What Is Continuous Current-Carrying?
Continuous current-carrying capacity (Iz) (or current-carrying ampacity in USA) 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.
Current-Carrying Capacities
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
| Type of insulation | Temperature limit a, d °C |
| Thermoplastic (PVC) | 70 at the conductor |
| Thermosetting (XLPE or EPR rubber) | 90 at the conductor b |
| 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 sheath b, c |
| 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. |
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| 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. |
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Note. In the USA, determination of current-carrying capacity for conductors is made in accordance with NFPA 70 – National Electrical Code.
How Is the Continuous Current-Carrying Capacity of a Conductor Selected?
Clause 523.2 of IEC 60364-5-52 states that:
The requirement of 523.1 [↑] 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.
IEC 60364-5-52-2009
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 [3] to a simplified form for their national rules. An example of one acceptable method of simplification is given in Annex C [3].
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.
Clause 523.3 of IEC 60364-5-52 states that:
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.
IEC 60364-5-52-2009
NOTE 3. In Germany, in addition the 24 h load diagram has to be taken into consideration.
Groups Containing More than One Circuit
The group reduction factors are applicable to groups of insulated conductors or cables having the same maximum operating temperature.
For groups containing cables or insulated conductors having different maximum operating temperatures, the current-carrying capacity of all the cables or insulated conductors in the group shall be based on the lowest maximum operating temperature of any cable in the group, together with the appropriate group reduction factor.
If, due to known operating conditions, a cable or insulated conductor is expected to carry a current not greater than 30 % of its grouped current-carrying capacity, it may be ignored for the purpose of obtaining the reduction factor for the rest of the group.
Number of Loaded Conductors
The number of conductors to be considered in a circuit are those carrying load current. Where it can be assumed that conductors in polyphase circuits carry balanced currents, the associated neutral conductor need not be taken into consideration. Under these conditions, a four-core cable is given the same current-carrying capacity as a three-core cable having the same conductor cross-sectional area for each line conductor. Four- and five-core cables may have higher current-carrying capacities when only three conductors are loaded.
This assumption is not valid in the case of the presence of third harmonic or multiples of 3 presenting a THDi (total harmonic distortion) greater than 15 %.
Where the neutral conductor in a multicore cable carries current as a result of an imbalance in the line currents, the temperature rise due to the neutral current is offset by the reduction in the heat generated by one or more of the line conductors. In this case, the neutral conductor size shall be chosen on the basis of the highest line current.
In all cases, the neutral conductor shall have a cross-sectional area adequate to afford compliance with 523.1 [↑].
Where the neutral conductor carries current without a corresponding reduction in load of the line conductors, the neutral conductor shall be taken into account in ascertaining the current-carrying capacity of the circuit. Such currents may be caused by a significant triple harmonic current in three-phase circuits. If the harmonic content is greater than 15 % of the fundamental line current, the neutral conductor size shall not be smaller than that of the line conductors. Thermal effects due to the presence of third harmonic or multiples of 3 and the corresponding reduction factors for higher harmonic currents are given in Annex E [3].
Conductors which serve the purpose of protective conductors only (PE conductors) shall not be taken into consideration. PEN conductors shall be taken into consideration in the same way as neutral conductors.
Conductors in Parallel
Where two or more live conductors or PEN conductors are connected in parallel in a system, either:
a) measures shall be taken to achieve equal load current sharing between them;
This requirement is considered to be fulfilled if the conductors are of the same material, have the same cross-sectional area, are approximately the same length and have no branch circuits along the length, and either
- the conductors in parallel are multi-core cables or twisted single-core cables or insulated conductors, or
- the conductors in parallel are non-twisted single-core cables or insulated conductors in trefoil or flat formation and have a cross-sectional area less than or equal to 50 mm2 in copper or 70 mm2 in aluminium, or
- if the conductors in parallel are non-twisted single-core cables or insulated conductors in trefoil or in flat formation and have a cross-sectional area greater than 50 mm2 in copper or 70 mm2 in aluminium, the special configuration necessary for such formations is adopted. These configurations consist of suitable groupings and spacings of the different phases or poles (see Annex H [3]) or
b) special consideration shall be given to the load current sharing to meet the requirements of 523.1 [↑].
This subclause does not preclude the use of ring final circuits either with or without spur connections.
Where adequate current sharing cannot be achieved or where four or more conductors have to be connected in parallel, consideration shall be given to the use of busbar trunking.
Variation of Installation Conditions Along a Route
Where the heat dissipation differs in one part of a route to another, the current-carrying capacity shall be determined so as to be appropriate for the part of the route having the most adverse conditions.
