the duration of electrical currents flowing through conductors in excess of their allowable continuous current-carrying capacities.

Clause 523.1 “Current-carrying capacities” of IEC 60364-5-52, in particular, states that

The current to be carried by any conductor for sustained periods during normal operation shall be such that the temperature limit of the insulation is not exceeded. This requirement is fulfilled by application of Table 52.1, for the types of insulation given in this table. The value of current shall be selected in accordance with 523.2 or determined in accordance with 523.3.

IEC 60364-5-52-2009

Table 52.1 of IEC 60364-5-52 shows the maximum allowable temperatures that conductors with different insulation can have.

Type of insulationTemperature limita,d °C
Thermoplastic (PVC)70 at the conductor
Thermosetting (XLPE or EPR rubber)90 at the conductorb
Mineral (thermoplastic (PVC) covered or bare exposed to touch)70 at the sheath
Mineral (bare not exposed to touch and not in contact with combustible material)105 at the sheathb,c
Table 52.1 – Maximum operating temperatures for types of insulation

Notes to the table:

a) The maximum permissible conductor temperatures given in Table 52.1 on which the tabulated current-carrying capacities given in Annex A are based, have been taken from IEC 60502 and IEC 60702 and are shown on these tables.

b) Where a conductor operates at a temperature exceeding 70 °C, it shall be ascertained that the equipment connected to the conductor is suitable for the resulting temperature at the connection.

c) For mineral insulated cables, higher operating temperatures may be permissible dependent upon the temperature rating of the cable, its terminations, the environmental conditions and other external influences.

d) Where certified, conductors or cable may have maximum operating temperature limits in accordance with the manufacturer’s specification.

NOTE 1. The table does not include all types of cables.

NOTE 2. This does not apply to busbar trunking systems or powertrack systems or lighting track systems for which the current-carrying capacity should be provided by the manufacturer according to IEC 60439-2 and powertrack systems to IEC 61534-1.

NOTE 3. For the temperature limit for other types of insulation, please refer to cable specification or manufacturer.

How Is the Continuous Current-Carrying Capacity of a Conductor Selected?

The requirement of 523.1 [3] is considered to be satisfied if the current for insulated conductors and cables without armour does not exceed the appropriate values selected from the tables in Annex B [3] with reference to Table A.52.3 [3], subject to any necessary correction factors given in Annex B [3]. The current-carrying capacities given in Annex B [3] are provided for guidance.

The two loaded conductors may be in a 2-wire AC circuit made by a phase conductor and a neutral conductor or two phase conductors as well as in a 2-wire DC circuit made by a pole conductor and a midpoint conductor or two pole conductors. The three loaded conductors may be in a 3- or 4-wire AC electric circuit made by three phase conductors or three phase conductors and the neutral conductor, respectively. In the latter case, the current flowing through the neutral conductor is neglected.

NOTE 1. It is recognized that National Committees may wish to adapt the tables of Annex B to a simplified form for their national rules. An example of one acceptable method of simplification is given in Annex C.

NOTE 2. It is recognized that there will be some tolerance in the current-carrying capacities depending on the environmental conditions and the precise construction of the cables.

The appropriate values of current-carrying capacity may also be determined as described in the IEC 60287 series, or by test, or by calculation using a recognized method, provided that the method is stated. Where appropriate, account shall be taken of the characteristics of the load and, for buried cables, the effective thermal resistance of the soil.

References

  1. IEC 60050-826-2022
  2. IEC 60364-1
  3. IEC 60364-5-52-2009

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