Wire Size Calculator

Selects the smallest KS-standard cross-section satisfying voltage drop, ampacity, ambient temperature derating (Ct), grouping factor (Cg), and insulation type (PVC/XLPE).

Result

Enter the values and click Calculate.

Ampacity by Wire Size (Korean KS Standard)

The most-searched values in one table. Based on installation method B1, 30°C ambient, PVC insulation — real sites derate this for temperature and grouping.

Copper

Cross-section	Single-phase	Three-phase	XLPE single
1.5 mm²	17.5 A	15.5 A	22 A
2.5 mm²	24 A	21 A	30 A
4 mm²	32 A	28 A	40 A
6 mm²	41 A	36 A	51 A
10 mm²	57 A	50 A	71 A
16 mm²	76 A	68 A	95 A
25 mm²	101 A	89 A	126 A
35 mm²	125 A	110 A	156 A
50 mm²	151 A	134 A	189 A
70 mm²	192 A	171 A	240 A
95 mm²	232 A	207 A	290 A
120 mm²	269 A	239 A	336 A
150 mm²	300 A	262 A	375 A
185 mm²	341 A	296 A	426 A
240 mm²	400 A	346 A	500 A
300 mm²	458 A	394 A	573 A

Aluminum

Cross-section	Single-phase	Three-phase
16 mm²	60 A	53 A
25 mm²	79 A	70 A
35 mm²	97 A	86 A
50 mm²	118 A	104 A
70 mm²	150 A	133 A
95 mm²	181 A	161 A
120 mm²	210 A	186 A
150 mm²	234 A	204 A
185 mm²	266 A	230 A
240 mm²	312 A	269 A
300 mm²	358 A	306 A

These are baseline values. Higher ambient temperature or grouped circuits reduce ampacity, so use the calculator above to apply correction factors for actual design. Aluminum is not used below 16mm². The XLPE column is an estimate (PVC single-phase value × 1.25), not a value published in the standard's tables.

Source: KEC 232.5 / KS C IEC 60364-5-52 Annex B (Tables B.52.2 and B.52.4, column B1)

What this tool does

The wire size calculator determines the proper cross-section (sq) from load current, run length, and allowable voltage drop. Based on KEC 232.5 (wire sizing) and KS C IEC 60364 ampacity tables, it presents the minimum size satisfying both overheating (ampacity) and voltage drop. Use it for electrical design, quotes, and KEC certification exam prep.

Who uses this

Branch circuit design: proper wire size per load
Long runs: upsizing for voltage drop
Motor circuits: sizing on rated current (check starting current separately)
Solar / EV charger: high-current circuit sizing plus voltage drop
KEC certification: practice wire sizing rules

How to use (4 steps)

1Enter load current (A). If you only know the load power (W or kW), convert it to current via voltage and power factor first (single-phase I = P/(V×pf); 3-phase I = P/(√3×V×pf), with P in watts).
2Enter one-way run length (m). Voltage drop is computed for the round trip, so enter one-way only.
3Choose allowable voltage drop (%). The limit depends on run length, so check the distance-based limits in Naesun Code 1410-1 (Korea's interior wiring code) before choosing.
4Click Calculate. You'll see ① ampacity-based minimum, ② voltage-drop-based minimum, ③ the larger (final selection).

Wire sizing formula (KEC 232.5)

Select the minimum cross-section satisfying both: ① Ampacity: wire ampacity × temperature factor (Ct) × grouping factor (Cg) ≥ load current Match from KS C IEC 60364 table (e.g., 2.5sq PVC ≈ 24A) ② Voltage drop: Single-phase: e = 35.6 × L × I / (1000 × A) 3-phase: e = 30.8 × L × I / (1000 × A) (e=drop V, L=one-way m, I=current A, A=area sq) → A ≥ 35.6 × L × I / (1000 × allowable drop V) Final = max(① result, ② result) Standard sizes (sq): 1.5, 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95, 120, 150, 185, 240, 300

Real examples

Example 1: Single-phase 220V, 16A, 20m, 3% drop

① Ampacity: 16A → 2.5sq (24A) ample. ② Drop: allowable 6.6V, A ≥ 35.6×20×16/(1000×6.6) = 1.73sq → 2.5sq ample. Final 2.5sq.