NOTE. This requirement can normally be neglected if heat dissipation only differs where the wiring is going through a wall of less than 0,35 m.
Single-Core Cables with a Metallic Covering
The metallic sheaths and/or non-magnetic armour of single-core cables in the same circuit shall be connected together at both ends of their run. Alternatively, to improve current-carrying capacity, the sheaths or armour of such cables having conductors of cross-sectional area exceeding 50 mm2 and a non-conducting outer sheath may be connected together at one point in their run with suitable insulation at the unconnected ends, in which case the length of the cables from the connection point shall be limited so that voltages from sheaths and/or armour to earth:
- a) do not cause corrosion when the cables are carrying their full load current, for example by limiting the voltage to 25 V, and
- b) do not cause danger or damage to property when the cables are carrying short-circuit current.
Current-Carrying Capacities Tables
The values in Tables B.52.2 to B.52.8 apply to cables without armour and have been derived in accordance with the methods given in the IEC 60287 series using such dimensions as specified in IEC 60502 and conductor resistances given in IEC 60228. Known practical variations in cable construction (e.g. form of conductor) and manufacturing tolerances result in a spread of possible dimensions and hence current-carrying capacities for each conductor size. Tabulated current-carrying capacities have been selected so as to take account of this spread of values with safety and to lie on a smooth curve when plotted against conductor cross-sectional area.
Table B.52.2 – Current-carrying capacities in amperes for methods of installation in Table B.52.1 – PVC insulation/two loaded conductors, copper or aluminium – Conductor temperature: 70 °C, ambient temperature: 30 °C in air, 20 °C in ground [3]
NOTE. In columns 3, 5, 6, 7 and 8, circular conductors are assumed for sizes up to and including 16 mm2. Values for larger sizes relate to shaped conductors and may safely be applied to circular conductors.
Table B.52.3 – Current-carrying capacities in amperes for methods of installation in Table B.52.1 – XLPE or EPR insulation, two loaded conductors/copper or aluminium – Conductor temperature: 90 °C, ambient temperature: 30 °C in air, 20 °C in ground [3]
NOTE. In columns 3, 5, 6, 7 and 8, circular conductors are assumed for sizes up to and including 16 mm2. Values for larger sizes relate to shaped conductors and may safely be applied to circular conductors.
NOTE. In columns 3, 5, 6, 7 and 8, circular conductors are assumed for sizes up to and including 16 mm2. Values for larger sizes relate to shaped conductors and may safely be applied to circular conductors.
NOTE. In columns 3, 5, 6, 7 and 8, circular conductors are assumed for sizes up to and including 16 mm2. Values for larger sizes relate to shaped conductors and may safely be applied to circular conductors.
Table B.52.6 – Current-carrying capacities in amperes for installation method C of Table B.52.1 – Mineral insulation, copper conductors and sheath – PVC covered or bare exposed to touch (see note 2) – Metallic sheath temperature: 70 °C, reference ambient temperature: 30 °C [3]
NOTE 1. For single-core cables the sheaths of the cables of the circuit are connected together at both ends.
NOTE 2. For bare cables exposed to touch, values should be multiplied by 0,9.
NOTE 3. The values of 500 V and 750 V are the rated voltage of the cable.
Table B.52.7 – Current-carrying capacities in amperes for installation method C of Table B.52.1 – Mineral insulation, copper conductors and sheath – Bare cable not exposed to touch and not in contact with combustible material Metallic sheath temperature: 105 °C, reference ambient temperature: 30 °C [3]
NOTE 1 – For single-core cables, the sheaths of the cables of the circuit are connected together at both ends.
NOTE 2 – No correction for grouping need be applied.
NOTE 3 – For this table reference method C refers to a masonry wall because the high sheath temperature is not normally acceptable for a wooden wall.
NOTE 4 – The values of 500 V and 750 V are the rated voltage of the cable.
NOTE 1 – For single-core cables the sheaths of the cables of the circuit are connected together at both ends.
NOTE 2 – For bare cables exposed to touch, values should be multiplied by 0,9.
NOTE 3 – De is the external diameter of the cable.
NOTE 4 – The values of 500 V and 750 V are the rated voltage of the cable.
NOTE 1 For single-core cables the sheaths of the cables of the circuit are connected together at both ends.
NOTE 2 No correction for grouping need be applied.
NOTE 3 De is the external diameter of the cable.
NOTE 4 The values of 500 V and 750 V are the rated voltage of the cable.
References
- IEC 60050-826-2022
- IEC 60364-1
- IEC 60364-5-52-2009
