How to Calculate the Minimum Conductor Size for a Three-Phase Motor Feeder?

What is the process of determining the minimum conductor size for a three-phase 240-volt feeder supplying power to multiple motors?

How can the total horsepower of the motors be converted to watts, and how is the current calculated to select the appropriate wire gauge according to NEC guidelines?

Answer:

To calculate the minimum conductor size for a three-phase 240-volt feeder powering multiple motors, convert the total horsepower to watts, determine the current, and then choose a wire gauge that can safely carry the current according to NEC guidelines. The calculated current for the motors is approximately 23.2 A, and typically, a 10 AWG conductor is rated for up to 30 A in free air. However, consulting NEC code is essential to ensure compliance.

Explanation:

A three-phase 240-volt feeder is going to supply power to one 5 hp 3-phase motor, three 2 hp 3-phase motors, and two 1 hp motors. The minimum size conductors permitted for this feeder must be determined based on the combined horsepower of the motors and the current they will draw.

First, we must convert the horsepower to watts because electric power is typically measured in watts. Using the fact that one horsepower is equivalent to 746 watts, we can calculate the total power requirement of the motors:

5 hp motor = 5 * 746 W = 3730 W

Three 2 hp motors = 3 * 2 * 746 W = 4476 W

Two 1 hp motors = 2 * 1 * 746 W = 1492 W

The total power required by the motors is 3730 W + 4476 W + 1492 W = 9698 W.

To determine the current, we use the formula I = P / (V * √3), where P is the total power in watts, V is the line-to-line voltage, and √3 is the square root of 3, which is approximately 1.732, to account for the three-phase power calculation. Therefore:

I = 9698 W / (240 V * 1.732) ≈ 23.2 A

Now that we have the current, we can consult the National Electrical Code (NEC) tables for conductor sizing, which indicate the ampacity of conductors of different AWG sizes. For a current of approximately 23.2 A, we would look for a wire gauge that can safely carry this current with the appropriate safety factor included.

Without the specific NEC charts, we cannot give a definite answer, but typically, a 10 AWG wire is rated for up to 30 A in free air, or up to 20 A when enclosed in conduit. The preference would be to size up if the current is close to or exceeds the limit to ensure safety and to account for potential increases in current due to starting the motors or other factors. Therefore, given the options provided (10 AWG, 8 AWG, 6 AWG, and 4 AWG), the minimum size conductors permitted for this feeder would be 10 AWG if the exact NEC guidelines and installation conditions are satisfied. However, it is important to consult the latest NEC code and tables, and possibly size up if necessary to ensure compliance and safety.