Okay, this is a tricky one, but we can break it down logically. Since you've isolated the load (the pole lights themselves) and are still blowing the contactor and tripping the breakers, the problem is definitely in the wiring between the panel and where the pole lights would connect. Here's a step-by-step troubleshooting approach, considering the shared neutral

1. Double-Check the Contactor Wiring and Coil Voltage:
Contactor Coil

Verify

absolutely that the contactor coil voltage is correct (277V as you stated). A lower voltage coil can burn out quickly, especially if continuously energized by 277V. Check the marking on the contactor itself. Contactor Wiring

Double-check the line and load side wiring on the contactor. Ensure the 277V circuits are connected to the line (incoming power) terminals, and that the wires leading out to the poles would be connected to the load terminals. It's possible the wires were accidentally switched.

Three-Pole Usage

Verify you are wiring the contactor as a three pole contactor and only sending one hot to each pole of the contactor and not bridging poles as that would be a problem.

2. Inspect the Shared Neutral

This is the most likely culprit given your symptoms. Since both circuits tripping together cause the issue, and you have a shared neutral, the neutral is probably the place to start looking.

Neutral Integrity

Visually inspect the neutral wire run from the panel to the contactor cabinet and then out towards the pole base. Look for:

Loose Connections

Check every connection point where the neutral is spliced or connected. A loose connection can create a high-resistance point that causes voltage drop and overload.

Damage/Nicks

Look for any signs of damage to the neutral wire insulation. Nicks or cuts can create a path to ground, especially under load.

Corrosion

Check for corrosion, especially in damp environments. Corrosion can increase resistance.

Continuity Test on Neutral

With all power OFF and circuits completely isolated, test the continuity of the neutral wire end-to-end from the panel neutral bus to the point where you would normally connect the neutral at the pole base. You should see very low resistance (close to 0 ohms). If you see high resistance, there's a break or poor connection somewhere. Also check the continuity between the neutral and the grounding conductor (should be very high resistance or OL).

Isolation Test

Disconnect the neutral wire at the panel neutral bus. Measure the resistance between the neutral wire (the one you disconnected) and ground at the panel. It should be infinite (very high resistance or OL - Over Limit). If you get a low resistance reading, the neutral is grounded somewhere along its run. This could be a nail through the wire, water intrusion, or rodent damage.

Multiple Neutrals

Make sure there is only one neutral conductor going from the contactor cabinet to the pole base. If there are two neutrals going to the same location on the pole lights it could cause interference.

3. Conduit Inspection



Conduit Condition

Inspect the conduit that houses the wires between the panel, contactor, and pole base. Look for:

Physical Damage

Dents, cracks, or crushed sections. Damage can pinch wires and cause shorts.

Water Intrusion

Signs of water inside the conduit. Water can create a conductive path.

Conduit Grounding

Ensure the conduit is properly grounded back to the panel.

4. Metering Under Load (with Caution)



If you can safely do so without further damage, with the load STILL DISCONNECTED, and using EXTREME CAUTION, monitor the voltage at the contactor output terminals when both circuits are energized. Look for: Voltage Imbalance

Is there a significant voltage difference between the two circuits? A voltage imbalance can indicate a problem with the neutral.

Voltage to Ground

Carefully measure the voltage from each contactor output terminal to ground. You should see 277V. If you see a lower voltage, it may indicate a short or fault to ground.

Neutral Current

This is tricky and requires a clamp meter capable of measuring AC current. Clamp the clamp meter around the neutral conductor only (not around any of the hot wires). With both circuits energized (but still with the pole lights disconnected!), measure the current flowing on the neutral. You should ideally see very little current (a few milliamps) or zero current. If you see significant current, that means current is returning on the neutral even with no load, which indicates a problem.

5. Test with Incandescent Bulbs (as a Safe Load)



Temporary Test Load

Instead of the pole lights, temporarily connect a few heavy-duty incandescent work lights as a test load. This is safer than immediately reconnecting the original lights. Make sure the bulbs are rated for 277V, if available. If not, use appropriate step-down transformers.

Observe

See if the circuits hold and the contactor operates correctly with the incandescent lights. If they do, the problem might actually be in the pole lights themselves, even though initial tests showed no shorts. Sometimes a fault only appears under full voltage and load.

6. Possible Causes and Explanations



Overloaded Neutral

While you're using 20A breakers, the neutral wire could be undersized for the combined load of both circuits. However, this usually results in overheating of the neutral, not immediate tripping and contactor failure with no load connected. However, worth investigating in the future.

Neutral Wire Problem

The most likely scenario is a break, high-resistance connection, or grounding of the neutral wire somewhere in the run between the panel and the load. Even a small amount of leakage current on the neutral can cause issues with the contactor and breakers.

Contactor Failure

You've replaced the contactor, but it's always possible you received another defective unit. Test with a known good contactor if possible.

Safety First



Work De-energized

Always work on electrical circuits with the power completely OFF.

Lockout/Tagout

Use proper lockout/tagout procedures to ensure the circuit remains de-energized.

Qualified Electrician

If you're not comfortable with these procedures, consult a qualified electrician.

In Summary:

The shared neutral is almost certainly the issue. Methodically inspect the neutral wire and its connections. Pay close attention to the isolation test of the neutral - a ground fault on the neutral can wreak havoc. Good luck! Let me know what you find. Flag for review