How do I size a circuit breaker for a specific load?

Follow these steps: (1) Calculate the full load current of the circuit — for a 3-phase load, FLC = kW / (sqrt(3) x V x power factor). (2) Apply a 125% safety factor per most electrical codes — the breaker rated current must be at least 1.25x the full load current for continuous loads. (3) Select the nearest standard breaker rating above that value. (4) Verify the breaker's breaking capacity exceeds the prospective fault current at the installation point. (5) Confirm the breaker's trip curve is appropriate for the load type (inductive, resistive, motor, etc.).

What size circuit breaker do I need for a 100 kW motor?

For a 100 kW, 400 V, 3-phase motor with 0.85 power factor: FLC = 100,000 / (1.732 x 400 x 0.85) = approximately 170 A. Apply the 125% rule: 170 x 1.25 = 212.5 A. Select the next standard MCCB frame: 250 A. However, motor circuits require special consideration for starting current (typically 6-8x FLC), so verify the MCCB's magnetic trip setting can withstand the inrush without nuisance tripping. A D-curve MCB or an MCCB with adjustable magnetic setting at 10-12x rated current is typically required.

What is the 125% rule for circuit breaker sizing?

The 125% rule (or 80% rule from the breaker's perspective) states that a circuit breaker should not be loaded beyond 80% of its rated current for continuous loads (loads running for 3 hours or more). Equivalently, the breaker's rated current must be at least 125% of the continuous load current. This comes from NEC Article 210 and IEC standards. For example, a 40 A continuous load requires a breaker rated at least 40 x 1.25 = 50 A. Some breakers are "100% rated" and can be loaded to full nameplate — but these are special (and more expensive) products.

How do I determine the prospective fault current at my installation?

The prospective fault current depends on the transformer rating, impedance, and the impedance of cables between the transformer and the breaker. For a rough estimate: Isc = Transformer kVA / (sqrt(3) x V x %Z/100). For a 1000 kVA, 400 V transformer with 6% impedance: Isc = 1,000,000 / (1.732 x 400 x 0.06) = 24,056 A or approximately 24 kA. This decreases with cable distance. For accurate values, request a fault level study from your utility or use short-circuit calculation software.

Can I use a 60 A breaker on a 60 A load?

Only if the load is non-continuous (runs less than 3 hours at a time). For continuous loads, you need a breaker rated at 60 x 1.25 = 75 A minimum — so an 80 A breaker would be required. Using a 60 A breaker on a 60 A continuous load will cause nuisance tripping because the thermal element operates at approximately 1.13x rated current per IEC standards, and sustained loading at 100% will gradually heat the bimetal toward that threshold.

How to Choose the Right Circuit Breaker: Sizing Guide

Circuit Breaker Sizing: A Systematic Approach

Choosing the right circuit breaker is not about picking a product from a catalog — it is about systematically matching the breaker's characteristics to the circuit it must protect. An oversized breaker fails to protect the cable from overheating. An undersized breaker nuisance-trips and disrupts operations. A breaker with insufficient breaking capacity can fail catastrophically during a fault.

This guide walks through the sizing process step by step, from determining the basic circuit parameters to verifying coordination with other protective devices in the system. It applies to all low-voltage circuit breakers: MCBs, MCCBs, ACBs, and extends to medium-voltage VCBs for completeness.

Step 1: Determine System Voltage

The breaker's rated voltage must be equal to or greater than the system voltage. This sounds obvious, but in practice, voltage mismatches occur in export projects where equipment designed for one market is installed in another.

System Type	Common Voltages	Breaker Type
Residential single-phase	230 V (Europe, Asia) / 120 V (Americas)	MCB 1P or 1P+N
Commercial 3-phase	400 V (Europe, Asia) / 480 V (Americas)	MCB 3P or MCCB 3P
Industrial 3-phase	400 V / 690 V	MCCB or ACB
Medium voltage distribution	6.6 kV / 11 kV / 33 kV	VCB or SF6 CB

Critical check: If the system voltage is 690 V, standard MCBs (rated 400 V max) cannot be used. You must select an MCCB or ACB rated for 690 V. For 480 V systems common in the Americas, verify the breaker's breaking capacity rating at 480 V — many breakers have different kA ratings at different voltages (e.g., 50 kA at 400 V but only 36 kA at 480 V).

Step 2: Calculate Full Load Current

The breaker's rated current (In) must be at least 125% of the continuous full load current (FLC) of the circuit. For non-continuous loads, In ≥ FLC is sufficient.

