How to Determine the Proper Size of Wire & Cable for Electrical Wiring Installation?

The following step by step guide will show you how to find the right size of cable and wire or any other conductor for electrical wiring installation with solved examples (in both British or English and SI System i.e. Imperial and Metric System respectively).

Keep in mind that it is very important to select proper wire size while sizing a wire for electrical installations. An inappropriate size of wire for bigger loads having high current may create chaos which leads to failure of the electrical equipment, hazardous fire and serious injuries.

Drop Voltage in Cables

We know that all conductors, wires and cables (except superconductors) have some amount of resistance.

This resistance is directly proportional to the length and inversely proportional to the diameter of conductor i.e.

Whenever current flows through a conductor, a voltage drop occurs in that conductor. Generally, voltage drop may be neglected for small length of conductors but in case of a lower diameter and long length conductors, we have to take into account the significant voltage drops for proper wiring installation and future load management.

According to IEEE rule B-23, at any point between power supply terminal and installation, voltage drop should not increase above 2.5% of the provided (supply) voltage.

Example:

If the supply voltage is 220V AC, then the value of allowable voltage drop should be;

Allowable Voltage Drop = 220 x (2.5/100) = 5.5V

Likewise, if the supply voltage is 120V AC, the allowable voltage drop should be no more than 3V (120V x 2.5%).

In electrical wiring circuits, voltage drops also occur from the distribution board to the different sub circuit and final sub circuits, but for sub circuits and final sub circuits, the value of voltage drop should be half of that allowable voltage drops (i.e. 2.75V of 5.5V as calculated above)

Normally, Voltage drop in tables is described in Ampere per meter (A/m) e.g. what would be the voltage drop in a one meter cable which carries one ampere current?

There are two methods to define the voltage drop in a cable which we will discuss below.

How to Find the Voltage Drop in a Cable?

To find voltage drop in a cable, follow the simple steps given below.

First of all, find the maximum allowable voltage drop.

Now, find the load current.

Now, according to the load current, select a proper cable (which current rating should be closer to the calculated load current) from table 1.

From Table 1, find the voltage drop in meter or 100 feet (what system you prefer) according its rated current.

(Stay cool :) We will follow both methods and systems for finding voltage drops (in meters and 100 feet) in our solved example for whole electrical installation wiring).

Now, calculate the voltage drop for the actual length of the wiring circuit according to its rated current with the help of the following formulas.

(Actual length of circuit x volt drop for 1m) /100 ===> to find Volt drop in per meter.

(Actual length of circuit x volt drop for 100ft) /100 ===> to find volt drop in 100 feet.

Now multiply this calculated value of volt drop by load factor where;

Load factor = Load Current to be taken by Cable/ Rated Current of Cable given in the table.

This is the value of Volt drop in the cables when load current flows through it.

If the calculated value of voltage drop is less than the value calculated in step (1) (Maximum allowable voltage drop), than the size of selected cable is proper

If the calculated value of voltage drop is greater than the value calculated in step (1) (Maximum allowable voltage drop), than calculate voltage drop for the next (greater in size) cable and so on until the calculated value of voltage drop became less than the maximum allowable voltage drop calculated in step (1).

How to Determine the Proper Cable & Wire Size for Given Load?

Below are solved examples showing how to find the proper Cable Size for Given Load.

For a given load, cable size may be found with the help of different tables but we should keep in mind and follow the rules about voltage drop.

Determining the size of cable for a given load, take into account the following rules.

For a given load except the known value of current, there should be 20% extra scope of current for additional, future or emergency needs.

From Energy meter to Distribution board, Voltage drop should be 1.25% and for final sub circuit, voltage drop should not exceed 2.5% of Supply voltage.

Consider the change in temperature, when needed, use temperature factor (Table 3)

Also, consider the load factor when finding the size of cable

When determining the cable size, consider the wiring system i.e. In an open wiring system, temperature would be low but in conduit wiring, temperature increases due to the absence of air.

Note: Keep in mind Diversity Factor in Electrical Wring Installation while selecting the proper sizeof cable for electrical wiring installation

Related Posts:

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How To Locate Faults In Cables? Cable Faults, Types & Causes

Why Coaxial Cables are Highly Insulated?

olved Examples of Proper Wire & Cable Size

Following are the examples of determining the proper size of cables for electrical wiring installation which will make it easy to understand the method of “how to determine the proper size of cable for a given load”.

Example 1... (Imperial, British or English System)

For electrical wiring installation in a building, total load is 4.5kW and total length of cable from energy meter to sub circuit distribution board is 35 feet. Supply voltages are 220V and temperature is 40°C (104°F). Find the most suitable size of cable from energy meter to sub circuit if wiring is installed in conduits.