Example 2: Same load, 80m (long run)

① Ampacity: 2.5sq still ample. ② Drop: A ≥ 35.6×80×16/(1000×6.6) = 6.9sq → need 10sq. Distance makes voltage drop govern. These examples use 3% (80m falls in the 5% band of the Naesun Code distance table — at 5% one size smaller would do). Final 10sq.

Example 3: 3-phase 380V 30A, 50m, 3% drop

① Ampacity: 30A → 4sq (28A) falls short → 6sq (36A). ② Drop: allowable 11.4V, A ≥ 30.8×50×30/(1000×11.4) = 4.05sq → 6sq. Final 6sq.

Frequently asked questions

Why check both ampacity and voltage drop?

Ampacity alone prevents overheating (fire), but long runs drop voltage so equipment underperforms. Short runs are governed by ampacity, long runs by voltage drop. Always select the larger for safety.

What allowable voltage drop should I use?

Naesun Code 1410-1 sets the allowable drop by run length. Check the distance-based limits in the text of Naesun Code 1410-1 itself (or the distance table in this site's wire-size guide) before entering a value. Excessive drop dims lighting, reduces motor torque, and increases heating.

Do PVC and XLPE cables have different ampacity?

Yes — the maximum conductor temperature differs: PVC (70°C) < XLPE (90°C). (XLPE = cross-linked polyethylene; in Korea the common XLPE power cable is called 'CV' cable: XLPE-insulated, PVC-sheathed.) XLPE carries more current at the same size. This calculator is based on the PVC table; if you choose XLPE insulation, it approximates ampacity as the PVC value × 1.25. For detailed design, check the dedicated XLPE ampacity table.

How do I match wire size with breaker rating?

Under KEC 212.4.1 the breaker rating must not exceed the cable ampacity (I_n ≤ I_Z). E.g., 20A breaker → a cable whose corrected ampacity is at least 20A. Ampacity for the same size changes with installation method, ambient temperature and grouping. This calculator assumes one standard installation method and corrects only for ambient temperature and grouping, so if your installation method differs, check that method's ampacity table directly. The breaker must trip before the wire overheats. Use this site's breaker calculator together.

Do I need correction for multiple wires in conduit?

Yes. Several circuits in one conduit shed heat poorly, so ampacity drops. Multiply the cable's ampacity by a reduction factor — 0.70 for 3 circuits, 0.65 for 4, 0.57 for 6, 0.50 for 9 (KS C IEC 60364-5-52 Table B.52.17). Enter the number of circuits in this calculator and it applies the factor automatically.

Copper vs aluminum?

Aluminum's conductivity is ~62% of copper, so under the same conditions its voltage drop is about 1.6× copper's, and it needs a larger cross-section for the same current. If you choose aluminum in this calculator, it multiplies the voltage drop by 1.6, checks ampacity against the aluminum table, and only recommends sizes from 16mm² up, where that table starts.

Cautions

•The calculator assumes one standard installation method. It accounts for conductor (copper/aluminum), insulation (PVC/XLPE), ambient temperature and number of circuits when you enter them; if your installation method differs, check that method's ampacity table directly.
•If several circuits share one conduit, enter the number of circuits — ampacity drops.
•If the ambient temperature is above 30°C, enter the actual temperature — boiler rooms and rooftops cut ampacity sharply.
•The cable's corrected ampacity must be at least the breaker rating (I_Z ≥ I_n).
•The final design must comply with KEC and be reviewed by an electrical safety manager.

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Last reviewed: 2026-10-03

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