Full load current formulas

Load Type	Formula	Example
Single-phase resistive	I = P / V	10 kW at 230 V = 43.5 A
Single-phase with PF	I = P / (V x PF)	10 kW at 230 V, PF 0.9 = 48.3 A
3-phase balanced	I = P / (√3 x V x PF)	100 kW at 400 V, PF 0.85 = 170 A
Motor (known HP)	I = HP x 746 / (√3 x V x η x PF)	50 HP at 400 V, η 0.92, PF 0.85 = 69 A
Transformer secondary	I = kVA x 1000 / (√3 x V)	1000 kVA at 400 V = 1,443 A

After calculating FLC, apply the 125% rule for continuous loads:

Minimum breaker rating = FLC x 1.25

Then select the next standard breaker rating above that value. Standard MCB ratings: 6, 10, 16, 20, 25, 32, 40, 50, 63, 80, 100, 125 A. Standard MCCB frame sizes: 100, 160, 250, 400, 630, 800, 1000, 1250, 1600, 2000, 2500 A.

Step 3: Verify Prospective Fault Current

The breaker's breaking capacity (Icu or Icn) must exceed the maximum prospective short-circuit current at the installation point. This is non-negotiable — it is a safety requirement in every electrical code worldwide.

Estimating prospective fault current

The prospective fault current at a transformer's secondary terminals can be estimated as:

Isc = (kVA x 1000) / (√3 x V x Zpu)

Where Zpu is the per-unit impedance of the transformer (typically 0.04 to 0.06 for distribution transformers).

Transformer (400 V secondary)	Impedance	Prospective Isc	Minimum Breaker Icu
250 kVA	4%	9 kA	10 kA (MCB possible)
500 kVA	5%	14.4 kA	18 kA (MCCB required)
1000 kVA	6%	24 kA	25 kA (MCCB required)
1600 kVA	6%	38.5 kA	40 kA (MCCB or ACB)
2500 kVA	6%	60 kA	65 kA (ACB or high-break MCCB)

The fault current decreases with cable length. As a rough rule of thumb, every 10 meters of 95 mm² copper cable at 400 V adds approximately 0.002 ohms of impedance, reducing the prospective fault current. At 50-100 meters from the transformer, a standard 10 kA MCB may be adequate even on a 1000 kVA transformer. However, always verify with a proper short-circuit calculation — rules of thumb are not acceptable for safety-critical decisions.

Step 4: Select the Breaker Type

Based on the load current, fault current, and application, select the appropriate breaker category:

Selection Criteria	MCB	MCCB	ACB	VCB (MV)
Current range	≤125 A	16-2,500 A	800-6,300 A	400-4,000 A
Voltage	≤440 V	≤690 V	≤690 V	3.6-40.5 kV
Breaking capacity	≤10 kA	18-200 kA	42-150 kA	16-50 kA
Trip adjustability	Fixed	Adjustable	Full LSIG	Relay-based
Selectivity	Not possible	Partial	Full (Cat B)	Full
Cost level	$	$$	$$$	$$$$

For the detailed MCB vs MCCB comparison, see our dedicated comparison guide. For ACB vs MCCB, see our ACB vs MCCB guide.

Step 5: Choose the Trip Curve or Trip Unit

For MCBs: Select B, C, or D curve

Curve B (3-5x In): Resistive loads — lighting, heating, cable protection. Use B-curve as the default unless the load has significant inrush current.
Curve C (5-10x In): General purpose — mixed loads, fluorescent lighting with electronic ballasts, small single-phase motors up to 2.2 kW. The most commonly specified curve worldwide.
Curve D (10-20x In): High inrush loads — 3-phase motors, transformers, welding machines, X-ray equipment, compressors. Required when motor starting current exceeds 5x FLC.

Not sure which curve a long or LED-heavy circuit can take? See how to choose between Type B, C and D MCB curves, including maximum cable length per curve.

For MCCBs: Configure the trip unit

With thermal-magnetic MCCBs, adjust:

Thermal (Ir): Set to 1.0x rated current for cables sized to full breaker rating; reduce to 0.8x if the cable is smaller or the load is well-defined.
Magnetic (Im): Set to 5-10x for general distribution; 10-14x for motor circuits to ride through starting inrush.

With electronic trip MCCBs, program the LSIG settings based on the coordination study. This requires the time-current curves of the upstream and downstream breakers to ensure proper selectivity.

Step 6: Verify Cable Protection

The breaker must protect the cable from overheating under both overload and short-circuit conditions. This requires:

Overload protection: The breaker's rated current (In) must satisfy: Ib ≤ In ≤ Iz, where Ib is the design current of the circuit and Iz is the current-carrying capacity of the cable (from IEC 60364 or national wiring rules).
Short-circuit protection: The breaker must clear the fault before the cable's I²t withstand limit is exceeded. The cable's let-through energy (I²t) at the maximum fault current must be less than k²S² (where k is the cable constant and S is the conductor cross-section in mm²). For copper PVC cables, k = 115.
Maximum disconnection time: For socket-outlet circuits, IEC 60364 requires disconnection within 0.4 seconds. For distribution circuits, 5 seconds. The breaker's time-current curve must confirm these times at the minimum fault current (typically the fault current at the far end of the cable).

