Yes, you are correct. The feeder cables should likely be #8 AWG copper. Here's the breakdown: MCA (Minimum Circuit Ampacity)

24.9A. This is the minimum current the circuit is designed to handle continuously.

MOCP (Maximum Overcurrent Protection)

30A. This is the maximum size of the overcurrent protection device (breaker or fuses) allowed to protect the condensing unit.

Engineer's Design

40A feeder breaker, #10 AWG wire to the fused disconnect (with a 30A fuse).

Why #10 AWG is likely insufficient


Ampacity of #10 AWG Copper

According to NEC Table 310.16 (assuming typical installation conditions - 75°C terminations are typical), #10 AWG copper wire has an ampacity of 30 amps.
2.

Overcurrent Protection Coordination

The 40A breaker is intended to protect the feeder conductors supplying the disconnect switch with the 30A fuses protecting the condenser unit. The wire needs to be sized such that the feeder breaker will protect the wire.3.

Code Compliance (NEC)

The general rule is the ampacity of the conductor must be equal to or greater than the noncontinuous load plus 125% of the continuous load. If the load is continuous (operates for 3 hours or more), the wire must be sized so its ampacity is at least 125% of the load. The MCA is 24.9A, therefore the minimum conductor ampacity is 31.125A (24.9 x 1.25 = 31.125).

Why #8 AWG is likely appropriate



Ampacity of #8 AWG Copper

#8 AWG copper wire has an ampacity of 40 amps (again, assuming typical installation conditions - 75°C terminations are typical).

Protection

A 40A breaker will protect the #8 AWG wiring.

Therefore, you are correct, the #10 AWG is too small. You need to use #8 AWG copper to match the 40A feeder breaker.Important Considerations and Recommendations



Specific Installation Conditions

The ampacity of wires is affected by several factors including:

Termination Temperature Rating

Are the terminals rated for 60°C, 75°C, or 90°C? This affects the allowable ampacity. Use the correct column in NEC Table 310.16.

Number of Conductors in Raceway or Cable

If multiple current-carrying conductors are bundled together, their ampacity needs to be derated according to NEC Table 310.15(C)(1).

Ambient Temperature

High ambient temperatures also require derating the wire ampacity.

Voltage Drop

While ampacity is the primary concern, voltage drop calculations should also be performed to ensure the motor receives adequate voltage at the end of the run.

Aluminum Wire

If aluminum wire is used, the ampacity is lower than copper for the same gauge. #6 AWG aluminum would likely be required.

Engineer Approval

Always bring this discrepancy to the attention of the engineer of record and get their approval in writing before making any changes to the design. There may be a specific reason why they chose #10 AWG (though it seems unlikely given the information you've provided), and you want to ensure you're not inadvertently creating another problem.In summary, based on the information provided, the feeder cable should be upgraded to #8 AWG copper. Get this confirmed by the engineer and ensure all installation conditions are considered.
Disclaimer: I am an AI chatbot and cannot provide professional engineering advice. Always consult with a qualified and licensed electrical engineer for specific design and installation guidance. Flag for review