You're right to be scratching your head! There's definitely something that needs further investigation. Here's a breakdown of what's likely happening and how to approach it

Why #4 CU on a 250A Breaker is Wrong (Normally)
Overcurrent Protection

Breakers are designed to protect the conductors feeding the load. A #4 CU conductor has an ampacity (allowable current carrying capacity) well below 250A. Putting it on a 250A breaker means the breaker wouldn't trip in time if the #4 wire were overloaded, potentially leading to overheating and a fire. The exact ampacity depends on insulation type and installation method, but it's likely in the 70-85A range.

NEC (National Electrical Code) Requirements

The NEC dictates conductor ampacity based on breaker size. Generally, you need a conductor rated to at least the breaker's amperage (with some exceptions).

Possible Explanations (And How to Investigate Each)


Incorrectly Sized Breaker (Most Likely)



Possibility

Someone replaced the original breaker with an oversized one at some point, perhaps because they thought they were experiencing nuisance tripping (but didn't address the underlying problem). Or maybe they made a mistake during a replacement.

How to Check



Original Drawings

Find the original electrical drawings (one-line diagrams, panel schedules) for the switchgear. They should specify the correct breaker size for that circuit.

Equipment Documentation

Look for the chiller's original nameplate data and installation instructions. These documents often specify the required overcurrent protection size.

Consult a Qualified Electrician/Engineer

If the documents aren't available or clear, bring in a professional electrical engineer or master electrician. They can perform a load calculation and determine the correct breaker size for the chiller.2.

Motor Circuit Conductor Rules & Inverse Time Breaker (Less Likely, but Possible)



Explanation

NEC Article 430 (Motors, Motor Circuits, and Controllers) allows for some flexibility in conductor sizing and overcurrent protection for motors. This is because motors have high inrush currents when starting. An "inverse time" breaker is designed to allow short-duration overloads, such as a motor starting current, to pass without tripping.

Why This Might Apply

A chiller uses a motor to run the compressor.

Important Considerations



Specific NEC Requirements

The NEC has very specific rules for motor circuit conductor sizing (typically allowing for 125% of the motor's full-load amps (FLA) to be used). The overcurrent protection also has specific maximum settings (often 150-250% of the motor FLA, depending on the type of breaker). This is a complicated area of the code.

Is it Still Correct?

Even if it was originally compliant with motor circuit rules, upsizing the chiller likely voids that compliance. A larger chiller will have a higher FLA, requiring different conductor and breaker sizes.

How to Check



Motor Nameplate

Find the original chiller motor's nameplate. It will list the FLA (Full Load Amps).

Calculations

Carefully perform motor circuit calculations according to NEC Article 430 using the NEW chiller's motor FLA. Determine the minimum conductor size and maximum overcurrent protection allowed. A qualified electrician is strongly recommended for this.

Breaker Type

Confirm the breaker is indeed an inverse time breaker suitable for motor circuits. Look for markings like "HACR" or "Motor Circuit Protector."

Documentation, Documentation, Documentation

Any motor circuit setup must be properly documented to justify the oversized breaker.3.

Continuous Load Derating (Less Likely in this Specific Scenario)



Explanation

If the chiller is considered a "continuous load" (operates at maximum current for 3 hours or more), the NEC requires conductors and overcurrent protection to be derated. This increases the required conductor and breaker size, making the #4 wire even more problematic.

Likelihood

Chillers are often considered continuous loads.

How to Check

Determine if the chiller's operating profile qualifies it as a continuous load. Consult the NEC for derating requirements.

Steps to Take


Safety First

Do not operate the chiller with the incorrectly sized wiring. This is a fire hazard. De-energize the circuit until the issue is resolved.2.

Gather Information

Collect all available documentation (electrical drawings, chiller specs, motor nameplates).3.

Qualified Professional

Engage a qualified electrician or electrical engineer. They can perform load calculations, verify compliance with the NEC, and recommend the correct breaker and conductor sizes for the new chiller.4.

Corrective Action



Replace the Breaker

If the breaker is oversized, replace it with the correct size as determined by your calculations and the NEC.

Upsize the Wiring

Replace the #4 CU wire with #2 CU, as you planned, to match the new chiller's requirements.

Check Other Circuits

Since you've found this issue on one circuit, thoroughly inspect all other circuits in the switchgear to ensure they are properly sized and protected.5.

Document Everything

Keep detailed records of all calculations, changes made, and the rationale behind the choices. This is essential for future maintenance and inspections.

In Summary:

It's highly probable that the 250A breaker with #4 CU wiring is a code violation and a safety hazard. The most likely cause is an incorrectly sized breaker. However, you must* thoroughly investigate all possibilities and consult with a qualified professional to ensure the entire system is safe and compliant with the NEC. Do not assume the other circuits are correct just because they look similar.Good luck! This is a serious situation, and getting it right is crucial. Flag for review