Calculating Motor Feeder Conductor Size for Multiple Motors

What is the motor feeder conductor size needed for a system with multiple motors of different horsepower and voltage ratings?

How do you calculate the total load in amperes for various motors and select the appropriate conductor size based on NEC regulations?

Answer:

The motor feeder conductor size for the described system involves summing the total current from all motors and selecting a conductor size that can handle the total load while meeting the 75 degrees C thermal ratings for aluminum THHN conductors.

Calculating the motor feeder conductor size for multiple motors with varying horsepower and voltage ratings requires precise calculations and adherence to NEC standards. To determine the appropriate conductor size, you must first calculate the total demand current by summing the individual currents of all motors. This total current is used to select a conductor size from the NEC tables that can safely carry the load without exceeding its ampacity.

In this scenario, with a total horsepower of 49 hp (45 hp from three-phase motors and 4 hp from single-phase motors), the total load in amperes is calculated to be 177 A. After determining the total current, the next step is to refer to the NEC Table 310.16 to find the suitable conductor size. In this case, a 4 AWG aluminum THHN conductor rated for a maximum of 195 A at 75 degrees C is the appropriate choice based on the calculated load.

It is essential to consider factors such as ambient temperature correction, derating due to conductor bundling, and continuous load factors when selecting the conductor size. Additionally, other factors like overload protection, short-circuit protection, and voltage drop considerations must be taken into account to ensure the safe and efficient operation of the motor system.