Size the feeder for a 3-phase motor: full-load current, the 125% continuous rating, the derated ampacity in a hot cabinet, then pick a cable that still carries it.
📋 Scenario
A 7.5 kW pump motor runs continuously on 400 V 3-phase. The cable runs inside a control cabinet at 40 °C, so its rating must be derated by a factor of 0.87. Work from the motor's full-load current up to the real installed cable size.
📋 Given
7.5 kW
400 V, 3-phase
0.85
0.88
40 °C → derate ×0.87
🧮 Useful relations
FLC = P / (√3 · V · pf · η)
Continuous rating = 1.25 · FLC
Required 30 °C ampacity = continuous rating / derating factor
ℹ️ NoteCable ampacities (PVC, 30 °C, illustrative): 1.5 mm² ≈ 17.5 A · 2.5 mm² ≈ 24 A · 4 mm² ≈ 32 A · 6 mm² ≈ 41 A.