Okay, let's break down why your EE is likely adding the extra 25% and where you might be missing the connection. You're close, but there's a key detail in how the NEC handles air conditioning.Here's the probable reasoning and the connections your EE is likely making

1. The Interplay of Articles 430 and 440 IS the Key:
You are correct: Article 440 is specifically for air-conditioning and refrigerating equipment. You are also correct: 440.6 tells you to use 430.22, 430.24, and other sections of Article 430. This is the connection! Article 440 uses Article 430's calculation methods for many things. It doesn't replace Article 430 entirely; it references it. Your EE's 25% is likely coming from: 440.33(A) Air-Conditioning and Refrigeration Equipment. (A) General. _The branch-circuit conductors to which one or more hermetic refrigerant motor-compressors are connected shall have an ampacity not less than 125 percent of the rated-load current or branch-circuit selection current, whichever is greater, of the largest motor-compressor plus the sum of the rated-load currents or branch-circuit selection currents of the other motor-compressors and other loads served by the circuit._2. The 25% is for the LARGEST Motor-Compressor

The 25% does not have anything to do with motor overload protection. The 25% is to account for the motor-compressors inrush current when determining ampacity of the feeder wires.

3. Where the Confusion Lies

You might be focusing solely on the motor overload protection (the 175% you used for the OCPD) which is based on the motor's full-load current. Your EE is calculating the feeder ampacity required to serve that motor-compressor, considering both the continuous load and potential for starting inrush.

In summary, think of it this way



OCPD (Overcurrent Protective Device) (Circuit Breaker or Fuses)

This protects the motor itself from overloads and short circuits. You correctly used 175% for this based on the motor's nameplate data and the NEC allowance. This is about protecting the MOTOR.

Feeder Ampacity (Wire Size)

This ensures the wires feeding the entire air conditioning unit (and possibly other loads) are large enough to handle the continuous load plus the potential starting current surge of the motor-compressor. This is about protecting the WIRING.

Example

Let's say the HVAC compressor motor's nameplate says: Rated-Load Current (RLA): 20 amps1.

OCPD Calculation (You already did this)

20 amps 1.75 = 35 amps. You'd likely use a 35A or 40A breaker (depending on standard sizes and specific OCPD type).
2.

Feeder Calculation (What your EE is doing)

20 amps 1.25 = 25 amps (for the largest compressor). Then, you add any other loads connected to that same feeder.If the only load on the feeder is the compressor, you'd need a wire ampacity of at least 25 amps. You'd then size the wire accordingly (e.g., 10 AWG copper, assuming the 75-degree C column of Table 310.16).

In conclusion, your EE is almost certainly correctly interpreting and applying the NEC. It's not just about protecting the motor; it's about ensuring the entire circuit can safely handle the load, including the motor-compressor's starting current.To resolve your "circular conversation," try this:1. Acknowledge that you understand Article 440 references Article 430's calculation methods.
2. Ask your EE to specifically point to the NEC section requiring the additional 25% calculation for feeder ampacity. 440.33(A) is likely the answer.
3. If they show you that section, the light bulb should go on, and you'll understand the difference between motor overload protection and feeder ampacity requirements. Flag for review