Commercial HVAC Retrofit: Rooftop Unit Upgrade

Project Overview

A 120,000 square foot retail complex in Atlanta, Georgia had experienced chronic HVAC problems for over a decade. The building housed 12 tenant spaces, each with its own rooftop HVAC unit, totaling 150 tons of cooling capacity. Property management had replaced compressors multiple times, dealt with constant service calls, and faced frustrated tenants complaining about inadequate cooling during summer months. When the decision was made to replace the aging equipment with new high-efficiency units, the electrical contractor conducted a thorough analysis that revealed the root cause of the problems: severe voltage drop in the original electrical installation.

The original electrical design, installed 25 years earlier when the building was constructed, used minimal conductor sizes that met code requirements at the time but provided inadequate voltage to the rooftop equipment. With runs of 300+ feet from the main electrical room to the roof, the undersized conductors resulted in voltage drop approaching 8% during peak cooling demand. This low voltage caused compressors to run hot, start hard, and fail prematurely.

This case study examines how the retrofit project addressed both the equipment replacement and the underlying electrical issues, resulting in a dramatic improvement in system performance and reliability.

Diagnosing the Problem

The voltage measurements revealed a chronic undervoltage condition. When voltage to a compressor is low, the motor must draw higher current to produce the required power output. This creates a destructive cycle: higher current causes additional I²R heating in the motor windings, which accelerates insulation breakdown and leads to premature failure. The higher current also increases voltage drop in the already undersized conductors, making the problem even worse during peak demand periods.

The Solution: Rooftop Sub-Panel

The engineering team evaluated two approaches: replacing the individual circuit conductors with larger sizes, or installing a sub-panel on the roof with a single large feeder from the main distribution. The sub-panel approach proved more cost-effective and provided superior voltage drop performance.

Results and Benefits

Evaluate Your HVAC Circuits

Experiencing HVAC reliability issues? Voltage drop might be the hidden cause. Use our calculator to evaluate your existing installations and plan upgrades.