Mike Dolt Enterprises
Mike Dolt Enterprises
User: emiller233, Location: pittsburgh, pa
Asked on: Thursday, June 3, 2021 at 9:13 am EDT
Answered on: Thursday, June 20, 2024 at 11:47 am EDT
Question: We have a project that needs CE. And our motor vendor (WEG) told us they don’t have a 60HP version, so we have to get a 75 HP and derate it down to 60HP.How do you handle the over current protection and wire sizing for this. Based off a 60HP or 75HP…?
Handling Over Current Protection and Wire Sizing for Derated Motor
Over Current Protection
Based on derated motor rating (60 HP)
Calculate the motor current at 60 HP using the formula:
Choose an overcurrent protection device (e.g., circuit breaker, fuse) with a trip rating slightly above the calculated motor current.Motor Current (A) = (Motor Power (HP) 746) / (Motor Voltage (V) Motor Power Factor)
Wire Sizing
Based on 75 HP motor rating (actual motor size)
Calculate the wire size required to carry the full load current of the 75 HP motor using the following formula:
Distance (ft)) / (Allowable Voltage Drop (%) Voltage (V)) Choose a wire size that is at least equal to the calculated size.Wire Size (AWG) = (Motor Current (A)
Considerations
The nameplate data on the 75 HP motor will provide the actual full load current rating. The allowable voltage drop percentage should be specified in the project requirements or local codes. Proper derating of the motor must be ensured to maintain the desired performance and prevent overheating. Consider using a Variable Frequency Drive (VFD) to control the motor speed and further reduce the current draw.
Example:
If the derated motor is 60 HP, the motor voltage is 480 V, and the motor power factor is 0.85, the following calculation can be used: Motor Current (A) = (60 746) / (480 0.85) = 113.6 ABased on this calculation, an overcurrent protection device with a trip rating of 125 A or 150 A would be suitable.For wire sizing, assuming a distance of 50 ft and an allowable voltage drop of 2%, the following calculation would apply: Wire Size (AWG) = (113.6 50) / (0.02 480) = 4 AWGTherefore, a 4 AWG wire would be required to carry the full load current of the 75 HP motor. Flag for review