Okay, this is a frustrating and complex issue. Let's break down the potential causes and troubleshooting steps. The fact that Eaton tested the breakers and found no issues is a key piece of information, which suggests the problem is external to the breakers themselves.Here's a systematic approach

1. Re-Examine the Load & Load Calculations (Most Important)
This is absolutely the most crucial step. Even though the service was upgraded 3 years ago, electrical usage patterns can change. People add more appliances, AC units run more often, and energy efficiency changes. Don't just rely on original calculations. Collect Data



Peak Demand Monitoring

This is essential. You need to know the actual current draw at different times of day. Use a recording ammeter (clamp-on or permanently installed) on each phase of the main service for at least a week, ideally longer. Capture data every few minutes. This will show you the true demand profile. Analyze this to see if you're exceeding 80% of the breaker's capacity at any point. Look for sudden spikes in demand.

Individual Unit Load Monitoring

If possible, install meters on a few representative units in each affected building to understand their individual consumption patterns. Look for appliances that might be turning on simultaneously.

Time of Day Correlation

Carefully log the exact time each breaker trips. Compare this to typical occupant activity. Is there a spike around 6-8 PM when people are cooking and running AC, or around midnight when ACs are cycling aggressively and water heaters kick on?

Verify the Calculations



Demand Factors

Are you using appropriate demand factors for multifamily dwellings in your load calculations? You need to make sure that the demand factors you're using are in accordance with the NEC and that they actually match the living situations in those apartments. If it is a lower income apartment complex the demand factors may not be accurate.

HVAC Load

In South Florida, HVAC is a major factor. Has there been an increase in the number or size of window AC units being used by residents? Are there code violations regarding window AC units?

Electric Vehicle Charging

Is anyone using EV chargers without authorization or proper installation? This can overload the system.

Water Heaters

How old are the water heaters? Old heaters can draw more current.

Lighting

While less likely, are there any large common area lighting loads that are kicking on at certain times (e.g., parking lot lights)?

Consider Harmonic Distortion

While less common, significant harmonic distortion can cause breakers to trip at lower-than-expected currents. This is more likely with a large number of electronic devices (computers, TVs, etc.). A power quality analyzer can measure harmonic distortion.

2. Neutral and Ground Issues (Very Important) Check for Shared Neutrals

Improper wiring with shared neutrals can cause neutral currents to overload and create voltage imbalances. This is a common issue in older buildings. Trace neutral wiring carefully to ensure it's properly isolated and sized.

Measure Neutral-to-Ground Voltage

Use a multimeter to measure the voltage between the neutral bus and the ground bus at the main service panel in each affected building. Ideally, this voltage should be very low (ideally less than 1 volt, but definitely less than 2 volts). Higher voltage indicates a problem with the neutral connection or excessive neutral current.

Check Neutral Connections

Carefully inspect all neutral connections in the main service panel, subpanels, and junction boxes. Look for loose connections, corrosion, or signs of overheating. Retorque all connections to the manufacturer's specifications.

Grounding System Integrity

Verify that the grounding system is properly installed and connected. Check the grounding electrode conductor to ensure it's intact and properly connected to the grounding electrode (ground rod, water pipe, etc.). Verify that the ground rod is still effective.

3. Voltage Fluctuations Voltage Monitoring

Install a recording voltmeter to monitor the voltage at the main service panel over a period of time. Look for voltage sags, surges, or imbalances.

Utility Issues

While FPL denies it, it's worth documenting any voltage fluctuations you observe and presenting them to FPL again. Be persistent. They might not be aware of a localized issue on their grid.

4. Breaker Installation and Environment Proper Installation

Ensure the breakers are properly installed in the panel and that the bus bar connections are tight. Loose connections can cause overheating and premature failure.

Ambient Temperature

High ambient temperatures can derate the breaker's capacity. Is the panel in a hot, unventilated area?

Vibration

Is there any significant vibration near the panel that could be contributing to breaker tripping?

5. Investigate the Refurbished Breakers Source of Refurbishment

Where did you get the refurbished breakers? Not all refurbished breakers are created equal. Some may not be properly tested or reconditioned.

Warranty

Do the refurbished breakers have a warranty?

Independent Testing

Even though Eaton tested the original breakers, consider having the refurbished breakers independently tested by a reputable third-party testing lab. This provides an unbiased assessment.

Troubleshooting Steps to Take Immediately


Install Recording Ammeters

This is the

first thing you should do. Without data, you're just guessing.2. Check Neutral-to-Ground Voltage

Easy to do and can point to significant problems.3.

Visually Inspect all connections in the panel.Important Considerations



NEC Compliance

Ensure that all wiring and installations are in compliance with the National Electrical Code (NEC).

Qualified Electrician

All electrical work should be performed by a licensed and qualified electrician.

Documentation

Keep detailed records of all inspections, tests, and repairs.

Why Refurbished Breakers Might Seem to Work Briefly and Then Fail



Marginal Condition

Refurbished breakers may have been near the end of their lifespan. The act of reinstalling them might have stressed them, leading to eventual failure.*

Underlying Problem

If there's an underlying issue in the electrical system (e.g., voltage fluctuations, harmonic distortion), it can damage or stress breakers, regardless of whether they're new or refurbished.

In summary, your most important step is to thoroughly investigate the loads on these buildings, especially considering that Eaton says the breakers are fine. Get good data with a recording ammeter. Also, don't ignore the possibility of neutral or grounding issues. Good luck! Let me know if you have any other questions. Flag for review