Industrial Motor Feeder: 500HP Chiller Installation

Project Overview

A major manufacturing facility in Houston, Texas required installation of a new 500HP centrifugal chiller to expand cooling capacity for their production process. The chiller's motor, operating at 480V three-phase, would be located in a new mechanical building approximately 600 feet from the main electrical distribution switchgear. This project presented significant voltage drop challenges that required careful engineering analysis to ensure reliable motor operation.

Large industrial motors present unique electrical challenges because they operate in two distinct modes: starting and running. During normal operation, the motor draws its rated full load current (FLC) continuously. However, during starting, the motor can draw 6 to 8 times its full load current for 10 to 15 seconds until it reaches operating speed. This high starting current creates a temporary but significant voltage drop that can affect the motor's ability to develop adequate starting torque and can disturb other equipment on the same electrical system.

The facility's engineering team recognized that this installation required more than a simple ampacity-based conductor selection. They needed to consider voltage drop during both running and starting conditions, evaluate the impact on other facility loads, and determine whether motor starting methods such as VFDs or soft starters should be incorporated into the design. This case study examines the complete engineering analysis that led to a successful installation.

Technical Requirements and Challenges

The 600-foot distance from the switchgear to the motor location creates substantial conductor resistance. For a motor of this size operating at 480V three-phase, the running current of 590 amps is significant. If conductors are sized only for ampacity, the voltage drop during normal operation could exceed acceptable limits, reducing motor efficiency and increasing operating costs. During starting, the situation becomes even more critical—the 3,540-amp starting current would cause dramatic voltage sag throughout the facility's electrical system.

The engineering team established design criteria requiring less than 3% voltage drop during normal running conditions and limiting starting voltage drop to prevent interference with other facility loads. They also needed to ensure the motor received at least 80% of nominal voltage during starting to develop adequate torque for the centrifugal compressor load.

Initial Analysis: Direct-On-Line Starting

The engineering team first analyzed a conventional direct-on-line (DOL) starting approach to understand the baseline requirements. Using 500 kcmil copper conductors, which provide adequate ampacity for the 590A load:

The analysis revealed two significant problems with the DOL approach. First, the running voltage drop of 3.3% exceeds the target 3% limit, resulting in slightly reduced motor efficiency and increased energy consumption over the 24/7 operating schedule. Second, and more critically, the nearly 20% voltage drop during starting would cause unacceptable voltage sag throughout the facility, potentially affecting production equipment, lighting, and other sensitive loads.

The Optimized Solution: VFD with Parallel Conductors

The engineering team developed an optimized solution incorporating two key elements: a Variable Frequency Drive (VFD) to eliminate the starting current problem, and parallel conductors to reduce running voltage drop while optimizing cost.

With the VFD managing starting current, the conductor sizing could be optimized for running conditions only. The team selected three sets of 500 kcmil copper conductors per phase (3 parallel runs), which reduces the effective resistance by a factor of three:

Economic Analysis

The economic analysis demonstrated compelling benefits for the optimized design. The reduced voltage drop translates directly to lower energy losses in the conductors. With 590A flowing through conductors 24/7, even small resistance reductions yield significant savings. The VFD also enables chiller capacity modulation, allowing the system to match cooling output to actual demand rather than cycling on and off, further reducing energy consumption.

Key Takeaways for Industrial Motor Feeders

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Planning a motor feeder installation? Our voltage drop calculator supports three-phase calculations for industrial applications, including parallel conductor configurations.