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Breaker Capacity Calculator

Load current × safety factor → smallest KS-standard rating that satisfies the requirement. Includes inverter+ELB nuisance-trip warning.

Load Information

Result

Enter load current and type, then click Calculate.

What this tool does

The Korean breaker sizing calculator computes MCB / MCCB rated current from load current, applying KEC 212.6 (Korean Electric Code breaker selection rules) and the standard 1.25 safety factor. It handles ordinary loads, motors, and inverter/VVVF loads with their respective adjustments. Use it for residential and commercial panel design, motor starter selection, and KEC certification exam preparation.

Who uses this

  • Residential / commercial panel design: select breaker for each branch circuit
  • Motor load: choose breaker rating that survives starting current 6-8× nominal
  • Inverter / VVVF: select B-type ELB to avoid nuisance tripping from HF leakage
  • Quote preparation: determine exact breaker rating before pricing
  • KEC certification exam: practice with standard breaker selection rules

How to use (4 steps)

  1. 1Choose load type: general (lighting/outlet), motor, or inverter/VVVF. Each applies different safety factors and additional considerations.
  2. 2Enter load current (A) or load capacity (kW/kVA). The tool auto-converts capacity to current using voltage, power factor, and phase count.
  3. 3Pick voltage (single-phase 220V / 3-phase 380V) and phase. Defaults are Korean standard LV.
  4. 4Click Calculate. You'll see the recommended breaker rating, type (MCB/MCCB/ELB), and the KEC citation.

Breaker rating formula (KEC 212.6)

General load: breaker rating ≥ load current × 1.25 (safety factor) Example: 12A load → 12 × 1.25 = 15A → select next standard 20A Motor load: consider starting current 6-8×; apply 1.25 to full-load current, then use ELCB (overcurrent-capable) instead of ELB 3-phase current conversion: I = P / (√3 × V × pf) Example: 7.5kW 380V pf 0.85 → 7,500 / (1.732 × 380 × 0.85) = 13.4A Standard ratings (KEC): MCB: 6, 10, 15, 16, 20, 25, 30, 32, 40, 50, 63A MCCB: 50, 60, 75, 100, 125, 150, 175, 200, 225, 250, 300, 400, 500, 600, 800A

Real examples

Example 1: Residential panel — living room lights + outlets (12A general)

12A × 1.25 = 15A. Next standard 20A MCB. KEC 212.6.2 recommends 16A or 20A for outlet circuits, so typically a 16A or 20A MCB.

Example 2: 3-phase 380V 7.5kW motor

Current = 7,500 / (√3 × 380 × 0.85) = 13.4A. 13.4 × 1.25 = 16.75A. Select 20A MCCB with thermal overload relay. Use D-curve to ride out the ~80A start.

Example 3: Inverter load — avoiding ELB nuisance trip

Inverters generate high-frequency leakage that trips standard ELBs. Use ELCB (overcurrent-rated) or inverter-compatible B-type ELB. Rating calculation is the same (current × 1.25).

Frequently asked questions

What's the difference between MCB, MCCB, and ELB?+

MCB (Miniature Circuit Breaker) = small breaker, residential 6-63A. MCCB (Molded Case) = industrial 50-800A. ELB (Earth Leakage Breaker) = protects against shock. Standard branch circuits use MCB + ELB combination.

Why a 1.25 safety factor?+

KEC 212.6.4 mandates 'breaker rating ≥ load current × 1.25'. The 1.25 ratio ensures the breaker trips before the wire overheats — a safety margin between wire ampacity and breaker rating.

Why are motor loads calculated differently?+

Motor starting current is 6-8× full-load current. A 1.25 factor alone would trip on start. Size the breaker by full-load current with 1.25, but use a D-curve or motor protection circuit breaker that can ride out the start current.

What does ELB sensitivity mean?+

ELB has separate ratings for overcurrent trip and ground-fault sensitivity. For shock protection use 30mA / 0.03s. For fire prevention use 100-300mA. Branch circuits: 30mA; main: 200mA.

Can I pick a non-standard rating?+

No. KEC 212.6.5 requires KS C 8321 standard ratings only. Round up to the next standard rating.

How do I match breaker rating with wire size?+

Wire ampacity ≥ breaker rating. Example: 20A breaker → wire ampacity ≥ 25A (e.g., 2.5sq IV or 4sq THHN). Check KEC 232.5 wire size table, or use this site's wire size calculator.

ELBs keep tripping on inverter loads — why?+

Inverter PWM output creates kHz-range leakage that confuses AC-only ELBs (50/60Hz detection). Fix: use a B-type ELB (inverter-compatible) or an inverter-specific breaker; alternatively, install an isolation transformer.

Cautions

  • •This calculator follows KEC 212.6 for general load breaker sizing. Special loads (welders, UPS, SMPS) need separate review.
  • •The 1.25 factor is a floor — for variable or future-growth loads, use 1.5 to 2.0.
  • •Always round up to the next standard rating, never down (which causes nuisance trips).
  • •ELB sensitivity (30mA, etc.) is decided separately by load type. This tool sizes overcurrent only.
  • •Final breaker selection requires KEC review by a certified electrical engineer.

Last reviewed: 2026-05-30

Breaker Capacity Calculator | Workmate