Cable Sizing & Voltage Drop Calculations Formula

Cable sizing is done based on three parameters:

1. Load current,
2. Short circuit current carrying capacity,
3. Voltage drop.

Below are given simple steps for performing cable sizing calculations.

Step 1 – Calculate Load Current:

Calculate the load current from the load data available. Formula is:

I=Load (in kW)/(sqrt(3)V(in kV)p.f) (A)

The current obtained has to derated since the cable is designed to operate at a defined temperature. Derating factors will be available in cable catalog.

Get the cable size corresponding to this derated current from the catalog.

Step 2 – Find Cable Size:

Next, find the cable size for the short circuit capacity of the system. Formula used is,

Cable size= (Fault current(in kA)*Fault clearing time)/K

where, K-constant depending on the conductor and insulation used. K=90 (for Al cables), K=140 (for Cu cables)

Step 2 is normally done for HT cables. For LT, if the circuit breaker used is ACB, then this step is done. For MCCB, this step is ignored.

Choose the cable size whichever is greater in step 1 or step 2.

Step 3 – Check / Calculate Voltage Drop:

Check for voltage drop with the cable size chosen.

Below is the formula to calculate the voltage drop of an electrical circuit based on the wire size and load current.  This calculator assumes that the circuit will operate in a normal condition room temperature with normal frequency. The actual voltage drop can vary depending on the condition of the wire, the conduit / raceway being used the temperature the connector the frequency etc.

VD=I×(2×L×R/1000)

Alternatively to compute the voltage drop and cable size, you require the following data.

1. The total load you wish to run with the concerned cable.
2. Corresponding current for that particular load. (To be on a safer side you can assume 2 times the load in kW or else you can apply the formula for power to get the exact current)
3. Once you know how much current the cable is gonna carry you can easily finalize the rating of protection device (ACB, MCCB, etc.,).
4. For the time being you can assume the standard cable size corresponding to your switchgear rating and as per your authority standards.
5. Length of the cable from the starting end to receiving end.
6. Voltage drop Ampere/Meter which you will get from the cable supplier (in mV).

Once you get all the above data, the voltage drop can be calculated as follows.

Voltage drop=(voltage drop ampere/meter) x length x current.

This will be in mV. Just divide by 1000 to get the voltage drop in volts.

The voltage drop from transformer to the load (lighting, motor) should not exceed 5%.

Also, if it is a motor load, the voltage drop for motor starting has to be checked. It has to be less than 15%.