How to Match an MCB to Cable Size and Load Current

An MCB should be selected to carry the normal load while protecting the cable from overload and short-circuit damage.

The basic selection rule is:

Ib ≤ In ≤ Iz

Where:

Ib is the design load current.
In is the rated current of the MCB.
Iz is the permitted current-carrying capacity of the cable after applicable correction factors.

In simple terms, the MCB rating must be high enough for the expected load but must not exceed the cable’s safe current-carrying capacity.

Should an MCB Be Selected by Load or Cable Size?

An MCB must be selected according to both.

The load determines the minimum current the circuit must carry. The cable determines the maximum current that can flow continuously without creating an overheating risk.

If the MCB rating is lower than the load current, nuisance tripping may occur.
If the MCB rating is higher than the cable capacity, the cable may overheat before the breaker trips.
If the cable and MCB are both oversized without considering the load, the installation may become unnecessarily expensive.

The correct process is to calculate the load current, select a suitable cable, apply the relevant cable correction factors, and then choose an MCB rating that satisfies Ib ≤ In ≤ Iz.

This principle is included in the Schneider Electric Electrical Installation Guide.

What Do Ib, In, and Iz Mean?

Symbol	Meaning	Example
Ib	Design current required by the load	13A
In	Rated current of the selected MCB	16A
Iz	Corrected current-carrying capacity of the cable	19A

In this example:

13A ≤ 16A ≤ 19A

The 16A MCB can carry the expected 13A load while remaining below the cable’s permitted current capacity.

For most fixed-rating MCBs, the overload trip setting is determined by the rated current. Unlike many industrial MCCBs, a standard MCB does not normally provide an adjustable long-time overload setting.

Step 1: Calculate the Design Load Current

The first step is to determine the maximum current expected during normal operation.

Single-Phase Load

For a single-phase AC load:

Ib = P ÷ (V × PF × η)

Where:

P is the load power in watts.
V is the supply voltage.
PF is the power factor.
η is the equipment efficiency.

For a purely resistive load, such as an electric heater, the power factor and efficiency may be close to 1.

Three-Phase Load

For a balanced three-phase load:

Ib = P ÷ (√3 × V × PF × η)

Motor and equipment nameplate current should be used when available because it normally provides a more reliable basis than estimating current from power alone.

The design current may also need to consider:

Continuous operating time
Simultaneous loads
Motor starting current
Transformer or LED-driver inrush current
Future load expansion
Local electrical regulations

Step 2: Select the Cable Size

After calculating the design current, select a cable with sufficient current-carrying capacity.

Cable ampacity depends on more than conductor cross-sectional area. The same cable size can have different permitted currents under different installation conditions.

Factor	Why It Matters
Conductor material	Copper and aluminum have different conductivity
Insulation type	PVC, XLPE, and other materials have different temperature limits
Installation method	Conduit, cable tray, wall, underground, and free-air installations dissipate heat differently
Ambient temperature	Higher temperature reduces heat dissipation
Cable grouping	Closely grouped loaded cables heat one another
Number of loaded conductors	More loaded conductors can increase thermal stress
Cable length	Long cables may require a larger size to control voltage drop
Local standard	Permitted ampacity and correction factors vary by market

For this reason, a statement such as “2.5mm² cable always requires a 20A MCB” is not universally correct.

The cable’s final permitted current, Iz, should be determined after all relevant correction or derating factors have been applied.

Step 3: Choose the MCB Rated Current

Once Ib and Iz are known, select a standard MCB rating between them.

Example 1: A Valid Selection

A single-phase resistive load has:

Design current: Ib = 13A
Corrected cable capacity: Iz = 19A

A 16A MCB can be selected because:

13A ≤ 16A ≤ 19A

Example 2: The MCB Is Too Small

A circuit has:

Design current: Ib = 18A
MCB rating: In = 16A
Corrected cable capacity: Iz = 24A

The cable capacity is sufficient, but the 16A MCB is below the design load. The breaker may trip during normal operation.

A 20A MCB may be considered because:

18A ≤ 20A ≤ 24A

Example 3: The Cable Is Too Small

A circuit has:

Design current: Ib = 18A
Proposed MCB rating: In = 20A
Corrected cable capacity: Iz = 17A

This selection is not acceptable because:

20A > 17A

The MCB may allow the cable to carry more current than its permitted capacity. The correct solution is not to install the 20A MCB automatically. The cable size, installation method, load, or circuit design must be reconsidered.

Step 4: Apply Cable and MCB Derating Factors

Both cables and thermal-magnetic circuit breakers can be affected by temperature and installation conditions.

MCBs installed side by side inside a closed distribution box may heat one another. High ambient temperature can also change the thermal tripping behavior of the breaker.

According to the Schneider Electric circuit-breaker selection guide, manufacturers provide correction data for circuit breakers operating outside their reference temperature or under grouped installation conditions.

Therefore, the selected MCB should be checked for:

Ambient temperature
Enclosure temperature
Adjacent loaded breakers
Ventilation
Continuous load
Manufacturer derating data

The corrected operating capacity of the MCB must still be sufficient for the actual load.

