User: JoeStillman, Location: West Chester, PA
Asked on: Thursday, July 3, 2025 at 11:04 am EDT
Answered on: Thursday, July 3, 2025 at 11:07 am EDT
Question: Three 3HP pool pump motors rated for 208/230V, 12.4/11.2A are fed from a 120/208V, 1Ø, 3W panel, each on a 20/2 GFI breaker. The breakers are tripping after the pumps run a little while. Contractor has tried...Replace breakers with non-GFI. Still trips.Replace 20/2 with 30/2. Still trips.Replace breakers with ones that have same label as pump/drive. Still trips.Put an amprobe on the branch circuit. Pump draws nameplate current.It's not tripping on startup. Why does a 20A breaker trip on a 12A load?I'm going out on Monday to take some power quality readings. Wish me luck.
This is a tricky situation, and good on you for going in with a power quality analyzer! Here's a breakdown of possible causes and what you should check with your readings
Possible Causes (Beyond the Obvious):
Voltage Drop Under Load
The 208V service could be sagging significantly under load, even if it's acceptable at no-load. A large voltage drop will cause the motors to draw more current to maintain the same power output (HP). This is the most likely culprit if the amprobe readings were only taken briefly.
Harmonics
Non-linear loads (like variable speed drives, if present) can introduce harmonic currents into the system. These harmonics don't show up on a simple amprobe reading but can cause significant heating and nuisance tripping of breakers.
High Neutral Current
In a 208V single-phase system, you can sometimes get high neutral current even with balanced loads. It's less common, but still worth checking.
Loose Connections
Loose connections anywhere in the circuit (panel, breakers, motor connections, etc.) can cause increased resistance, leading to heat buildup and premature breaker tripping.
Motor Overheating
The motors themselves could be overheating due to poor ventilation, a failing capacitor (if capacitor start/run motors), or bearing issues. While not directly related to the electrical feed initially, overheating will cause the motor to draw more current.
Ambient Temperature
If the panel is located in a hot environment, the breakers may be derated and tripping at a lower current than their rating.
Breaker Derating Over Time
Breakers can become more sensitive and prone to tripping over time, especially if they have experienced repeated overloads. However, since the contractor replaced the breakers multiple times, this is less likely, but still a remote possibility.
Backfeed from other sources
Check to see if there is a secondary supply backfeeding onto the grid during operation of the pool pumps.
What to Check with Your Power Quality Analyzer
Here's what you need to measure and analyze:1.
Voltage (L-N) and Current (L) - Real-Time Recording
Record for an extended period
Ideally, record voltage and current on each branch circuit feeding the pumps before they start, during startup, and for as long as they run until tripping occurs.
Look for Voltage Sag
Watch for a significant voltage drop when the pumps start and as they run. Note the actual voltage at the panel and at the motor terminals if possible.
Current Fluctuations
Check for any unusual current spikes or fluctuations beyond the nameplate rating.2.
Harmonics Analysis
THD (Total Harmonic Distortion) of Voltage and Current
High THD indicates significant harmonic content. A high THD of current is most concerning.
Individual Harmonic Components
Identify the dominant harmonic frequencies (e.g., 3rd, 5th, 7th).
Neutral Current
Measure and record neutral current. Compare it to the phase currents. Excessively high neutral current is a problem.3.
Power Factor
Measure power factor at the panel and at the motor terminals. A low power factor (significantly below 0.85) indicates reactive power, which increases current draw. This could point to an issue with the motors or the capacitors (if applicable).
4. Temperature (If Possible)
Use an infrared thermometer to check the temperature of the breakers, motor windings, and connections while the pumps are running. Overheating is a critical clue.
Troubleshooting Steps Based on Readings
Voltage Drop
If voltage is dropping significantly, you have an undersized service, undersized wiring, or excessive distance from the panel. Options: Upgrade the service. Run larger gauge wire to the pumps. Consider a voltage boost transformer.
Harmonics
If harmonics are high, you need to mitigate them. Options: Install harmonic filters. Use motors with lower harmonic distortion.
High Neutral Current
Check for unbalanced loads elsewhere in the panel or improper neutral connections.
Overheating
If breakers or motors are overheating, address the cause: Improve ventilation. Replace faulty motors or capacitors. Tighten all connections.
Important Safety Considerations
Lockout/Tagout
Always de-energize the circuit before working on electrical equipment.
Personal Protective Equipment (PPE)
Use appropriate PPE, including safety glasses, gloves, and insulated tools.*
Qualified Personnel
Ensure you are qualified to perform electrical measurements and troubleshooting. If not, consult with a licensed electrician.
Good luck! Take thorough readings, document everything, and you'll likely find the root cause. Flag for review