The Scientific Method for Calculating the Cross-Section of an Electrical Cable and Selecting the Appropriate Circuit Breaker

 

The Scientific Method for Calculating the Cross-Section of an Electrical Cable and Selecting the Appropriate Circuit Breaker

The cable cross-section is calculated using the power formula:

p = 3 * I * V * cosΦ

Where cosΦ = 0.8, which is the power factor.

From this, the current value can be determined. The formula for calculating voltage drop is:

V.D = mv * l * i / 1000

Where:

V.D: represents the value of the voltage drop.

MThe Scientific Method for Calculating the Cross-Section of an Electrical Cable and Selecting the Appropriate Circuit Breaker

The cable cross-section is calculated using the power formula:

p = 3 * I * V * cosΦ

Where cosΦ = 0.8, which is the power factor.

From this, the current value can be determined. The formula for calculating voltage drop is:

V.D = mv * l * i / 1000

 

Where:

V.D: represents the value of the voltage drop.

MV: represents the voltage drop per meter for the cable type.

Note: The voltage drop varies from one cable to another depending on the following factors:

a- The cable's resistivity

b- The cable's length

c- The current flowing through the cable.

However, this formula is the internationally accepted or commonly used formula.

Today, we will study how to calculate the cross-sections of wires and cables and how to select the appropriate circuit breakers. First: It is necessary to calculate the total load in kilowatts and use the power formula:

p = 3 * V * I * cosΦ

Where p is the total kilowatts. Its calculation: V is the phase voltage, which is 380 volts, and cosΦ varies from country to country. Therefore, the current I is calculated, which is the total current drawn. Based on this, we choose the circuit breaker with a standard value higher than the calculated current. It is recommended to allow for a 10% increase in the breaker's amperage.

What are the standard values ​​for wires?

1.5 And 2 mm² cable: 10 A circuit breaker

2 and 3 mm² cable: 16 A or 20 A circuit breaker

4 mm² cable: 20 A or 25 A circuit breaker

6 mm² cable: 25 A or 32 A circuit breaker

10 mm² cable: 32 A or 40 A circuit breaker

16 mm² cable: 40 A circuit breaker

25 mm² cable: 50 A or 63 A circuit breaker

35 mm² cable: 80 A circuit breaker

50 mm² cable: 100 A circuit breaker

70 mm² cable: 125 A or 160 A circuit breaker

95 mm² cable: 160 A or 200 A circuit breaker

120 mm² cable: 200 A or 250 A circuit breaker A 150 mm² cable cross-section requires a 250 A circuit breaker.

A 185 mm² cable cross-section requires a 250 A or 300 A circuit breaker.

A 240 mm² cable cross-section requires a 300 A circuit breaker.

A 300 mm² cable cross-section requires a 400 A circuit breaker.

Single-phase: We use the following formula: P = V * I * 0.8

Where:

For every 1 kW, the circuit breaker is rated at 5 A.

Three-phase: We use the following formula: P = V * I * 0.8 * √3

Where:

For every 1 kW, the circuit breaker is rated at 2 A.

Determining the cable cross-section:

The cable cross-section is determined by following these steps:

First: Calculate the load current.

Single-phase:

Current equals power / voltage

I = actual current = (VA / 220)

Three-phase:

Current equals power divided by (√3 * voltage)

I = actual current = (VA / 380 * (3)^1/2)

Second: Determining the Circuit Breaker Current

Circuit Breaker Current = 1.25 * Load Current

IC.B = 1.25 * Actual Current

Third: Selecting the Circuit Breaker

The circuit breaker is selected from its respective tables by choosing the highest value calculated in step two.

IC.B rated = next standard rating above IC.B

Fourth: Determining the Cable Current

Cable Current = 1.2 * Circuit Breaker Current

IC = cable current = 1.2 * IC.B rated

Fifth: How to Select the Cable

The value calculated in step four is taken and the cable tables are consulted.

Most offices use tables from specific companies. The cable's current rating must be greater than or equal to the current value calculated in step four. The cable rating cross-section area = next cross-section with current IC.r ≥ IC. As for the wire cross-sectional area, it is usually pre-defined according to the load you intend to use:

1.5 mm² wire can handle a current of 18 amps

2.5 mm² wire can handle a current of 21 amps

4.0 mm² wire can handle a current of 27 amps

6.0 mm² wire can handle a current of 35 amps

10 mm² wire can handle a current of 48 amps

16 mm² wire can handle a current of 65 amps

25 mm² wire can handle a current of 88 amps

35 mm² wire can handle a current of 110 amps

50 mm² wire can handle a current of 140 amps

70 mm² wire can handle a current of 175 Amp

A 95 mm² wire can withstand a current of 215 Amp

A 120 mm² wire can withstand a current of 255 Amp

A 185 mm² wire can withstand a current of 340 Amp

This study presents a simplified view, but the wire cross-section of the cable depends on several factors, most importantly the cable length, the material used in its construction (copper, aluminum, or a mixture in varying proportions), and the cable's location (overhead or underground), among others.

