Balanced three-phase systems use a different voltage relationship from DC and single-phase circuits. The standard resistive estimate uses √3 × current × one-way length × conductor resistance.
Use line voltage and line current
For a typical UK low-voltage three-phase system, use the line-to-line supply voltage and the line current supplying the balanced load.
Real circuits include reactance
The simplified resistive method is useful for an initial estimate. For larger conductors, long runs or loads with a significant phase angle, use impedance data and the full design method required for the installation.
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Professional guidance
This article provides general information only. Electrical installation design and verification should be completed by a competent person using the current regulations and manufacturer data.