How to Find the Suitable Size of Cable & Wire for Electrical Wiring Installation?

How to Find the Suitable Size of Cable & Wire for Electrical Wiring Installation?

In the realm of electrical installations, one crucial element often dictates the success of an entire project: the selection of the right cable and wire size. Whether you are working on residential wiring or overseeing a complex industrial setup, the size of cables and wires used can profoundly impact the efficiency, safety, and longevity of the electrical system.

The task of determining the suitable cable and wire size, however, can be a confusing and difficult technical journey. Undersized wires can lead to overheating and potential hazards, while oversized cables might result in wastage and inefficiency. Striking the delicate balance requires a blend of mathematical precision, knowledge of electrical standards, and an understanding of the unique demands posed by each application.

In this article, we shall learn all about how to calculate cable size and wire size for your electrical wiring installations.

Cables and how they can impact voltage drop

It is common knowledge that all conductors (other than some superconductors), cables, and wires are characterised by some resistance. It is important to note that resistance is inversely proportional to the conductor’s diameter and directly proportional to its length.

Therefore, R ∝ L/a … [Laws of resistance R = ρ (L/a)]

The conductor experiences a voltage drop whenever an electrical current passes through it. This drop may be negligible for conductors that are of smaller length. But in the case of longer conductors and conductors with a narrow diameter, considering voltage drops is important if correct installation and safety are to be ascertained.

The IEEE rule B-23 mandates that voltage drop should never be larger than 2.5% of the supply voltage at any place between installation and power supply.

Let us take a look at an example:

If supply voltage is 160V AC, then voltage drop can be allowed up to:

Permissible Voltage Drop = 160 x (2.5/100) = 4V

Voltage drops in cables can be measured using two systems of measurement.

The FPS (foot pound system) utilizes a length-based (100 ft) system to calculate voltage drop.

The SI (system international and metric system) calculates voltage drop in amperes per meter (A/m).

Nowadays, you can search for cable current carrying capacity calculator on the web to access several such free tools.

How do I calculate the voltage drop in a cable?

The steps for calculating voltage drop in the cable are as follows:

First, measure the highest permissible voltage drop.

Second, calculate the load current.

Third, you must know that the most appropriate cable will be one that has a current rating closest to the load current calculated. Several charts with cable sizes and corresponding current ratings are easily available online. Use such a chart to select a suitable cable.

Fourth, use the chart to also locate voltage drop in meters or 100ft, according to your system of preference.

Fourth, use the chart to also locate voltage drop in meters or 100ft, according to your system of preference.

Fifth, use the following cable size and current rating formula for calculating voltage drop along the actual length of the wiring circuit.

  1. If you want to calculate the drop in voltage per meter, (Actual length of circuit x volt drop for 1m) /100
  2. If you want to find the drop in voltage in 100ft (Actual length of circuit x volt drop for 100ft) /100

After calculating the above, you have to multiply the value you get with the load factor -

Load factor = Load Current to be taken by Cable/ Rated Current of Cable (which should be given in the chart you are referring to).

Now, you have arrived at the voltage drop value of the cable when the load current passes through it.

If this value is less than the maximum permissible voltage drop, then the size of the cable selected should be okay.

However, if the value is higher than the value of the maximum permissible voltage drop, then the voltage drop for the next greater-sized cable according to the referred chart has to be calculated. This process has to be continued until the value of the voltage drop becomes less than the maximum permissible voltage drop value.

How can I determine precise wire and cable size for a certain load?

First, you need to keep in mind that cable sizes corresponding to given loads can be ascertained with the help of standardized tables and charts available online but certain rules regarding voltage drop must be heeded.

There must be a 20% additional scope of current for emergency or future requirements for a given load other than the known current value.

Voltage drop must be limited to 1.25% from the energy meter to the distribution board while it should not be more than 2.5% of the supply voltage for the final sub-circuit.

Temperature variations have to be considered as per needs. Temperature factor charts are also easily available online and can be referred to.

You also need to keep in mind the load factor when trying to estimate cable size.

Two other factors that need to be considered in the determination of cable size are the type of wiring system and the diversity factor in installation.

Let us now look at a solved example using a wire size calculation formula to better understand this.

Example of wiring size calculation:

The total load for the installation of electrical wiring in a building is 5.5kW and the total length of cable from the meter to the distribution board is 40 feet. Supply voltage is 220V and temperature is recorded at 35°C. Calculate the most appropriate cable size from meter to sub-circuit for wiring to be installed in conduits.


Total Load = 5.5kW =5.5 x 1000W = 5500W

20% additional load = 5500 x (20/100) = 1100W

Total Load = 5500W + 1100W = 6600W

Total Current = I = P/V = 6600W /220V =30A

Now select the size of cable for a load current of 30A from your referred chart or table. This should be around (7/0.044) (34 Amperes). Accordingly, we can use (7/0.044) cable.

Thereafter, you need to match the (7/0.044) cable with the charted temperature factor. This value should be around 0.97 at 35°C and the current carrying capacity of (7/0.044) is 34A. Therefore, the current carrying capacity of this cable at 35°C could be calculated as:

Current rating for 35°C = 34 x 0.97 = 32.98 A

As the calculated value (32.98 A) at 35°C is less than that of the current carrying capacity of (7/0.044) cable which is 34A, this cable size (7/0.044) is also appropriate with the given temperature.

Next, you need to calculate the voltage drop for 100 feet for this (7/0.044) cable from cable size charts with corresponding current ratings and voltage drops available online. This value should be around 5.7V. Since in this case, the length of the cable is 40 feet, the voltage drop for 40 feet cable would be calculated as:

Actual Voltage drop for 40 feet = (5.7 x 40/100) x (30/34) = 2.01V

And Allowable voltage drop = (2.5 x 220)/100 = 5.5V

Therefore, we can see that the actual voltage drop (2.01V) is less than that of the maximum permissible voltage drop of 5.5V. Hence, the most suitable cable size is (7/0.044) for the given load.

Get your cables at the Schneider Electric eShop!

Schneider Electric has been a trusted name in the market for electric cables and wires for over two decades. For those who want to avoid the hassles of complicated calculations, visit the Schneider Electric eShop for expert consultation on cable sizes best suited to your exact requirements.

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