Step 5: Select the Correct MCB Trip Curve

The trip curve determines the instantaneous magnetic tripping range of the MCB. It does not replace the rated-current calculation.

Trip Curve	Instantaneous Trip Range	General Load Direction
B curve	Approximately 3–5 × In	Resistive loads and circuits with low inrush current
C curve	Approximately 5–10 × In	Mixed commercial loads and moderate inrush current
D curve	Approximately 10–20 × In	Loads with high starting or inrush current

A higher trip curve should not be selected only to stop nuisance tripping. The circuit’s starting current, minimum fault current, cable protection, and required disconnection time must also be verified.

For motor circuits, an MCB may provide short-circuit and cable protection, but separate motor overload protection may still be required.

Step 6: Check the MCB Breaking Capacity

Rated current and breaking capacity are different specifications.

Rated current determines the normal current the MCB can carry.
Breaking capacity determines the maximum prospective short-circuit current the MCB can safely interrupt.

The MCB breaking capacity must be equal to or higher than the prospective short-circuit current at the installation point.

For example, depending on the calculated fault level, a project may require a 4.5kA, 6kA, 10kA, 15kA, or 25kA MCB.

A higher breaking capacity does not allow the use of a smaller cable or a higher rated current. Each parameter must be selected independently.

Step 7: Check Voltage, Poles, and Applicable Standards

Before completing the selection, verify:

Rated operational voltage
AC or DC application
Number of poles
System earthing arrangement
IEC/EN or applicable national standard
Terminal capacity
Busbar compatibility
Required accessories
Isolation requirements

The number of poles should be selected according to the system configuration and local installation requirements. A 1P, 1P+N, 2P, 3P, 3P+N, or 4P device should not be chosen based on physical panel space alone.

Practical MCB Selection Checklist

Selection Item	Question to Confirm
Load current	What is the maximum normal operating current?
Cable capacity	What is the cable’s corrected current-carrying capacity?
MCB rating	Does the selection satisfy Ib ≤ In ≤ Iz?
Trip curve	Does it suit the load’s inrush or starting current?
Breaking capacity	Is it at least equal to the prospective short-circuit current?
Voltage	Is the MCB rated for the actual system voltage?
Poles	Does the pole configuration match the circuit?
Environment	Have temperature and grouping factors been considered?
Standard	Does the device comply with the project’s required standard?

Common MCB and Cable-Sizing Mistakes

Selecting the MCB Only by Equipment Power

Power alone may not reflect power factor, efficiency, inrush current, or actual nameplate current.

Using a Fixed Cable-Size Table Without Correction Factors

Cable ampacity changes according to installation method, temperature, grouping, insulation, and local regulations.

Increasing the MCB Rating to Stop Tripping

Installing a larger MCB without checking the cable capacity can remove the cable’s intended overload protection.

Ignoring Voltage Drop

A cable may have adequate thermal capacity but still produce excessive voltage drop over a long distance.

Confusing Trip Curve with Rated Current

Changing from a B curve to a C or D curve changes the instantaneous magnetic response. It does not increase the cable’s permitted current.

Ignoring MCB Breaking Capacity

A correctly selected current rating is not enough if the breaker cannot safely interrupt the available short-circuit current.

CNC MCB Options for Different Circuits

CNC Electric supplies miniature circuit breakers with multiple:

Rated currents
Pole configurations
B, C, and D trip curves
Breaking capacities
Terminal and busbar arrangements
Residential, commercial, and industrial application options

The CNC YCB9-80M/H MCB provides multiple current ratings, pole configurations, and tripping characteristics, with 6kA and 10kA breaking-capacity options.

You can also explore the complete CNC MCB range to compare products for different final-distribution requirements.

Frequently Asked Questions

Should the MCB rating equal the load current?

Not necessarily. The MCB rating is normally selected from a standard current rating that is equal to or higher than the design load current, provided it does not exceed the corrected cable capacity.

Can I install a larger MCB if the existing breaker keeps tripping?

Not without determining why the breaker is tripping and verifying the cable capacity. Frequent tripping may be caused by overload, inrush current, a fault, incorrect trip-curve selection, high temperature, or loose connections.

Can a 32A MCB be used with a 2.5mm² cable?

There is no universal answer based on cable size alone. Conductor material, insulation, installation method, ambient temperature, cable grouping, local standards, and other conditions must be checked before confirming the permitted current.

Does a higher breaking capacity protect the cable better from overload?

No. Breaking capacity relates to the maximum short-circuit current the breaker can interrupt. Overload protection depends mainly on the MCB rated current, trip characteristics, and cable capacity.

Does changing from a B curve to a C curve increase the MCB ampere rating?

No. A B16 and C16 MCB both have a rated current of 16A. Their instantaneous magnetic tripping ranges are different.

Why does an MCB trip when the load current is below its rating?

Possible causes include high ambient temperature, heat from adjacent breakers, short-duration inrush current, loose connections, harmonics, intermittent overloads, or an electrical fault.

Conclusion

Correct MCB selection requires more than choosing the nearest ampere rating.

The design load, cable capacity, installation conditions, trip curve, breaking capacity, voltage, poles, and applicable standards must all be considered.

The essential relationship is: