You've got a good grasp of the situation and are thinking through it correctly! Here's a breakdown of why you're likely right and the key points to consider

Your Understanding is Correct:
Motor OCPD Sizing

You correctly calculated the maximum OCPD size per 430.52 for an inverse time breaker (250% of FLC = 77A).

Standard Size Breakers

You correctly identified that the standard breaker sizes that fall around this are 70A and 80A.

240.4(B) and Conductor Ampacity

You understand that 240.4(B) generally allows you to go to the next standard OCPD size if the conductor ampacity is sufficient. This is the critical point where the original poster's logic breaks down.

Conductor Ampacity Concerns

You're right to be concerned about the conductor size. #8 THHN copper (the most likely choice) is typically rated for 55A (75°C column of Table 310.16). Even a #6 THHN is usually only rated for 75A.

Why the Original Poster is Likely Incorrect

The original poster is probably relying solely on 240.4(B) without considering the requirement that the conductor ampacity must be equal to or greater than the selected OCPD size. If the conductor ampacity is less than 80A, they cannot use an 80A breaker, period.

The Correct Approach


Calculate Max OCPD

As you did, 250% of 30.8A = 77A.
2.

Determine Minimum Conductor Ampacity

Section 430.22 generally requires the conductors to have an ampacity of at least 125% of the motor's FLC (30.8A x 1.25 = 38.5A).
3.

Select Conductor Size

Choose a conductor size with an ampacity of at least 38.5A (from step 2) that is appropriate for the application (THHN in conduit is common). #8 THHN is likely to meet the ampacity but check your local code and application and may not if derating is needed.
4.

Apply 240.4(B) Carefully

If the conductor ampacity is

less than 77A (your calculated maximum), you cannot use an 80A breaker. You must use a 70A breaker, as this is the next lower standard size. If the conductor ampacity were, for some reason, greater than or equal to 77A (which is highly unlikely in this scenario), you could potentially use an 80A breaker if it's allowed by other code sections. However, that would be unusual.

Important Considerations and Exceptions



Motor Protection (Overload Relays)

The OCPD you are discussing is primarily for short circuit and ground fault protection. Motors also need overload protection (running overcurrent), which is typically provided by separate overload relays (heaters) in the motor starter. This is covered under 430.31 and following sections.

Specific Motor Type/Application

There are some exceptions in Article 430 that could allow for higher OCPD settings, but they are for specific motor types or applications (e.g., motors with specific locked-rotor characteristics, torque requirements, etc.). These are less common. Unless you have a specific reason to believe one of these exceptions applies, stick to the standard rules.

Derating

Always consider derating factors for conductor ampacity (ambient temperature, number of conductors in a conduit, etc.) as this could change your conductor selection.

Voltage Drop

Don't forget to calculate voltage drop to ensure it's within acceptable limits.*

Local Amendments

Always check for local code amendments, as they can sometimes modify the NEC requirements.

In conclusion, you are very likely correct. Unless there are very specific circumstances and a good reason to use a larger breaker, the 70A breaker is the correct choice based on your information. Flag for review