Without further information on the % of each harmonic, you could only assume that most of the distortion will be triple-n harmonics.

Table 4Aa of BS 7671:2018+A2:2022 indicates that the neutral can be sized simply for (line current x 1.0) with 3rd harmonic content of 5 %. 

The above advise is current guidance for UK installations.

For triple-n harmonics exceeding 33 %, the factor is applied to a calculated neutral current, not the line current.

Example calculations are given in 5.5.2 in Appendix 4 to BS 7671:2018+A2:2022.

thank you for your response Sir, so from BS 7671 I can see it uses "assumptions" of wether the load MAY or MAY NOT produce harmonics. Does this mean I just have to "predict" (for lack of a better word) if a load would produce harmonic distortions?

thank you for your response Sir, so from BS 7671 I can see it uses "assumptions" of wether the load MAY or MAY NOT produce harmonics. Does this mean I just have to "predict" (for lack of a better word) if a load would produce harmonic distortions?

The manufacturer should provide information on harmonic distortion of equipment one way or another, especially in the UK or the EU. This might be:

• For relevant equipment, conform to the relevant conducted harmonic emission standard (you will then be able to assume the harmonic distortion does not exceed the limits of the standard); or
• For equipment where there is no standard, or the use of a standard is optional, the actual harmonic emissions from type tests.

In the UK, and in a number of EU countries, there are limits for total combined harmonic distortion of installations - in the UK the installation designer must demonstrate conformity to ENA Engineering Recommendation G5 (current version I believe is G5/5)