Okay, let's break down this situation and analyze your thoughts. You're on the right track with several points.

Here's a breakdown of the problem and possible solutions



The Problem: "Buzzing" Wires in Conduit on Startup This usually indicates vibration due to loose wires, excessive inrush current causing the wires to momentarily pull against each other or the conduit, or (less likely) a harmonic issue. Information We Have

Relatively new installation (less than 6 months). Complaint: Wires "buzz" in conduit on condensing unit startup. Apparent wiring: #10 THHN on a 30A breaker. You haven't been to the site yet.

Your Thoughts & My Commentary


#10 THHN is fine

Generally,

correct. #10 THHN is typically rated for 30 amps per NEC code (National Electrical Code), provided the conditions of use permit. Important Qualification

You need to verify all conditions. NEC 310.15 addresses ampacity. This includes factors like:

Termination Temperature Rating

Ensure the equipment terminals are rated for 75°C. If so, #10 THHN is likely fine. However, if terminations are only rated for 60C, you'd need to use the 60C ampacity for #10, which is 25A.

Number of Current-Carrying Conductors in Raceway

More than 3 current carrying conductors in the same conduit requires ampacity adjustment factors.

Ambient Temperature

High ambient temperatures require ampacity correction factors.

Voltage Drop

While not a safety issue, excessive voltage drop on startup can contribute to increased inrush current and put more stress on the system.2.

Breaker needs to be the correct brand for the panel



Absolutely Correct! Using the wrong breaker in a panel is a significant safety hazard and a code violation. The breaker MUST be listed (UL, etc.) and designed to be used in that specific panel. Different brands have different bus bar configurations and tolerances. Follow up

The service tech needs to confirm the correct breaker is installed.3.

"Soft Start" Kit



Good Idea! A soft start kit significantly reduces the inrush current on startup. This can: Eliminate or reduce the "buzzing" wire phenomenon. Reduce stress on the compressor motor, potentially extending its life. Reduce voltage sag on the electrical system during startup.

Steps to Take


Go to the Site & Investigate Thoroughly

You can't rely solely on the service tech's initial report. You need to see this for yourself.2.

Verify Wiring and Components



Wire Size and Type

Confirm it is #10 THHN. Look at the markings on the wire insulation.

Breaker

Confirm it is the correct brand and model for the panel. Check the panel's labeling. Verify the breaker rating (30A).

Connections

Inspect all connections (at the breaker, at the disconnect, and at the condensing unit). Look for: Loose connections Corrosion Burn marks (signs of overheating) Proper torque on connections (use a torque screwdriver/wrench to verify).

Conduit

Inspect the conduit run. Make sure it's properly supported. Are there any sharp bends or kinks that could be stressing the wires?

Grounding

Verify proper grounding of the condensing unit and the electrical system.3.

Measure Voltage

Check the voltage at the condensing unit under load (while the unit is running). Note the voltage at startup. Significant voltage drop can contribute to the problem.4.

Consider Other Possible Causes



Contactor Chatter

A failing contactor in the condensing unit can sometimes cause a buzzing or humming sound. Rule this out.

Loose Mounting

Check the condensing unit's mounting. If it's loose, the vibration of the compressor could be amplified and transmitted through the electrical connections.

Refrigerant Charge

While less likely to cause buzzing in the wires, an incorrect refrigerant charge can put excessive strain on the compressor, which could indirectly contribute to vibration.

Recommendations (In Order of Priority)


Verify Breaker Correctness

This is the most important. A wrong breaker is a safety hazard. Replace it immediately if incorrect.2.

Inspect Wiring & Connections

Thoroughly inspect all wiring and connections. Tighten any loose connections, replace corroded connections, and ensure proper torque.3.

Consider a Soft Start Kit

Even if the above steps resolve the buzzing, a soft start kit is a good idea to reduce stress on the compressor and electrical system.4.

Re-evaluate Wire Size if Necessary

If after investigation, you find conditions that require derating the ampacity of #10 THHN (e.g., more than 3 current-carrying conductors in conduit, high ambient temperature, or low temperature rated terminations), consider upsizing the wiring to #8 AWG or larger.

Important Safety Considerations



De-energize the circuit before working on any electrical components. Use proper PPE (Personal Protective Equipment), including safety glasses and insulated gloves. If you're not comfortable working on electrical systems, hire a qualified electrician.

By thoroughly investigating the situation and addressing the potential causes, you should be able to resolve the buzzing wire issue and ensure the safety and reliability of the condensing unit's electrical system. Good luck! Flag for review