Cable Size Calculation: Single-Phase and Three-Phase (Calculator)

Cable cross-section is not chosen on a single criterion. The correct approach is to apply two criteria separately and take the larger result.

1. Ampacity

The current the cable can carry without overheating. The design current is calculated first:

• Single-phase: I = P / (U · cos φ)
• Three-phase: I = P / (√3 · U · cos φ)

Ambient temperature and grouping corrections are then applied. At 40 °C a PVC cable loses 13% of its capacity; with 4 circuits grouped together it loses 35%. These two factors multiply — ignoring them leaves the cable overheating in service.

2. Voltage drop

ΔU = b · [ρ·(L/S)·cos φ + λ·L·sin φ] · I   (b = 2 single-phase, √3 three-phase)

This criterion becomes decisive as the run lengthens. On long runs a cross-section that is adequate for ampacity must often be increased by one or two steps because of voltage drop.

The effect of cos φ

The lower the power factor, the higher the current for the same active power. A load at cos φ = 0.7 draws roughly 36% more current than one at cos φ = 0.95 — cable, breaker and losses all grow accordingly. Power factor correction feeds straight into cable cost here.

Single-phase or three-phase?

For the same power, three-phase current is √3 times lower. At higher powers a three-phase supply means both a thinner cable and less voltage drop.

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