Harmonic Distortion limite and Neutral cable sizing

Question

Please I would like to know what percentage of cross section area should my neutral cable be with respect to the live conductor for a total harmonic distortion limit of less then 5% at nominal frequency of 50Hz

AJJewsbury · Accepted Answer

Sounds like something for section 5.5 of appendix 4 of BS 7671. You probably need a few more details (like how well loads are balanced across phases) to get a complete answer, but as I read it an ordinary full size N (i.e. 100% of L) will cope with up to 15% distortion. 
 - Andy.

gkenyon · Answer

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.

gkenyon · Answer

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)

gkenyon · Answer

AJJewsbury said: Think again - there can't be current in the N if there's no N conductor. 
 That's an interesting perspective, but I'm not sure that's always the case either .... if there are single-phase 400 V loads (or broken three-phase 400 V delta loads) connected alongside fully balanced single-phase 230 V loads, and 400 V TP&N loads, in a three-phase 4-wire system, harmonics can still travel in the 4-wire neutral upstream of a 3-wire circuit, and unbalance even in only delta-connected or line-line connected loads could cause neutral current based on volt-drop causing unbalance in the star-connected and single-phase loads.

Harmonic Distortion limite and Neutral cable sizing

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