Okay, this is a tricky problem, but let's break it down and figure out what's most likely happening. The fact that it only trips when both circuits are energized, even with the load completely disconnected, is the key clue. Here's a breakdown of potential causes and troubleshooting steps

Most Likely Culprits (in order of probability):

1. Shared Neutral Overload

This is the

most likely culprit. Even with the loads disconnected, there's likely still a small amount of leakage current flowing in the disconnected conductors (due to capacitive coupling, insulation imperfections, or other factors). When both circuits are energized, these small leakage currents add up in the shared neutral and exceed the neutral's capacity. This can lead to overheating and ultimately, failure of the contactor or even the neutral conductor itself. This is exacerbated because the contactor is only rated for 30A when the conductors are 20A. Why it Blows the Contactor

The neutral being overloaded can cause voltage imbalances on the hot legs. This can stress the contactor, potentially causing it to weld shut or fail. Overheating is also a possibility.

Troubleshooting



Completely Isolate the Neutral



Temporarily disconnect the shared neutral conductor completely at the contactor cabinet. Then power up both circuits. If the breakers still trip, then it isn't solely the neutral overload. This is dangerous and should only be done after all conductors are properly disconnected. Measure Neutral Current

If you can safely and temporarily connect an ammeter to the neutral wire at the contactor panel with both circuits energized (and the loads disconnected), see how much current is actually flowing. Even a few amps can indicate an issue.

Check for "Ghost Voltages"

Use a high-impedance voltmeter to check for voltages between neutral and ground, and between each hot leg and neutral. Even with loads disconnected, induced voltages can sometimes be present.

Check Neutral Connections

Ensure that every neutral connection is tight and clean, both in the panel and at the lights. A loose connection can create a high-resistance point, causing voltage drop and heat.2.

Contactor Coil Voltage Issue



Possibility

Even though you have a 277V coil contactor, there's a small chance the supply voltage to the coil is fluctuating when both circuits are energized, causing the contactor to chatter or draw excessive current before the breakers trip. This can damage the contactor's internal mechanisms.

Troubleshooting



Monitor Coil Voltage

Connect a voltmeter to the contactor coil terminals (A1 and A2) and monitor the voltage while you energize both circuits and cover the photocell. See if the voltage dips significantly or becomes unstable just before the breaker trips. A good quality meter will be needed that can read the voltage changes.

Check Control Circuit Wiring

Inspect the wiring of the photocell and the contactor coil circuit. Look for loose connections, corrosion, or damaged insulation. Even slight resistance in the control circuit can cause problems.3.

Harmonics/Non-Linear Loads (Less Likely without loads, but possible)



Explanation

Even though the load is disconnected, are there other devices (e.g., drivers or sensors) connected to these circuits that might be generating harmonics? Harmonics can overload the neutral and cause breakers to trip.

Troubleshooting

Requires specialized equipment (harmonic analyzer), but less likely if you've disconnected everything at the pole.4.

Crossed Wiring/Accidental Paralleling (Less Likely, but check)



Possibility

Could there be a wiring error somewhere that's unintentionally paralleling the two circuits? For example, is one side of a contactor coil (the A2 terminal for instance) connected to both hot phases rather than the neutral? Or, more likely, is there an issue with the way the neutral is connected in the cabinet that is causing a voltage imbalance or short circuit?

Troubleshooting



Double-Check Wiring Diagrams

Carefully verify that the wiring of the contactor, photocell, and breakers exactly matches the intended design. Use a reliable wiring diagram. Trace every wire.

Point-to-Point Continuity Testing

With the power OFF, use a multimeter to check continuity between all wires at the contactor cabinet to ensure that the circuits are truly isolated as intended.5.

Faulty Breakers



Possibility

Although less likely since both breakers trip, it's possible one or both of the breakers are faulty and are tripping at a lower current than their rating.

Troubleshooting



Test Breakers

Use a breaker tester to verify that the breakers are tripping at their rated current. Alternatively, swap the breakers with known good ones of the same type and rating.

Actionable Steps to Take


Prioritize Neutral Investigation

Focus heavily on the shared neutral. This is the most likely culprit. Disconnect the neutral at the source.2.

Voltage Monitoring

Monitor the voltage at the contactor coil and at the breakers while energizing the circuits.3.

Wiring Diagram Verification

Triple-check the wiring against a reliable diagram.4.

Visual Inspection

Carefully inspect all wiring for damage, corrosion, or loose connections.5.

Isolate Components

Isolate sections of the circuit one at a time to narrow down the source of the problem.

Safety First! Work De-energized

Whenever possible, work with the power off. Use proper lockout/tagout procedures.

Qualified Electrician

If you are not a qualified and experienced electrician, do not attempt to troubleshoot this problem. High voltage can be deadly.

Use Proper PPE: Wear appropriate personal protective equipment (PPE), including safety glasses, gloves, and insulated tools.By systematically investigating these potential causes, you should be able to identify the source of the problem and get your parking lot lights working again. Good luck! Flag for review