As an industrial motor user, sizing your cables for running three-phase motors is an important part of your job. The size of the cable will directly affect the performance of the motor, and can ultimately determine the success or failure of your projects. If you’re getting ready to install a new motor, understanding how to properly size the cable is essential.
When it comes to cable sizing for three-phase motors, there are two main factors to consider: current and voltage. The current is affected by the motor’s power rating, and the voltage is affected by the motor’s electrical characteristics. Understanding these two variables is key to correctly calculating the size of the cable needed.
The first step in calculating the size of the cable for a three-phase motor is to determine the full load current (FLC). The FLC is calculated by multiplying the motor’s power rating by 1.25 and then dividing the result by the voltage. For example, if you have a 3-phase motor with a power rating of 10 kW and a voltage of 400V, the FLC would be calculated as follows: 10 kW * 1.25 / 400V = 31.25 Amps.
Once you have the FLC, you can use it to calculate the minimum cross sectional area of the cable. To do this, you’ll need to refer to the current-carrying capacity table for the type of cable you’re using. The table will provide the cross sectional area of the cable that’s required to carry the specified current. For example, if the FLC is 31.25 Amps and you’re using a copper cable, then the minimum cross sectional area required for the cable would be 4mm2.
In addition to the current-carrying capacity table, you’ll also need to take voltage drop into account. Voltage drop is the difference between the input and output voltages of the circuit and should be kept to a minimum. In general, it’s recommended that the voltage drop should not exceed 3% of the total voltage. To calculate the voltage drop, you can use the following equation: Vdrop = I*R/1000, where I is the current in Amps and R is the resistance of the cable in Ohms.
Finally, after calculating the minimum cross sectional area of the cable and the maximum voltage drop, you can use the tables provided in the National Electric Code to select the appropriate cable size. The tables list the available sizes for a given cable type along with the corresponding current-carrying capacity and voltage drop. Once you’ve selected the correct size, you’re ready to purchase the cable and begin installing your motor.
Sizing your cables for three-phase motors is a critical part of any industrial motor installation. By following the steps outlined above, you can ensure that you have the correct cable size for the job and can get your motor up and running quickly and safely.
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