How to Find The Suitable Size of Cable & Wire for Electrical Wiring Installation? With Examples

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How to Find The Suitable Size of Cable & Wire for Electrical Wiring Installation? With Examples

Remember that it's far very crucial to choose proper wire size while sizing a wire for electrical installations. An inappropriate size of wire for heavy loads current can also create chaos which results in failure of the electrical system, hazardous fire and serious injuries.

Voltage Drop in Cables

Whenever the current flows through the conductor, there will be a voltage drops in that conductor. Normally, voltage drop may be neglected for small length of conductors but in case of a lower diameter and long length conductors we should consider a significant voltage drops for proper wiring installation and future load management.

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

Example:

Let us assume , the supply voltage is 230V AC, then the value of permissible voltage drop should be;

• Permissible Voltage Drop = 230 x (2.5/100) = 5.75V

Similarly, if the supply voltage is 110V AC, the Permissible Voltage Drop  should be not more than 2.75V ( 110V x 2.5%).

In electrical wiring circuits, for sub circuits the value of voltage drop should be half of that permissible voltage drop.

Normally, the voltage drop is expressed in Ampere per meter (A / m)

Tables & Charts for Proper Cable & Wire Sizes

Below are the important tables which you should follow for determining the proper size of cable for Electrical Wiring Installation

How to calculate Voltage Drop in a Cable?

Step no. 1:         Calculate the maximum allowable  voltage drop.

Step no. 2:          Calculate the load current

Step no. 3:          After finding the load current select a proper cable from table 1

Step no. 4:          from table 1 find the voltage drop of the cable and multiply with the length of cable

Step no. 5:          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.

Step no. 6:          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

Step no. 7:          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).

Example :

For Electrical wiring in a building, Total load is 5kW and the length of the cable from Main panel to sub circuit is 40 feet. Supply voltages is  230V and temperature is 40°C. Find the suitable size of cable which is going through conduits .

Solution:-

    Ø Total Load = 5kW

Ø                           Let us assume at max 20% overload occurs i.e. 1.2*5kW=6kW or 6000W

Now for 6000W load current will flow i.e. 6000/230= 26.08A

ØNow we have to select the size of cable from table 1 for 26.08A current which is 7/0.036 (28                      Amperes). It means we can use 7/0.036 cable according to table 1.

Ø                         Now check the selected (7/0.036) cable with temperature factor in Table 3, so the temperature                factor is 0.94 (in table 3) at 40°C  and current carrying capacity of (7/0.036) is 28A, therefore,                current carrying capacity of this cable at 40°C (104°F) would be

Current rating for 40°C  = 28 x 0.94 = 26.32 Amp.

                  Maximum current carrying capacity is 26.32 A and actual is 26.08A.Hence   this size of cable                  (7/0.036) is also suitable.

Ø                          Now find the voltage drop for 100 feet for this (7/0.036) cable is 7V, But in our case, the length of cable                 is 40 feet. Therefore, the voltage drop for 40 feet cable would be;

                Actual Voltage drop for 40 feet = (7 x 40/100) x (26.08/28) = 2.608V

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

             Here The Actual Voltage Drop (2.608V) is less than that of maximum allowable voltage drop of 5.5V.                  Therefore, the most suitable cable size is (7/0.036) for that given load.