How to Choose a Circuit Breaker Based on Cable Cross-Section?

When selecting a circuit breaker for an electrical installation, one of the most common mistakes is choosing the breaker solely based on the power of the connected equipment.

It is equally important to consider the cable cross-section, cable type, installation method, ambient temperature, grouping of cables, and expected load.

The main purpose of a circuit breaker is to protect electrical wires and cables against overload and short circuits.

IEC 60364-4-43 specifies requirements for the protection of conductors against the effects of overload and short circuits.

The Main Principle – The Circuit Breaker Must Protect the Cable

One of the most important principles when selecting a circuit breaker is:

IB ≤ In ≤ IZ

where:

• IB – design load current;

• In – rated current of the circuit breaker;

• IZ – continuous current-carrying capacity of the cable under the actual installation conditions.

This means that the rated current of the circuit breaker must not exceed the permissible continuous current of the cable.

Therefore, it is not correct to simply use rules such as:

2.5 mm² = 20 A

or

4 mm² = 25 A

These values may be suitable under typical conditions, but not for every installation.

Cable Cross-Section and Circuit Breaker Selection Table

The following table is indicative and intended for typical 230/400 V building electrical installations using copper cables.

* Power is calculated approximately using P = U × I, at 230 V and cosφ = 1. This is not the actual permissible power of the cable. The final selection of the circuit breaker and cable must be based on the specific installation conditions.

Important: this table is not a universal circuit breaker selection table. It is intended only as a quick reference.

The actual continuous current-carrying capacity of a cable must be determined based on the specific installation method and the applicable correction factors. IEC 60364-5-52 current-carrying capacity tables distinguish between different installation methods.

How Much Current Can a Cable of Different Cross-Section Carry?

To understand why selecting a circuit breaker is not as simple as it may seem, it is useful to look at the permissible current of the cable.

For example, IEC 60364-5-52 tables provide different current-carrying capacities for copper conductors with PVC insulation and three loaded conductors at 30 °C, depending on the installation method.

These values are examples from standard tables. In an actual installation, they may need to be adjusted using temperature, grouping, installation and other applicable correction factors.

Therefore, the same 2.5 mm² cable may have a permissible current of, for example, 18 A, 20 A, 21 A or 24 A depending on the installation method.

This is exactly why a circuit breaker cannot be selected based solely on cable cross-section.

1.5 mm² Cable – 10 A, 13 A or 16 A?

1.5 mm² copper cable is very commonly used for lighting circuits.

Depending on the installation, the following circuit breakers may be used:

• B10 A;

• B13 A;

• B16 A.

However, a 16 A circuit breaker is not automatically suitable for every 1.5 mm² installation.

For example, if the cable's continuous current-carrying capacity under the actual installation conditions is only 15 A, a 16 A circuit breaker would not satisfy the requirement:

In ≤ IZ

In such a situation, an appropriate protection solution must be selected.

2.5 mm² Cable – 16 A or 20 A?

This is one of the most frequently asked questions.

2.5 mm² copper cable is widely used in building electrical installations, particularly for socket circuits.

Depending on the installation conditions, the following may be used:

• B16 A;

• B20 A.

However, a 20 A circuit breaker may only be used if the cable's permissible continuous current under the actual conditions is at least 20 A and all other protection requirements are also satisfied.

For example, if the cable is installed inside thermal insulation or together with many other loaded cables, its permissible current may be significantly reduced.

4 mm² Cable – 25 A or 32 A?

4 mm² cable is commonly used for higher-power electrical circuits.

Depending on the installation method, the following may be suitable:

• B20 A;

• B25 A;

• B32 A.

For example, if the calculated permissible current of the cable under the actual installation conditions is 27 A, a 32 A circuit breaker would not be the correct choice simply because the cable is 4 mm².

In this case, a circuit breaker with a rated current that does not exceed the permissible current of the cable must be selected.

6 mm² Cable – 32 A?

6 mm² copper cable is commonly used for:

• electric cookers;

• electric heaters;

• higher-power socket circuits;

• outbuilding supply circuits;

• other higher-power electrical equipment.

Typical circuit breakers may include:

B25 A, B32 A or B40 A

However, the final selection must still be based on the actual installation conditions.

10 mm² Cable – 40 A, 50 A or 63 A?

10 mm² cable is often used for higher-power circuits and connections between distribution boards.

Depending on the specific conditions, the following may be suitable:

• 40 A;

• 50 A;

• 63 A.

For example, if the cable's continuous current-carrying capacity under the actual installation conditions is 52 A, a 63 A circuit breaker is not automatically permitted simply because it is the next available standard rating.

In this case, an appropriate circuit breaker, such as 50 A, should be selected, or the cable cross-section should be increased.

When Is a Larger Cable Cross-Section Required?

A larger cable cross-section may be required in several situations.

1. High load current

The higher the load, the larger the required cable cross-section may be.

2. Long cable runs

Voltage drop must be taken into account on long cable runs.

For example, a 30–50 metre cable run may require a larger cable than a short run carrying the same load.

3. Cables installed together

If many loaded cables are installed in the same conduit, tray or bundle, heat dissipation is reduced.

A grouping correction factor must therefore be applied.

4. High ambient temperature

Higher ambient temperatures reduce the cable's permissible continuous current.

5. Cable installed in thermal insulation

Heat dissipation may be significantly reduced, which lowers the permissible current.

