Sizing wires under KEC 232.5 — a field guide

A 4mm² cable in a Korean factory boiler room turned brown in under a month. The design said it was fine. The mistake? Korean KEC tables assume 30°C ambient, and the boiler room ran near 50°C. That single missing factor was the difference.

Here are the traps to avoid when sizing wires under Korean KEC 232.5 and Naesun Code 1410-1.

Two conditions must be satisfied at the same time

Wire sizing has two governing conditions: ampacity (KEC tables) and voltage drop (Naesun 1410-1). Pick the larger cross-section. The common mistake is checking only one.

Short distance with high current → ampacity governs. Long distance → voltage drop governs. 30 meters is the rough tipping point.

Don't forget the two derating factors

KEC tables assume 30°C and a single circuit. Real sites rarely match. Two derating factors apply.

Ct — ambient temperature

PVC at 40°C: ×0.87. At 50°C: ×0.71. Boiler rooms and rooftop trays — this is decisive. Reference: KS Table B.52.14.

Cg — circuit grouping

Three circuits in one tray: ×0.70. Four: ×0.65. Nine: ×0.50. Multiply Ct × Cg. At 40°C with 4 circuits, ampacity drops to 0.87 × 0.65 = 0.566 — about half. Ignoring this is dangerous.

Voltage drop limits by distance

• ≤ 60m: 3%
• ≤ 120m: 5%
• ≤ 200m: 6%
• > 200m: 7%

These are for the trunk after service entry. Inside dwellings, design for 2% to be safe. Sensitive loads (servers, medical equipment): tighten to 1%.

Bottom line

Three things. Check both ampacity and voltage drop. Apply Ct·Cg. Treat KEC as a floor, not a ceiling — go one size up when in doubt. The wire size calculator handles all three automatically. Still, an electrical safety manager must review before construction.