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I = P / (√3 · U · cos φ · η), with P the rated output on the nameplate. Example: 90 W, 400 V, star (Y), cos φ 0.80, η 85 %: I = 90 / (1.732 · 400 · 0.80 · 0.85) ≈ 0.19 A. A 7.5 kW motor at 400 V with cos φ 0.85 and η 90 % draws about 14 A. The nameplate current is the best figure; the formula is for checking or when it is missing.
Calculate it in the tool above: Motor current from the nameplate
DC: I = P / U. Single-phase: I = P / (U · cos φ). Three-phase: I = P / (√3 · U · cos φ), with U the line voltage. 2 kW at 230 V with cos φ 1.0 is 8.7 A; 10 kW three-phase at 400 V with cos φ 0.85 is 17 A; 60 W at 12 V DC is 5 A.
Calculate it in the tool above: Current, power and impedance
The nameplate gives two voltages, e.g. 230/400 V Δ/Y. Connect in delta when the grid's line voltage equals the lower one and in star when it equals the higher one. A 230/400 V motor on a 400 V grid is connected in star; a 400/690 V motor on 400 V in delta – and only that motor can be started star-delta. The same load takes three times the power in delta as in star.
Calculate it in the tool above: Star/delta (Y/Δ) connection
IEC 60364-5-52 annex G recommends 3 % for lighting and 5 % for other loads from a public low-voltage supply; the US NEC recommends 3 % for a branch circuit and 5 % in total; Australia and New Zealand allow 5 % (AS/NZS 3000). On boats ISO 13297 and ABYC E-11 allow 3 % for critical DC loads and 10 % for others; on ships 6 % (IEC 60092-352). The national rules always apply.
Calculate it in the tool above: Voltage drop in a cable
Qc = P · (tan φ1 − tan φ2). Raising 50 kW from cos φ 0.75 to 0.95 needs 50 · (0.882 − 0.329) ≈ 27.7 kvar. At 400 V 50 Hz in delta, each of the three capacitors is Qc / (3 · 2π · 50 · 400²) ≈ 183 µF. Aim for 0.95–0.98, not higher, and use detuned banks where there are frequency converters.
Calculate it in the tool above: Power factor correction (capacitor)
Leisure boats: 12 V or 24 V DC from batteries, and 230 V 50 Hz (Europe) or 120/240 V 60 Hz (USA) from shore power or an inverter (ISO 13297, ABYC E-11). Ships: mostly 440 V 60 Hz three-phase, also 400 V 50 Hz and 690 V, as insulated (IT) systems without neutral and with earth-fault monitoring, lighting at 230 V or 115 V through transformers; large ships 6.6 kV or 11 kV (IEC 60092, SOLAS).
