Breaker Capacity Calculator

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

Result

Enter load current and type, then click Calculate.

Breaker Rating by Load Current (Korean KS Standard)

The most-searched values in one table. Each cell applies the load-type safety factor to the load current, then picks the smallest KS standard rating at or above it.

Load current	General (×1.25)	Motor DOL (×1.5)	Motor Y-Δ (×2.0)
5 A	10 A	10 A	10 A
10 A	16 A	16 A	20 A
15 A	20 A	25 A	32 A
20 A	25 A	32 A	40 A
25 A	32 A	40 A	50 A
30 A	40 A	50 A	63 A
40 A	50 A	63 A	100 A
50 A	63 A	75 A	100 A
60 A	75 A	100 A	125 A
75 A	100 A	125 A	150 A
100 A	125 A	150 A	200 A
125 A	175 A	200 A	250 A
150 A	200 A	225 A	300 A
200 A	250 A	300 A	400 A

KS Standard Ratings (A)

MCB (miniature, up to 63A): 6 · 10 · 16 · 20 · 25 · 32 · 40 · 50 · 63
MCCB (molded case): 15 · 20 · 30 · 40 · 50 · 60 · 75 · 100 · 125 · 150 · 175 · 200 · 225 · 250 · 300 · 350 · 400 · 500 · 600 · 800 · 1000 · 1200 · 1600 · 2000

These are first-pass selections based on safety factor alone. Actual design must also stay within the cable ampacity and coordinate with upstream breakers (selectivity) and starting characteristics. Use the calculator above to check margin and warnings.

Source: KEC 212.6 / KS C 8324 (MCB) · KS C 8453 (MCCB) · ×1.25 / ×1.5 / ×2

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)

1Choose load type: general (lighting/outlet), motor, or inverter/VVVF. Each applies different safety factors and additional considerations.
2Enter load current (A) or load capacity (kW/kVA). The tool auto-converts capacity to current using voltage, power factor, and phase count.
3Pick voltage (single-phase 220V / 3-phase 380V) and phase. Defaults are Korean standard LV.
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.