Calculating THHN Feeder Conductor Size for Multiple Motors

What size THHN feeder conductor is required for two 230 VAC, 7.5 HP motors, one 115 VAC, 1.5 HP short time motor, and one 230 VAC, 0.75 HP motor?

What is the formula to calculate the total load current for all the motors?

Answer:

The size of the THHN feeder conductor required for two 230 VAC, 7.5 HP motors, one 115 VAC, 1.5 HP short time motor, and one 230 VAC, 0.75 HP motor is 3 AWG.

When dealing with multiple motors of varying sizes and voltages, it is important to calculate the total load current to determine the appropriate size of the THHN feeder conductor needed to supply power to all the motors.

To calculate the total load current for all the motors, we can use the formula:

I = (HP x 745.7) / (V x 1.73)

Using this formula, we calculate the total load current for each motor:

  • For two 230 VAC, 7.5 HP motors: (7.5 x 745.7) / (230 x 1.73) = 29.7 A
  • For one 115 VAC, 1.5 HP short time motor: (1.5 x 745.7) / (115 x 1.73) = 8.85 A
  • For one 230 VAC, 0.75 HP motor: (0.75 x 745.7) / (230 x 1.73) = 4.41 A

Adding up all the load currents gives us a total load current of 42.96 A. Referring to the NEC Table 310.15(B)(16), we find that the minimum THHN feeder conductor size required is 3 AWG.

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