How to Select a Circuit Breaker – Practical Example

Suppose we need to install a 230 V socket circuit.

The expected load is:

P = 3.5 kW

We calculate the current:

I = P / U

I = 3500 / 230 ≈ 15.2 A

The calculated load current is therefore approximately 15.2 A.

A 16 A circuit breaker could be considered.

Now we need to select the cable.

Suppose we use a 3×2.5 mm² copper cable.

If its permissible current under the actual installation conditions is greater than 16 A, the condition:

IB ≤ In ≤ IZ

may be satisfied:

15.2 A ≤ 16 A ≤ IZ

However, other necessary checks must also be carried out, such as voltage drop and fault protection/automatic disconnection requirements.

Circuit Breaker Characteristics – B, C or D?

The rated current of the circuit breaker is not the only parameter that matters.

The appropriate trip characteristic must also be selected.

B Characteristic

Type B circuit breakers are commonly used for:

• lighting;

• socket circuits;

• household electrical appliances;

• circuits without high inrush currents.

C Characteristic

Type C circuit breakers are suitable for circuits where higher short-duration inrush currents may occur.

For example:

• electric motors;

• transformers;

• some HVAC equipment;

• high-power power supplies.

D Characteristic

Type D circuit breakers are used for circuits with very high short-duration inrush currents.

It is important to understand:

C16 is not "more powerful" than B16.

Both circuit breakers have a rated current of 16 A, but their instantaneous tripping characteristics are different.

Circuit Breaker Rating and Cable Cross-Section

For quick reference, the following principle can be used:

This table is not a design table. It is intended only as a quick reference.

The actual circuit breaker selection must be based on the cable's permissible current under the specific installation conditions.

How to Calculate Load Current?

For a single-phase 230 V circuit:

I = P / U

For example:

3 kW / 230 V ≈ 13 A

5 kW / 230 V ≈ 21.7 A

7 kW / 230 V ≈ 30.4 A

These values are approximate and apply to 230 V active loads with cosφ ≈ 1.

For motors, transformers, LED drivers and other loads, the power factor and inrush current must also be taken into account.

Three-Phase Loads

For a balanced three-phase load in a 400 V system, the current can be approximately calculated using:

I = P / (√3 × U × cosφ)

If cosφ = 1:

Again, the circuit breaker cannot be selected based solely on this table.

The actual equipment, power factor, starting currents, cable cross-section, installation method and other parameters must also be considered.

Voltage Drop

Cable selection must take into account more than just cable heating.

For long cable runs, voltage drop must also be calculated.

This is particularly important for:

• garages;

• outbuildings;

• pumps;

• electric motors;

• EV charging equipment;

• electric boilers;

• high-power heaters.

For example, if an EV charger is located 50 m from the distribution board, simply selecting a 32 A circuit breaker is not sufficient. Voltage drop and the thermal loading of the cable must also be checked.

Short-Circuit Protection

The circuit breaker's ability to interrupt the prospective short-circuit current must also be checked.

Circuit breakers may have, for example:

• 4.5 kA;

• 6 kA;

• 10 kA;

short-circuit breaking capacities.

The appropriate value must be selected according to the prospective short-circuit current at the installation point.

Common Mistakes

❌ Always use a 20 A breaker for a 2.5 mm² cable

Not always.

❌ Always use a 16 A breaker for a 1.5 mm² cable

Not always.

❌ If the circuit breaker does not trip, the cable is safe

Not necessarily. The circuit breaker's operating characteristics and the cable's thermal loading are closely related.

❌ C16 is more powerful than B16

No. Both have a rated current of 16 A.

❌ You can simply install a larger circuit breaker

If the cable is not designed for the higher current, this can result in dangerous overload.

❌ The cable cross-section is selected based only on the circuit breaker

The cable must be selected based on the load and all relevant installation conditions, and the protective device must then be coordinated with the cable.

Correct Circuit Breaker Selection Sequence

To select an appropriate circuit breaker, the following sequence should be used:

1. Determine the load

Calculate the expected operating current IB.

2. Select the cable

Determine the required cable cross-section.

3. Determine the permissible cable current

Based on the cable type and installation method, determine IZ.

4. Apply correction factors

For example:

• ambient temperature correction factor;

• cable grouping correction factor;

• installation method correction factor;

• other applicable correction factors.

5. Select the circuit breaker rating

The following condition must be satisfied:

IB ≤ In ≤ IZ

6. Select the trip characteristic

B, C or D depending on the characteristics of the load.

7. Check voltage drop

Especially for long cable runs.

8. Check short-circuit conditions

Verify that the protective device provides the required protection and that its breaking capacity is sufficient.

Conclusion

A circuit breaker should not be selected solely based on the cable cross-section.

The correct approach is:

Load → cable cross-section → installation method → permissible cable current → circuit breaker → voltage drop → short-circuit verification.

The simple rule "1.5 mm² = 16 A, 2.5 mm² = 20 A, 4 mm² = 25 A" may be a useful rough guideline for everyday situations, but it is not a universal design rule.

The permissible current of a cable is significantly affected by its installation method, ambient temperature, cable grouping and other factors.

A safe electrical installation starts not with selecting the circuit breaker, but with correctly calculating and selecting the cable.

If you are unsure about the correct cable or circuit breaker, especially for high-power equipment, long cable runs or complex electrical installations, the calculations should be carried out by a qualified electrical installation professional.