Okay, let's break down this AC unit circuit and apply the NEC principles. You're right to question the sizing, as it's not always intuitive.

Key Information



Source

3-Pole, 100A fused switch in distribution board (with 90A fuses installed).

Load

AC Unit. FLA (Full Load Amps): 40A MCA (Minimum Circuit Ampacity): 50A Unfused 3-Pole 100A disconnect at the AC unit.

Answering Your Questions


Continuous Load and the 125% Factor



Yes, HVAC equipment is often considered a continuous load. The NEC defines a continuous load as a load that is expected to operate at its maximum current for 3 hours or more. HVAC systems usually fit this definition. The MCA should include the 125% factor for continuous loads. The MCA is the minimum ampacity of the circuit conductors. It's intended to account for the continuous load and provide some margin for other things. Double-Check the Calculation

To verify, let's see if the MCA of 50A aligns with a continuous load calculation based on the FLA: 40A (FLA) 1.25 = 50A This matches the MCA provided.
2.

Wire Ampacity and the 90A Fuse



No, you do not necessarily need a wire ampacity of at least 90A. This is where the protection and circuit conductor ampacity rules need to be coordinated. Here's how the NEC applies: NEC 440.22(A)

This section directs you to use the larger of: 125% of the AC unit's compressor rated-load current (RLA), or The AC unit's full-load current (FLA) Add to that the full-load current of all other motors/loads supplied by the circuit. In your case, MCA is the design, so it would be 50A.

NEC 240.4(B) Next Higher Standard Overcurrent Device Rating typically doesn't apply to HVAC equipment. Article 440 takes precedence. This is why the 90A fuse seems oversized. NEC 440.6(A) Overcurrent protection requirements for single motor-compressor equipment. (1) Use the branch-circuit short-circuit and ground-fault protective device shall be capable of carrying the starting current of the motor. NEC 440.22 Overcurrent Protection. Motor-compressor branch-circuit overcurrent protection shall be selected in accordance with 440.22(A) or (B), as applicable. (A) General. The motor-compressor branch-circuit overcurrent device shall be capable of carrying the starting current of the motor. A protective device having a trip current exceeding 175 percent of the motor-compressor rated-load current or branch-circuit selection current, whichever is greater, shall be permitted where the starting current of the motor is not sufficient to trip the protective device. So, you would need to verify the AC unit specifics on the equipment itself. The Wire ampacity requirements



NEC 310.15(B)(16) Allowable Ampacities of Insulated Conductors this is your starting point, but it is subject to the derating factors in 310.15(B)(1) through (7). Select a conductor with an ampacity equal to or greater than the AC unit's MCA. The conductors must be protected at their ampacity per the device protection requirements.Summary


Verify the MCA (Minimum Circuit Ampacity)

It seems correct at 50A based on 40A FLA 1.25 = 50A.
2.

Conductor Sizing

Choose conductors with an ampacity of at least 50A after any derating for ambient temperature, conduit fill, etc. Use 75°C termination columns for equipment.3.

Overcurrent Protection

The 90A fuses seem oversized. Verify AC unit specs for correct overcurrent protection, not to exceed 175% of RLA or branch-circuit selection current, whichever is greater.

Recommendation



Coordinate with the AC Unit Specs

The most important step is to review the AC unit's installation manual or nameplate. This document will likely specify the maximum overcurrent protection allowed for the unit.

Recalculate

Armed with the manufacturer's recommended maximum overcurrent protection, double-check the calculations to ensure compliance with the NEC and the unit's specifications.

Consider Engineering Review

Given the potential for incorrect protection, an engineering review of the design would be beneficial.

Important Considerations



Disconnect Switch (at AC Unit)

The 100A unfused disconnect switch at the AC unit is simply a means of disconnect for servicing the equipment. It doesn't provide overcurrent protection. The overcurrent protection is provided by the fuses in the panel. The disconnect switch only needs to be rated equal to or greater than the unit's MCA.

Harmonics

Consider potential harmonic currents on the circuit, especially if the AC unit uses variable frequency drives (VFDs). You may need to upsize the neutral conductor and use appropriately rated equipment.

Disclaimer:* I am an AI Chatbot. This information is for general guidance only and does not constitute professional electrical engineering advice. Always consult with a licensed electrical engineer and follow the latest edition of the NEC for your specific application and jurisdiction. Flag for review