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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).

Load Conditions

Cable Specification

Installation Environment

Result

Enter the values and click Calculate.

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: cable selection considering starting current
  • Solar / EV charger: high-current circuit sizing plus voltage drop
  • KEC certification: practice wire sizing rules

How to use (4 steps)

  1. 1Enter load current (A). If you only know capacity (kW), convert via voltage and power factor first (I = P/(V×pf), √3 for 3-phase).
  2. 2Enter one-way run length (m). Voltage drop is computed for the round trip, so enter one-way only.
  3. 3Choose allowable voltage drop (%). KEC interior code: 3% for feeders, total 5% including branch circuits.
  4. 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 ≥ load current × safety factor Match from KS C IEC 60364 table (e.g., 2.5sq IV ≈ 27A) 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

Real examples

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

① Ampacity: 16A → 2.5sq (27A) 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. Final 10sq.

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

① Ampacity: 30A → 4sq (36A) or 6sq. 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.

Where do the 3% / 5% drop limits come from?+

Korean interior code: feeder ≤3%, feeder+branch ≤5% (runs ≤60m). Beyond 60m, relaxed rules apply. Excessive drop dims lighting, reduces motor torque, and increases heating.

Do IV, HIV, CV wires have different ampacity?+

Yes — insulation temperature differs. IV (60°C) < HIV (75°C) < CV/XLPE (90°C). CV carries more current at the same size. This calculator uses standard IV/HIV; CV needs a separate table.

How do I match wire size with breaker rating?+

Wire ampacity ≥ breaker rating. E.g., 20A breaker → ampacity ≥ 25A (2.5-4sq). 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. Multiple circuits in one conduit reduce heat dissipation, lowering ampacity. Apply factors like 0.7 for ≤3 circuits, 0.6 for 4-6. This calculator assumes a single circuit, so upsize for bundled runs.

Copper vs aluminum?+

Aluminum's conductivity is ~61% of copper, needing ~1.6× the cross-section for the same current. This calculator uses copper (Cu). For aluminum, size up at least one step.

Cautions

  • •Uses copper, single circuit, IV/HIV insulation. Aluminum, bundled runs, CV need separate correction.
  • •Multiple circuits in conduit require ampacity derating factors.
  • •Ambient over 30°C requires additional derating.
  • •Must match breaker rating (ampacity ≥ breaker rating).
  • •Final design requires KEC review by a certified electrical engineer.

Last reviewed: 2026-05-30

Wire Size Calculator | Workmate