Okay, let's break down this problem and address your confusion about the service factor. Service factor is considered, but in a specific way.

Understanding the Problem and the Code

We're calculating the maximum overcurrent protection (breaker) for a feeder circuit that supplies multiple motors. The NEC (National Electrical Code) provides rules to ensure the breaker doesn't trip during motor starting currents while still providing adequate overload and short-circuit protection.

NEC Rules that are Applicable Motor Full-Load Amps (FLA)

We need to find the FLA for each motor type. We'll typically use NEC tables for this (specifically Table 430.248 for single-phase and Table 430.250 for three-phase).

Largest Motor

The largest motor's FLA is treated differently than the others.

Sum of All Motors

Add up the FLA of all motors.

Inverse Time Breaker

The code (NEC 430.52(C)(1)) allows for an inverse time breaker sized up to a percentage of the largest motor's FLA, plus the sum of the FLA of the remaining motors. This percentage is usually 250% (or sometimes even higher, depending on the specific motor type).

The Problem

Here's how to solve the problem step-by-step:1. Find Motor FLAs



5 hp, 230V, Single-Phase

From NEC Table 430.248, a 5 hp, 230V single-phase motor has an FLA of 28 amps.

1.5 hp, 120V, Three-Phase

From NEC Table 430.250, a 1.5 hp, 208V three-phase motor has an FLA of 5.4 amps. You’ll need to adjust for the 120 volts for this motor.
2.

Adjust for 120 Volt Three-Phase Motors

The FLA table value is based on 208V. We need to adjust this to 120V by using a ratio:FLA_adj = FLA_table (Voltage_table / Voltage_actual)FLA_adj = 5.4 (208 / 120) = 9.36 amps Each 1.5 hp, 120V, three-phase motor has an FLA of 9.36 amps.
3.

Identify Largest Motor

The 5 hp single-phase motor (28 amps) is the largest.4.

Calculate the Overcurrent Protection

NEC 430.52(C)(1) states that you can use an inverse time breaker up to 250% of the largest motor's FLA, plus the sum of the other motors' FLAs. Overcurrent Protection = (2.50 Largest Motor FLA) + (Sum of Other Motor FLAs) Overcurrent Protection = (2.50 28) + (9.36 + 9.36) Overcurrent Protection = 70 + 18.72 = 88.72 amps5.

Choose the Next Standard Breaker Size

The standard breaker sizes are 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, etc. You must round up to the next standard size. Therefore, an 88.72 amp calculation requires a

90 amp breaker.

Why Service Factor is NOT Directly Used in This Calculation

The service factor of a motor indicates the amount of overload a motor can handle intermittently. It allows the motor to operate at a higher horsepower than its nameplate rating for short periods without overheating. The service factor is already taken into account when the motor is designed, so you do not need to include it in this calculation.

In Summary

The correct answer is (d) 90A.The service factor is NOT directly used in this specific feeder overcurrent protection calculation. The calculation focuses on the running current (FLA) and a multiplier for the starting current of the largest motor. Flag for review