Step 7: Check Selective Coordination

In a properly coordinated system, only the breaker closest to the fault trips, while all upstream breakers remain closed. This requires the downstream breaker's total clearing time to be less than the upstream breaker's non-trip time at every fault current level.

Coordination approaches:

Current-based selectivity: The upstream breaker's magnetic trip setting is higher than the downstream breaker's let-through current. Works for MCB-MCB and MCB-MCCB combinations at moderate fault levels.
Time-based selectivity: The upstream breaker has a short-time delay (S function) that holds it closed while the downstream breaker clears. Requires Category B breakers per IEC 60947-2.
Energy-based selectivity: The upstream breaker can withstand the let-through energy of the downstream breaker without tripping. Achieved with current-limiting MCCBs paired with properly selected downstream devices.
Zone-based selectivity (ZSI): Breakers communicate via a restraint signal — the upstream breaker delays only if it receives a signal from the downstream breaker indicating the fault is in its zone. Fastest and most reliable, but requires compatible breakers and wiring.

Step 8: Consider Environmental and Installation Factors

Real-world conditions affect breaker performance. Apply derating factors as needed:

Ambient temperature: MCBs and MCCBs are calibrated at 40°C (IEC standard reference). At higher temperatures, the thermal element trips at lower currents. At 50°C, apply approximately 0.9x derating; at 60°C, approximately 0.8x. In tropical or high-temperature environments, size the breaker one step above the calculated requirement.
Altitude: Above 2,000 m, air density decreases and dielectric strength drops. Apply voltage derating above 2,000 m — typically 1% per 100 m. The breaker's breaking capacity may also be reduced. See our altitude derating guide for medium-voltage equipment.
Grouping: Multiple breakers mounted side by side in an enclosed panel generate mutual heating. Apply grouping derating factors from the manufacturer's data. Typical derating: 0.8x for 4 adjacent MCBs in an enclosed distribution board at 40°C ambient.
Pollution degree: In dusty, humid, or corrosive environments, the breaker's insulation requirements increase. Industrial environments are typically Pollution Degree 3 (conductive dust possible); outdoor installations may require Pollution Degree 4.

Practical Sizing Examples

Example 1: Office lighting circuit

Load: 20 x 36 W LED panels = 720 W at 230 V single-phase. FLC = 720 / 230 = 3.13 A. Apply 125%: 3.91 A. Select: B6 MCB (6 A, B-curve). Breaking capacity: 6 kA sufficient (downstream of a small transformer, cable length >30 m). Cable: 2.5 mm² PVC is adequate.

Example 2: Industrial motor feeder

Load: 75 kW, 400 V, 3-phase motor, PF 0.85, η 0.93. FLC = 75,000 / (1.732 x 400 x 0.85 x 0.93) = 137 A. Apply 125%: 171 A. Prospective fault current: 35 kA (30 m from 1000 kVA transformer). Select: 250 A frame MCCB with Icu ≥36 kA. Trip setting: Ir = 160 A (thermal), Im = 12x (magnetic, to withstand 6x FLC starting current). Motor starting current: 137 x 6.5 = 891 A for approximately 8 seconds — the MCCB's magnetic trip at 12 x 160 = 1,920 A will not nuisance trip during starting.

Example 3: Main incoming breaker for a building

Transformer: 2000 kVA, 400 V, 6% impedance. Secondary FLC: 2000 x 1000 / (1.732 x 400) = 2,887 A. Prospective fault current: 2000 / (1.732 x 0.4 x 0.06) = 48.1 kA. Apply 125%: 3,609 A. Select: 4000 A ACB with Icu ≥50 kA, Category B with LSIG electronic trip. For the upstream medium-voltage protection, a 12 kV VCB protects the primary side of the transformer.

Circuit Breaker Sizing Checklist

Use this checklist for every circuit breaker selection. All eight items must be verified:

☐ Ue ≥ system voltage — rated operational voltage covers the actual system voltage
☐ In ≥ 1.25 x FLC — rated current covers 125% of continuous load (or 100% for non-continuous)
☐ Icu ≥ Isc — breaking capacity exceeds prospective fault current
☐ Trip curve matches load type — B/C/D for MCBs; adjustable thermal/magnetic for MCCBs
☐ Cable is protected — breaker trips before cable I²t limit is exceeded
☐ Disconnection time met — 0.4 s for socket circuits, 5 s for distribution circuits (IEC 60364)
☐ Selectivity verified — downstream breaker trips before upstream breaker
☐ Derating applied — ambient temperature, altitude, grouping corrections applied

Need Help Sizing Circuit Breakers for Your Project?

NAIJI Electric's engineering team can assist with circuit breaker selection, coordination studies, and complete switchgear panel design. We manufacture low-voltage switchgear, medium-voltage switchgear, and vacuum circuit breakers — delivering complete power distribution solutions from a single factory.

Contact NAIJI Electric — circuit breaker manufacturer since 2001, ISO 9001 / CE / CCC certified, factory direct, 30-60 day delivery.