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BS 7671 Harmonic Derating & Neutral Sizing Calculator

Under non-linear loads (LED drivers, IT server racks, variable speed drives), triplen harmonic currents (3rd, 9th, 15th...) do not cancel out at the star point of a three-phase system. Instead, they sum additively in the neutral conductor. This tool calculates the required rating factor (Ch) and detects when cable sizing must shift from the phase to the neutral conductor in accordance with BS 7671:2018+A4:2026 Table 4A3.

Statutory Precision: When 3rd harmonic content exceeds 33%, BS 7671 requires the cable cross-sectional area to be selected based on the neutral current rather than the line current.

Calculated Design Values

Neutral Current (In)0.0 A

Table 4A3 Factor ()1.00

Governing Design SizingPhase Current

Effective Design Current (It)0.0 A

Recommended Minimum Conductor16 mm²Based on BS 7671 Appendix 4 tabulated current capacity.

BS 7671:2018+A4:2026 Table 4A3 Rule Framework

BS 7671:2018+A4:2026 Appendix 4 Section 5.5, Table 4A3

AM2 PORTFOLIO & DESIGN AUDIT FORMATTRANSPARENT FIRST-PRINCIPLES MATHS

Under triplen harmonic conditions, the star point does not cancel neutral currents. Table 4A3 governs cable sizing thresholds:

0% to 15% Harmonics: Ch = 1.0. Neutral conductor is not loaded beyond balance limits. Sized on phase current.
>15% to 33% Harmonics: Ch = 0.86. Thermal heating derating applied to 4-core cable. Sized on phase current (It = Ib / 0.86).
>33% to 45% Harmonics: Ch = 0.86. Neutral current exceeds line current (In = 3 × I3). Sized on neutral current derated by 0.86 (It = In / 0.86).
>45% Harmonics: Ch = 1.0. Cable is purely neutral-governed. Phase conductors carry significantly less heat than neutral. Sized on neutral current (It = In).