You've correctly calculated the minimum ratings according to the excerpts you provided. However, you're missing the purpose of each device. Motor Branch Circuit Protection (31.3.1)

This protects the motor itself from overloads and short circuits. It's deliberately sized higher than the motor's FLC to allow for the inrush current during motor starting. Motors draw a very high current (typically 6-8 times FLC) for a short period during startup. If the branch circuit protection was too low, it would nuisance trip/blow every time the motor started. This is why the 175% (fuse) or 250% (breaker) allowance is crucial.

Disconnect Switch (30.2.1 and 30.
2.2(c))

This is primarily for safety and maintenance. It provides a means to safely de-energize the circuit for servicing the motor and associated equipment. Its sizing is based on the continuous load it will carry. You are adding the transformer load to the motor load to determine the size.

The Key Difference: Overload vs. Short Circuit Branch Circuit Protection (Fuses/Breakers)

Primarily protect against short circuits and overloads. The high percentage allowance (175%/250%) is only for overloads. Fuses and breakers will still trip very quickly on a short circuit, which is a far more serious fault condition. A short circuit could cause instantaneous damage or start a fire.

Disconnect Switch

Primarily to disconnect power for safety. It's rated to carry the continuous load and handle occasional overloads within its rating. It may or may not have built-in overload protection (depending on whether it's a fused or non-fused disconnect). A fused disconnect does provide overload protection, but it is sized for both disconnect and overload protection, not just overload protection like the motor branch circuit protection.

Why the Smaller Disconnect Doesn't Always Trip First

You're right; in some overload scenarios, the disconnect's fuse could trip before the branch circuit fuse. However, consider these factors:1. Fuse Time-Current Characteristics

Both fuses have a time-current curve. A smaller fuse will trip faster for a given overload. However, the shape of the curve matters. A dual-element, time-delay fuse (often used for motor circuits) is designed to withstand temporary overloads (like starting inrush) without tripping, while still providing fast protection against higher fault currents.2.

Coordination (Selective Coordination)

Ideally, electrical systems are designed for selective coordination. This means that the nearest overcurrent protective device to a fault should trip, minimizing the impact on the rest of the system. In a well-coordinated system, the branch circuit protection (70A fuse in your example) would be faster for higher fault currents than the 60A fuse in the disconnect. This is achieved by carefully selecting the fuse types and sizes based on their time-current characteristics.3.

Purpose and Priority

The branch circuit protection's primary purpose is to protect the motor. While the disconnect can provide some overload protection, its primary purpose is safe disconnection. It’s acceptable for the motor branch circuit protection to trip first in many overload scenarios, as the motor is the most valuable component at risk.

In Summary

You are missing the fact that the Branch Circuit Protection, especially when using a Dual-Element Time-Delay Fuse, is designed specifically for a motor circuit and should be able to handle large overloads for short periods of time (IE: Motor In-Rush). This is not true for all circuits or devices, and is why the Branch Circuit protection is so high. Flag for review