Cable Rating Reduction Factor due to Harmonic Currents

The presence of harmonic current in a power system increases the current in the neutral conductor due to phase current imbalance (refer to Fig. 1b).
Current Waveforms

To ascertain that the current-carrying capacity of the circuit is sufficient factoring the presence of harmonics in the system, Table 4Aa (BS7671 - 2011) provides some factors in determining the cable sizes

Table 4Aa - Rating factors for triple harmonics in four-core and five-coure cablesa
Third harmonic content of line current*
%
Rating factor
Size selection is based on
line current
Size selection is based on
neutral current
0 - 15 1.0 -
>15 - 33 0.86 -
>33 - 45 - 0.86
>45 - 1
*NOTE: The third harmonic content expressed as total harmonic distortion.
aReference: BS 7671 - 2011

How to use the Table 4Aa.

Case 1
Assuming a design current carrying capacity of circuit to 25A with a third harmonic of 32%.
As the third harmonic current is less than 33%, the governing current for cable selection should be the line current.
25/0.86 = 29A (This is the minimum current-carrying capacity of the cable phase conductors to satisfy Table 4Aa.
Case 2
Assuming a design current carrying capacity of circuit to 25A with a third harmonic of 44%.
As the third harmonic current is greater than 33%, the governing current for cable selection should be the neutral current.
25/0.86 = 29A (This is the minimum current-carrying capacity of the of cable neutral conductor to satisfy Table 4Aa.

Note: The minimum current carrying capacity stated herein does not include other derating factors such as grouping, temperature, etc.

References:

  1. Cable Heating Effects due to Harmonic Distortion in Electrical Installations
  2. Jonathan Blackledge
    Dublin Institute of Technology, [email protected]
    et al.
6975 = 6969 = 6983

About the Author

Ver Pangonilo

A Filipino Engineer, Registered Professional Engineer of Queensland (RPEQ) – Australia and Professional Electrical Engineer (PEE) – Philippines with extensive experience in concept select, front-end engineering, HV & LV detail design, construction and commissioning of Hazardous and Non-Hazardous Area electrical installations in water and waste water pipeline and pumping facilities, offshore platforms, hydrocarbon process plants and pipelines including related facilities. Hazardous area classification and design certification (UEENEEM015B, UEENEEM016B, UEENEEM017B).

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