Okay, this sounds like a frustrating situation with several potential issues compounding the problem. Here's a breakdown of what's likely happening and what steps to take

Analysis of the Symptoms
Low Voltage Overall

198V phase-to-phase at the service entrance is already low for a 208V system. This is a significant starting point, and the utility company should be notified. They need to verify their transformer is providing adequate voltage.

Voltage Drop on the Roof

A further drop to 187V, 180V, and 180V on the roof is unacceptable. This indicates a serious voltage drop issue somewhere in the wiring between the service entrance and the rooftop unit.

Unbalanced Voltages

The phase-to-phase voltages are significantly unbalanced (187V vs. 180V vs. 180V). This is a strong indicator of a failing connection, high resistance in one or more phases, or potentially a winding issue in the compressor motor.

Voltage Drop Under Load

The L2-L3 voltage dropping rapidly when the motor starts is critical. This confirms the problem is load-dependent and likely due to a high resistance connection that worsens under load.

Lack of Ground Wire

The absence of a ground wire in the conduit is a serious safety hazard and a code violation. The equipment is not properly grounded.

High voltage to ground with no ground wire

High voltage to ground on the chassis is dangerous and needs to be resolved before further testing.

Troubleshooting Steps1. Safety First! Lockout/Tagout

Ensure the disconnect switch feeding the RTU is locked out and tagged out. Never work on energized equipment unless absolutely necessary and with proper PPE.

Verify Power is Off

Double-check with a meter that the power is indeed off at the RTU before opening any panels or touching any wires.

Electrical Hazard

The voltage to ground readings, combined with the lack of a proper ground wire, create a dangerous touch potential. Treat the unit with extreme caution until properly grounded.2.

Document Everything Record all voltage readings (L-L, L-N, L-G) at each point in the system. Write them down! Include the date and time, and what equipment was running (or not running) when you took the readings. Take pictures of the wiring, connections, disconnect, and any other relevant details.3. Check the Disconnect (Again!) Visual Inspection

Even if it's new, visually inspect the disconnect. Look for signs of overheating (burnt or discolored terminals, melted plastic).

Tightness

Verify all connections in the disconnect are tight. Use a properly insulated screwdriver and apply firm pressure. Don't overtighten.

Contact Condition

If possible, with the power off, inspect the contacts inside the disconnect. Are they clean and making good contact?4.

Trace the Wiring From the Breaker to the Disconnect

Follow the wires from the breaker in the panelboard all the way to the disconnect. Look for any splices, junction boxes, or other points where the wiring could be compromised.

Conduit Condition

Inspect the conduit for damage. Dents, cracks, or other damage could indicate damaged wiring inside.

Inside the RTU

Once inside the RTU, carefully trace the wiring from the disconnect to the compressor contactor and other components.5.

Voltage Drop Testing (Under Load) If safe to do so after the unit is grounded, and with appropriate PPE, start the compressor. Carefully measure the voltage at various points in the circuit while the compressor is running. This will help pinpoint the location of the voltage drop. Measure voltage across the disconnect (line-side to load-side on each phase). A significant voltage difference indicates a problem within the disconnect itself. Measure voltage at the compressor contactor terminals. Be extremely cautious when working with energized equipment!6. Resistance Testing (Power Off) After de-energizing and locking out the circuit, use an ohmmeter to check the resistance of each phase of the wiring. Measure the resistance of each conductor (L1, L2, L3) from the breaker to the disconnect. Measure the resistance through the disconnect (line-side to load-side) with the disconnect closed. You should see very low resistance (ideally near zero ohms). Compare the resistance values. A significantly higher resistance on one phase compared to the others indicates a problem in that conductor. Compressor Windings

Check the resistance of the compressor motor windings. Consult the compressor manufacturer's specifications for the expected resistance values. Open windings or shorted windings can cause unbalanced voltages and motor failure.7.

Address the Grounding Issue Immediately Install a proper ground wire in the conduit. This is essential for safety. Bond the RTU chassis to the ground wire. Verify the grounding system is effective. Use a low-resistance ohmmeter to measure the resistance between the RTU chassis and the building's grounding electrode system. It should be very low (less than 1 ohm).

Possible Causes & Solutions Utility Voltage Issue

The low incoming voltage is a major concern. Contact the utility company and report the problem.

Loose Connections

Loose connections are the most common cause of voltage drops and unbalanced voltages. Tighten all connections at the breaker, disconnect, contactor, and compressor terminals.

Corroded Connections

Corrosion can increase resistance. Clean any corroded connections with a wire brush and apply an anti-oxidant compound.

Damaged Wiring

Damaged or undersized wiring can cause voltage drops. Replace any damaged wiring. Verify the wiring is properly sized for the load and the distance.

Failing Disconnect

The disconnect may have internal damage or worn contacts. Replace the disconnect if it's suspect.

Compressor Motor Issues

A failing compressor motor with winding shorts or open circuits can cause unbalanced voltages and draw excessive current. Test the motor windings. If the motor is bad, replace it.

Contactor Problems

The contactor may have burned or pitted contacts, leading to high resistance. Replace the contactor if it's suspect.

Important Considerations Licensed Electrician

This work involves electrical systems and should be performed by a qualified and licensed electrician.

HVAC Technician Collaboration

Work closely with the HVAC technician to diagnose the compressor motor and other components.*

Code Compliance

Ensure all work is performed in accordance with local electrical codes.

In Summary

You have a combination of problems: low incoming voltage, voltage drop in the wiring, unbalanced voltages, a lack of grounding, and a heatwave putting extra stress on the system. Address the grounding immediately. Then, systematically troubleshoot the wiring, starting with the disconnect and working back towards the breaker. Contact the utility company about the low voltage. If the wiring and connections are good, the compressor motor itself may be the problem. Good luck! Flag for review