Yes, your calculations and component selection seem generally correct based on the information provided, but it's crucial to verify your assumptions against the specific installation conditions and relevant codes (primarily the NEC - National Electrical Code in the US). Let's break it down

Your Calculation and Reasoning:
293 amps x 1.25 = 366.25 amps

This is the standard calculation to account for continuous loads. NEC Section 430.22(A) requires that the conductors supplying a motor be rated at not less than 125% of the motor's full-load current. Since you stated that the "full package current" is 293A, it's likely that this includes the motor.

500 kcmil copper THHN @ 75 degrees = 380 amps

This is correct, based on NEC Table 310.16.

400 amp Circuit Breaker

This is also correct, following NEC Section 430.52, you could potentially use a circuit breaker size up to 250% of the full load current. In this case, 293A 250% = 732.5A. Since this is the maximum breaker size allowed. You could use a 400 amp breaker as long as it can handle the motor inrush current.

Important Considerations & Verification Needed


"Full Package Current" Definition

This is the most critical point. You MUST be absolutely sure what "full package current" means in the context of your air compressor. Does it ONLY include the motor? Or does it include other loads within the compressor package (e.g., control circuits, heaters, fans)? If it's only the motor's full-load current, then your calculations are probably valid. If it includes other loads, you need to identify those individual loads, add them together, apply the 125% factor to the motor's current only, and then add the non-continuous loads. Look at the nameplate of the air compressor. It should provide a "minimum circuit ampacity" (MCA) or a "maximum overcurrent protection" (MOCP) rating. These numbers are very useful and should be considered. The MCA number will usually be very close to the 125% factor you already used.2.

Motor Inrush Current

Motors draw a significantly higher current when starting (inrush current). This is essential for breaker selection. You'll need to determine if the 400 amp breaker will hold through the motor start. If the motor is marked with a Code Letter indicating its locked-rotor current, you can refer to NEC Table 430.7(B) and 430.52 to determine the appropriate breaker size. The motor's inrush current is a temporary surge, but the breaker must be able to withstand it without tripping. If a 400 amp breaker keeps tripping, you may need to increase it.
3.

Voltage Drop

Check for voltage drop over the length of the wire run. Excessive voltage drop can damage the motor and other equipment. The NEC recommends limiting voltage drop to 3% for power circuits and 5% overall. Use a voltage drop calculator (many are available online) to ensure your 500 kcmil conductors are adequate for the distance of the run at 293 amps.4.

Installation Method

The ampacity of conductors also depends on how they are installed (e.g., in conduit, direct burial, in free air). NEC Table 310.15(B)(16) provides ampacities based on installation method and insulation type (THHN, in your case). If the conductors are bundled with other conductors, derating factors may apply according to NEC Table 310.15(B)(3)(a). Make sure to account for all applicable derating factors.
5.

75 Degree vs. 90 Degree Terminals

The terminals on your circuit breaker and the air compressor itself may have temperature ratings (typically 60°C or 75°C). Even though THHN is rated for 90°C, you MUST use the ampacity column in Table 310.16 corresponding to the lowest* temperature rating of the terminals. Assuming you are using 75°C rated equipment, this calculation is correct.6.

Service Factor

If the air compressor motor has a service factor greater than 1.15, there could be adjustments to the calculations.
7.

Local Codes and AHJ (Authority Having Jurisdiction)

Always check with your local electrical inspector or the AHJ. They have the final say and may have additional requirements.

Recommendation


CONFIRM the "Full Package Current" definition. Look at the air compressor's nameplate.2. Calculate voltage drop.3. Review all applicable NEC sections (especially Article 430 for Motors) and your local electrical codes.4. Consult with a qualified electrician if you are unsure about any of these steps. Electrical work is dangerous, and mistakes can be costly and even life-threatening.By carefully considering these factors and consulting with qualified professionals, you can ensure a safe and code-compliant installation for your air compressor. Flag for review