Note: The previous study was based on copper wires with a length not exceeding 40 meters.: represents the voltage drop per meter for the cable type.

Note: The voltage drop varies from one cable to another depending on the following factors:

a- The cable's resistivity

b- The cable's length

c- The current flowing through the cable.

However, this formula is the internationally accepted or commonly used formula.

Today, we will study how to calculate the cross-sections of wires and cables and how to select the appropriate circuit breakers. First: It is necessary to calculate the total load in kilowatts and use the power formula:

p = 3 * V * I * cosΦ

where p is the total kilowatts. Its calculation: V is the phase voltage, which is 380 volts, and cosΦ varies from country to country. Therefore, the current I is calculated, which is the total current drawn. Based on this, we choose the circuit breaker with a standard value higher than the calculated current. It is recommended to allow for a 10% increase in the breaker's amperage.

What are the standard values ​​for wires?

1.5 and 2 mm² cable: 10 A circuit breaker

2 and 3 mm² cable: 16 A or 20 A circuit breaker

4 mm² cable: 20 A or 25 A circuit breaker

6 mm² cable: 25 A or 32 A circuit breaker

10 mm² cable: 32 A or 40 A circuit breaker

16 mm² cable: 40 A circuit breaker

25 mm² cable: 50 A or 63 A circuit breaker

35 mm² cable: 80 A circuit breaker

50 mm² cable: 100 A circuit breaker

70 mm² cable: 125 A or 160 A circuit breaker

95 mm² cable: 160 A or 200 A circuit breaker

120 mm² cable: 200 A or 250 A circuit breaker A 150 mm² cable cross-section requires a 250 A circuit breaker.

A 185 mm² cable cross-section requires a 250 A or 300 A circuit breaker.

A 240 mm² cable cross-section requires a 300 A circuit breaker.

A 300 mm² cable cross-section requires a 400 A circuit breaker.

Single-phase: We use the following formula: P = V * I * 0.8

Where:

For every 1 kW, the circuit breaker is rated at 5 A.

Three-phase: We use the following formula: P = V * I * 0.8 * √3

Where:

For every 1 kW, the circuit breaker is rated at 2 A.

Determining the cable cross-section:

The cable cross-section is determined by following these steps:

First: Calculate the load current.

Single-phase:

Current equals power / voltage

I = actual current = (VA / 220)

Three-phase:

Current equals power divided by (√3 * voltage)

I = actual current = (VA / 380 * (3)^1/2)

Second: Determining the Circuit Breaker Current

Circuit Breaker Current = 1.25 * Load Current

IC.B = 1.25 * Actual Current

Third: Selecting the Circuit Breaker

The circuit breaker is selected from its respective tables by choosing the highest value calculated in step two.

IC.B rated = next standard rating above IC.B

Fourth: Determining the Cable Current

Cable Current = 1.2 * Circuit Breaker Current

IC = cable current = 1.2 * IC.B rated

Fifth: How to Select the Cable

The value calculated in step four is taken and the cable tables are consulted.

Most offices use tables from specific companies. The cable's current rating must be greater than or equal to the current value calculated in step four. The cable rating cross-section area = next cross-section with current IC.r ≥ IC. As for the wire cross-sectional area, it is usually pre-defined according to the load you intend to use:

1.5 mm² wire can handle a current of 18 amps

2.5 mm² wire can handle a current of 21 amps

4.0 mm² wire can handle a current of 27 amps

6.0 mm² wire can handle a current of 35 amps

10 mm² wire can handle a current of 48 amps

16 mm² wire can handle a current of 65 amps

25 mm² wire can handle a current of 88 amps

35 mm² wire can handle a current of 110 amps

50 mm² wire can handle a current of 140 amps

70 mm² wire can handle a current of 175 Amp

A 95 mm² wire can withstand a current of 215 Amp

A 120 mm² wire can withstand a current of 255 Amp

A 185 mm² wire can withstand a current of 340 Amp

 

This study presents a simplified view, but the wire cross-section of the cable depends on several factors, most importantly the cable length, the material used in its construction (copper, aluminum, or a mixture in varying proportions), and the cable's location (overhead or underground), among others.

Note: The previous study was based on copper wires with a length not exceeding 40 meters.