How to Determine the Proper Size of Wire & Cable for Electrical Wiring Installation?

The following step by step guide will show you how to find the right size of cable and wire or any other conductor for electrical wiring installation with solved examples (in both British or English and SI System i.e. Imperial and Metric System respectively).

Keep in mind that it is very important to select proper wire size while sizing a wire for electrical installations. An inappropriate size of wire for bigger loads having high current may create chaos which leads to failure of the electrical equipment, hazardous fire and serious injuries.

Drop Voltage in Cables

We know that all conductors, wires and cables (except superconductors) have some amount of resistance.

This resistance is directly proportional to the length and inversely proportional to the diameter of conductor i.e.

Whenever current flows through a conductor, a voltage drop occurs in that conductor. Generally, voltage drop may be neglected for small length of conductors but in case of a lower diameter and long length conductors, we have to take into account the significant voltage drops for proper wiring installation and future load management.

According to IEEE rule B-23, at any point between power supply terminal and installation, voltage drop should not increase above 2.5% of the provided (supply) voltage.

Example:

If the supply voltage is 220V AC, then the value of allowable voltage drop should be;

Allowable Voltage Drop = 220 x (2.5/100) = 5.5V

Likewise, if the supply voltage is 120V AC, the allowable voltage drop should be no more than 3V (120V x 2.5%).

In electrical wiring circuits, voltage drops also occur from the distribution board to the different sub circuit and final sub circuits, but for sub circuits and final sub circuits, the value of voltage drop should be half of that allowable voltage drops (i.e. 2.75V of 5.5V as calculated above)

Normally, Voltage drop in tables is described in Ampere per meter (A/m) e.g. what would be the voltage drop in a one meter cable which carries one ampere current?

There are two methods to define the voltage drop in a cable which we will discuss below.

How to Find the Voltage Drop in a Cable?

To find voltage drop in a cable, follow the simple steps given below.

First of all, find the maximum allowable voltage drop.

Now, find the load current.

Now, according to the load current, select a proper cable (which current rating should be closer to the calculated load current) from table 1.

From Table 1, find the voltage drop in meter or 100 feet (what system you prefer) according its rated current.

(Stay cool :) We will follow both methods and systems for finding voltage drops (in meters and 100 feet) in our solved example for whole electrical installation wiring).

Now, calculate the voltage drop for the actual length of the wiring circuit according to its rated current with the help of the following formulas.

(Actual length of circuit x volt drop for 1m) /100 ===> to find Volt drop in per meter.

(Actual length of circuit x volt drop for 100ft) /100 ===> to find volt drop in 100 feet.

Now multiply this calculated value of volt drop by load factor where;

Load factor = Load Current to be taken by Cable/ Rated Current of Cable given in the table.

This is the value of Volt drop in the cables when load current flows through it.

If the calculated value of voltage drop is less than the value calculated in step (1) (Maximum allowable voltage drop), than the size of selected cable is proper

If the calculated value of voltage drop is greater than the value calculated in step (1) (Maximum allowable voltage drop), than calculate voltage drop for the next (greater in size) cable and so on until the calculated value of voltage drop became less than the maximum allowable voltage drop calculated in step (1).

How to Determine the Proper Cable & Wire Size for Given Load?

Below are solved examples showing how to find the proper Cable Size for Given Load.

For a given load, cable size may be found with the help of different tables but we should keep in mind and follow the rules about voltage drop.

Determining the size of cable for a given load, take into account the following rules.

For a given load except the known value of current, there should be 20% extra scope of current for additional, future or emergency needs.

From Energy meter to Distribution board, Voltage drop should be 1.25% and for final sub circuit, voltage drop should not exceed 2.5% of Supply voltage.

Consider the change in temperature, when needed, use temperature factor (Table 3)

Also, consider the load factor when finding the size of cable

When determining the cable size, consider the wiring system i.e. In an open wiring system, temperature would be low but in conduit wiring, temperature increases due to the absence of air.

Note: Keep in mind Diversity Factor in Electrical Wring Installation while selecting the proper sizeof cable for electrical wiring installation

Related Posts:

How to calculate the value of resistor for LEDs (with different types of LED’s circuits)

How To Locate Faults In Cables? Cable Faults, Types & Causes

Why Coaxial Cables are Highly Insulated?

olved Examples of Proper Wire & Cable Size

Following are the examples of determining the proper size of cables for electrical wiring installation which will make it easy to understand the method of “how to determine the proper size of cable for a given load”.

Example 1... (Imperial, British or English System)

For electrical wiring installation in a building, total load is 4.5kW and total length of cable from energy meter to sub circuit distribution board is 35 feet. Supply voltages are 220V and temperature is 40°C (104°F). Find the most suitable size of cable from energy meter to sub circuit if wiring is installed in conduits.

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