Electrical · Field guide
Sizing wires under KEC 232.5 — a field guide
A practical guide to KEC 232.5 (KEC = Korea Electro-technical Code) and Naesun Code 1410-1 (Naesun = Korea's interior wiring code, 내선규정), told through real site cases. Last updated 2026-10-03.
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 one missing correction factor was the whole problem.
Here are the traps to avoid when sizing wires under Korean KEC 232.5 and Naesun Code 1410-1. Follow them in order and it becomes clear why one line of a table is never the whole answer.
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. Where one hands over to the other depends on the current and voltage. Running the calculator at a 3% allowable drop, voltage drop takes over from about 25m for 220V single-phase 2-wire at 30A, about 47m for 220V single-phase 2-wire at 100A, and about 75m for 380V three-phase 3-wire at 30A.
Examples (copper, PVC, 30°C, single circuit, 3% allowable voltage drop)
220V single-phase 2-wire, 100A, 5m → 25mm² (ampacity governs — 25mm² is rated only 101A in the table, barely above the 100A load, so the calculator suggests checking one size up)
220V single-phase 2-wire, 30A, 80m → 16mm² (voltage drop governs — 80m falls in the 5% band of the table below, but the examples here use 3% for simplicity; at 5% it would be 10mm²)
Don't forget the two derating factors (Ct·Cg)
KEC tables assume 30°C and a single circuit. Real sites rarely match. Two derating factors apply.
Ct — ambient temperature
PVC at 40°C: multiply the cable's ampacity by 0.87. At 50°C: ×0.71. In boiler rooms and rooftop trays this factor can decide the result. Reference: Table B.52.14 of KS C IEC 60364-5-52. The site's wire size calculator applies it automatically once you enter the insulation type and ambient temperature.
Cg — circuit grouping
Three circuits in one tray: multiply ampacity by 0.70. Four: ×0.65. Nine: ×0.50. When circuits pile into one tray at a power panel, ampacity falls toward half quickly.
Apply both factors to the table ampacity together. At 40°C with 4 circuits that is 0.87 × 0.65 ≈ 0.57 — just over half the table value. Ignoring this is dangerous.
What changes with XLPE?
XLPE (cross-linked polyethylene) insulation allows a maximum conductor temperature of 90°C versus 70°C for PVC, so it carries more current than PVC. Where ampacity sets the size, the same current can therefore come out one cross-section smaller.
One caveat. The site's calculator approximates XLPE as the PVC table × 1.25. For detailed design, read the XLPE values directly from the ampacity tables in Annex B of KS C IEC 60364-5-52.
Aluminum conductors in the Korean market
Aluminum's share is rising across the industry, but Korean residential work is still mostly copper. On feeders and service entrances of 100A and above, though, aluminum shows up often.
At the same cross-section, aluminum conducts only about 62% as well as copper, so under the same conditions its voltage drop is about 1.6 times copper's, and its ampacity comes from a separate aluminum table, not the copper one. If you choose aluminum, the site's calculator only recommends sizes from 16mm² up — the range its aluminum ampacity table covers.
Allowable voltage drop by distance (Naesun Code 1410-1)
| Distance | Allowable drop |
|---|---|
| ≤ 60m | 3% |
| > 60m – 120m | 5% |
| > 120m – 200m | 6% |
| > 200m | 7% |
These apply from the service entrance through the feeder. Inside the customer's premises, branch circuits are usually designed to stay within 2%, and circuits feeding sensitive equipment (server rooms, medical devices) are tightened to 1% — both are design practice, not code values. The examples on this page all use 3% for simplicity.
Frequently asked questions
Q. 220V single-phase 2-wire, 30A, 50m. What size do I need?
Allowable drop 3% = 6.6V. A_min = (35.6 × 50 × 30) / (1000 × 6.6) = 8.09mm² → next KS standard size 10mm². On ampacity, 10mm² carries 57A single-phase, so 30A is no strain. Result: 10mm². Even at 50°C, as in a boiler room, the calculated result stays 10mm² (57A × 0.71 ≈ 40A ≥ 30A). Whether to go one size up from there is a design-margin call based on future load and installation conditions, not something the calculation demands.
Q. How do I size the cable for an inverter (VFD) load?
Size it on the inverter's rated current, the same as any other load. The catch is the ELB (earth-leakage breaker, i.e. an RCD): the inverter's high-frequency leakage current can make an ordinary ELB trip with no real fault. The fix is the right ELB type, not removing the ELB or raising its trip threshold on your own — use a type made for inverter circuits (Type F or Type B, IEC 62423), and take both the type and the rated residual current from the inverter maker's recommendation.
Q. Isn't following the KEC table enough?
KEC is the minimum safety standard — a pass, not an optimum. Even after applying the temperature and grouping corrections, installers often go one or two sizes above what the table gives, to leave room for future load. Cable is hard to replace once it is pulled, so building in headroom up front usually beats re-pulling it later.
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 works out the first two automatically and warns you when the margin is thin, so you can consider the next size up. Results are for reference only — an electrical safety manager must review the design before construction.