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).

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.