This is a complex scenario, and your concerns are valid. Let's break down the issues and address your questions

Understanding the Problem

You're trying to achieve a 65kA SCCR (Short-Circuit Current Rating) for your panel while protecting 14 motor starter circuits with a 10kA SCCR. The challenge is to protect the low-rated starters from fault currents exceeding 10kA without causing nuisance trips on the upstream breaker.

Addressing Your Questions



1. How would sizing be done to ensure they are coordinated to trip in the correct sequence based on the fault current?

This is the crucial and most complex aspect. You need a proper coordination study to ensure selective coordination. Here's the general process, but always consult with a qualified electrical engineer experienced in coordination studies: Obtain the Time-Current Curves (TCCs)

Collect the TCCs for all protective devices: Upstream 400A adjustable breaker at the MCC (with its various trip settings if possible). Proposed fuses (both the main fuses – if you use them – and the branch circuit fuses). Branch circuit breakers. Motor starter overloads (although these are for overloads, they can contribute to tripping behavior). Consider the prospective fault current at the location of the panel. This will be determined by the utility/service entrance and the transformer impedance.

Plot the Curves

Plot all the TCCs on the same graph. The Y-axis is time (usually in seconds or milliseconds), and the X-axis is current (in amps).

Analyze Coordination

The goal is to ensure that, for any fault current: The branch circuit fuses trip before the branch circuit breakers. This protects the starter. The branch circuit breakers trip before the main fuses (if present) and the upstream MCC breaker. This isolates the fault and minimizes disruption.

Sizing Considerations



Fuses

Fuses are typically sized at 125-200% of the motor's full-load amps (FLA). The exact percentage depends on the motor characteristics and the fuse type. You might need to adjust the fuse size to achieve proper coordination, but make sure it still provides adequate motor protection. Fast acting fuses are commonly used for this application.

Breakers

Breakers are sized for motor starting inrush current.

Adjustable MCC Breaker

The trip settings of the MCC breaker will significantly impact coordination.

Prospective Fault Current

Understanding this is the first step.

Consult a Coordination Study

A professional coordination study uses software to perform these calculations, analyze the results, and recommend the optimal device settings and sizes. This is critical for ensuring safety and reliability. A study should also include the impact of motor starting, which creates temporary current surges.

Why Coordination is Difficult



Multiple Devices

You have many devices in series, each with its own TCC.

Starter SCCR

The 10kA starter SCCR is your most significant constraint.

Motor Starting

Inrush current during motor starting can cause nuisance trips if devices aren't properly selected and coordinated.

2. If I used a main breaker as a disconnect that was rated for 65kA SCCR, would I still need the fuses at all? Not Necessarily, IF... Using a main breaker rated for 65kA SCCR as a disconnect could eliminate the need for the branch circuit fuses. However, this is only true if the branch circuit breakers alone provide sufficient protection to the 10kA SCCR starters and you can achieve selective coordination with the upstream MCC breaker. The Key Condition: Series Ratings

You would need to establish a UL Listed series rating between the main breaker in your panel and the branch circuit breakers. A series rating confirms that the combination of the main breaker and the branch circuit breakers can safely interrupt fault currents up to 65kA, even though the individual branch breakers might have a lower interrupting rating. The series rating must explicitly cover the types of breakers you plan to use.

How Series Ratings Work

The higher-rated main breaker limits the current to the lower-rated branch breakers during a fault.

Challenges with Relying on Series Ratings



Limited Combinations

Series ratings are specific to certain combinations of breakers from the same manufacturer. You can't mix and match breakers from different brands and assume a series rating exists.

Upstream Breaker Settings

The series rating is dependent on the settings of the upstream main breaker. Any change in these settings may invalidate the series rating.

Verification

You must verify the existence of a UL Listed series rating in the manufacturer's documentation. Don't assume it exists!

Why Fuses Might Still Be Preferred



Simplicity

Fuses are simpler to understand and apply for coordination in many cases.

Coordination

Fuses can offer better coordination characteristics than breakers in certain scenarios.

Certainty

A correctly selected and coordinated fuse is a very reliable way to protect the starters and achieve the desired SCCR.

Flexibility

Fuses provide greater flexibility in selecting components from different manufacturers.

Cost

Fuses may be a more cost-effective solution.

Recommendations


Consult with a Qualified Electrical Engineer

This is essential. A professional engineer can perform a coordination study and recommend the best approach. This is especially important because of the low 10kA SCCR of the motor starters and the high available fault current.2.

Consider Fuses

Given the 10kA SCCR of the starters, using fuses in each branch circuit is often the most straightforward and reliable solution.3.

Verify Series Ratings (If Applicable)

If you choose to rely on series ratings, carefully verify the existence of a UL Listed series rating between your main breaker and your branch circuit breakers. Get this in writing from the manufacturer.4.

Document Everything

Keep detailed records of your calculations, coordination study, and the series ratings (if applicable).5.

Clearly Label

Label the panel to indicate that it relies on a specific series rating (if that's the approach you take) and include the model numbers of the breakers used.

In summary: While technically possible to use only a main breaker with a series rating and no fuses, it's usually more straightforward and reliable to use fuses in each branch circuit. Always consult a qualified electrical engineer to perform a coordination study. Flag for review