Wire Size and Fuse Ratings for 3-Phase Induction Motors

Motors are usually protected by both fuses (or circuit breakers) and by heater coils in a magnetic starter. Fuses open the circuit quickly in case of a massive overload or short circuit. Heater coils provide a delay and open the circuit if the average current, over a period of time, is greater than the circuit is designed for.

In some cases it may be necessary to use delayed action fuses. These provide a very short delay to prevent blowing a fuse during the short interval while the motor is accelerating up to its normal speed after being started.

Current values in this chart are approximate, and are compiled from data published by several motor manufacturers. They may be a little high or low for a specific motor. For selection of magnetic starter heater coils, it is better to follow the nameplate current rating of the actual motor to be used rather than depending on this or any other table.

Wire and fuse sizes are listed for reference only, and may vary with type of insulation, number of conductors in a cable and other factors. For a new design, requirements of the NEC (National Electrical Code) should be followed. Copies of the code can be ordered through most book stores. Other local ordinances may also apply.

For selecting ampere rating of heater coils for magnetic motor starters, select the standard coil closest to the rating on the motor nameplate. If motor operates in cold environment, the coil with next lower rating may be preferred. If in hot environment, the coil with next larger current rating may work better.

For additional information, see Design Data Sheet 11. See also Design Data Sheet 33 and Design Data Sheet 49 for effect of high and low voltage on electric motors. More detailed information on magnetic motor starters including wiring diagrams, will be found in the Womack book “Electrical Control of Fluid Power".

TEMPERATURE CONVERSIONS - FAHRENHEIT AND CELSIUS

Enter one of the columns marked “Temp” with the temperature, either Fahrenheit or Celsius, which you wish to convert. If converting to Celsius, read the equivalent value in the column to the left. If converting to Fahrenheit, read the equivalent value in the column to the right. The table was calculated from these formula:

°F = [°C × 9/5] + 32
or
°C = 5/9 × [°F - 32]

Enter one of these columns with the temperature you wish to convert

© 1990 by Womack Machine Supply Co. This company assumes no liability for errors in data nor in safe and/or satisfactory operation of equipment designed from this information.