Cable Sizing Calculator — Medium Voltage

For 34.5 kV, the wire shields and tape armor produce circulating current losses. In a manual calculation, this is a messy formula involving shield resistance and spacing. Elena’s mental shortcut: “Assume 5% loss.” Factor: 0.95.

The Last Manual Calculation

Elena: “That’s why a real MV calculator checks thermal & mechanical stress, not just ampacity.”

Three conductors touching (trefoil) traps heat. Correction: 0.90.

Mark asks, “Why not just use an online calculator?”

( 0.92 \times 0.88 \times 0.96 \times 0.90 \times 0.95 = 0.664 )

She emails the results to procurement. The software license renews at midnight. But Elena doesn’t need it. She built her own. Six months later, a competitor’s site uses a generic calculator, undersizes a 34.5 kV cable, and suffers a thermal runway failure. Elena’s client sends her a bottle of bourbon and a note: “Your ‘manual’ calculator paid for itself 50x over.”

Mark: “So we need a cable with base rating of 201A / 0.664 = 303A minimum.”