The experts at NICEIC consider the effect of ambient conditions and installation methods when determining minimum live conductor sizes.
BS 7671 often specifies a minimum cross-sectional area for live conductors and for the purposes of resistance to mechanical damage likely in a particular application or environment.
In most cases, the limiting factor is current-carrying capacity. However, any quoted minimum size will not have taken into account any external factors which may reduce this.
Whilst the specified minimum size might be adequate in some cases, often it will be necessary to take into account a range of other influences that could affect current-carrying capacity such as:
● ambient conditions in relation to external influences as detailed in Section 522 of BS 7671
● installation methods, conductor arrangements, presence of insulation materials and the like covered in Section 523.
Voltage drop as described in Section 525 can also be the limiting factor when determining minimum conductor size, but is not considered in this article.
Correcting the current-carrying capacity of a cable for ambient conditions
The cross-sectional area (csa) of live conductors must not be less than the minimum sizes stated in the relevant part of BS 7671, such as Table 52.3 (general), Table 55.2 (auxiliary circuits) and, where applicable, in various sections in Part 7 covering special installations/locations (524.1).
The current to be carried by a conductor for sustained periods during normal operation shouldn’t result in the appropriate temperature limit specified in Table 52.1 of BS 7671 being exceeded (523.1).
Table 4A2 of Appendix 4 lists the different installation methods and their related reference method codes recognised by BS 7671.
The installation method affects the rate of heat dissipation and hence the current-carrying capacity of a cable. For example, the current-carrying capacity of a cable clipped direct to a surface is generally higher than that of the same cable enclosed in conduit or trunking.
The current-carrying capacity of a cable given in Appendix 4 normally relates to the maximum temperature at which the conductor can operate continuously without causing damage to the cable insulation or sheath, terminations, connected equipment and surroundings such as building materials.
For the purposes of BS 7671, ambient temperature is the temperature of the medium surrounding a non-sheathed or sheathed cable when it is not loaded (523.4). The current-carrying capacity of a cable reduces as the ambient temperature surrounding the cable increases.
The current-carrying capacities tabulated in Appendix 4 are based on the following ambient temperatures:
● 30 °C – Non-sheathed and sheathed cables in air, irrespective of the installation method.
● 20 °C – Buried cables, either directly in soil or in ducts in the ground.
Where ambient temperature differs from these values, the appropriate rating factor (Ca) given in Tables 4B1 (for cables in air) and 4B2 (for buried cables) should be applied to the tabulated current-carrying capacities set out in Tables 4D1A to 4J4A.
Groups of more than one circuit
Where cables are grouped in close proximity, their current-carrying capacity is reduced. The grouping factor (Cg) outlined in Tables 4C1 to 4C6 of Appendix 4 applicable to a range of cable types and installation methods should be applied.
Group reduction factors are applicable to groups of non-sheathed or sheathed cables having the same maximum operating temperature.
For groups containing non-sheathed or sheathed cables having different maximum operating temperatures, the current-carrying capacity of all the cables in the group should be based on the lowest maximum operating temperature of any cable in the group together with any appropriate group reduction factor.
It should be noted that where the external distance between adjacent cables exceeds twice their overall diameter, as illustrated in Fig 1, it is not necessary to apply a correction factor for grouping.
Number of loaded conductors
When calculating current-carrying capacity, only those conductors carrying load current should be considered.
Neutral conductors of balanced polyphase circuits do not need to be taken into consideration.
A neutral conductor should be considered as a loaded conductor where the third harmonic, or multiple of third harmonic, presents a total harmonic distortion greater than 15 % of the fundamental line current (523.6.1).
Where the neutral conductor in a multicore cable carries current, due to an imbalance in 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 conductor size should be chosen based of the highest line current. In all cases the neutral conductor should have an adequate cross-sectional area to ensure that the conductor temperature limit is not exceeded (523.6.2).
Cables in thermal insulation
Wherever possible, cables should be placed so that they are not covered by insulation. Where this is not possible, the cross-sectional area of the cable should be selected taking into account the tabulated current-carrying capacities given in Appendix 4, for cables installed in a wall or above a ceiling. It should be noted that where these tabulated values are used, it is not necessary to apply a rating factor (Ci) for thermal insulation.
Where a single cable is totally surrounded by thermal insulation over a length of 0.5 m or more, the current-carrying capacity must be taken as 0.5 times the current carrying capacity for that cable clipped direct to a surface and open (Ci = 0.5) (523.9).
Where a cable of up to 10 mm² is installed totally surrounded in thermal insulation for less than 0.5 m, the appropriate derating factor given in Section 2.6 of Appendix 4 should be applied to determine its current-carrying capacity.
Regulation 524.2.1 states that the neutral conductor of a circuit should have a cross-sectional area not less than that of the associated line conductor(s) in:
● Single-phase, 2-wire circuits in all cases, and
● Polyphase and single-phase three-wire circuits where the cross-sectional area of the associated line conductors is less than or equal to:
■ 16 mm² for copper conductors, or
■ 25 mm² for aluminium conductors
● Situations, such as where there is significant harmonic current in a three-phase circuit, where the neutral conductor carries current without a corresponding reduction in current in the associated live conductors (523.6.3).
The cross-sectional area of the neutral conductor may be smaller than the associated live conductors where all of the following criteria in Regulation 524.2.3 are met:
i.) the maximum current in the neutral conductor in normal service, including any harmonic current component, does not exceed the current-carrying capacity of the reduced cross-sectional area neutral conductor, and
ii.) the neutral conductor is protected against overcurrent in accordance with Regulation 431.2; that is, overcurrent detection appropriate to the neutral conductor’s cross-sectional area is provided, arranged to disconnect the live conductors, but not necessary the neutral conductor, and
iii.) in any case, the cross-sectional area of the neutral conductor should be at least:
● 16 mm² for copper conductors, or
● 25 mm² for aluminium conductors.
A number of factors must be taken into account when determining the minimum cross-sectional area of live conductors.
Although BS 7671 often states a minimum conductor size based on mechanical strength and durability, in many cases this minimum size will be insufficient when needing to consider the ambient conditions for the installed conductors which must be corrected in accordance with Section 2 of Appendix 4.
Additionally, when determining the corrected current-carrying capacity of a conductor, any and all relevant factors affecting this should be taken into consideration.
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