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 currentcarrying 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
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. ^{a}Reference: 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 currentcarrying 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 currentcarrying 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:
 Cable Heating Effects due to Harmonic Distortion in Electrical Installations
 Jonathan Blackledge
Dublin Institute of Technology, [email protected]
et al.