Okay, let's break down the situation and outline the steps you need to take to ensure proper coordination and safety.

Understanding the Problem

You have a main fused disconnect supplying a meter bank, which in turn feeds multiple residential panels. The available fault current at the main disconnect is a significant 16,606 amps. You need to make sure that:1. The fuses in the main disconnect can adequately interrupt the available fault current.2. The main fuses and the 100A residential breakers are properly coordinated. This means that under fault conditions, you want the downstream 100A breaker to trip before the upstream main fuse blows. This minimizes the extent of the outage.

Steps to Take1. Verify Fuse Interrupting Rating



Check the Fuse's AIC Rating

Look at the fuse's marking. Class T fuses are inherently current-limiting and have a very high interrupting rating (typically 200,000 amps or more). However, you MUST verify this on the fuse itself!



Compare AIC Rating to Available Fault Current

The fuse's AIC rating must be greater than or equal to the available fault current at its location (16,606 amps in your case). Since Class T fuses typically have a 200kA or higher AIC, they most likely meet this requirement.

If the fuse's AIC rating is less than the available fault current, you MUST replace the fuse with one that has a sufficient AIC rating. This is a critical safety issue.2.

Gather Fuse and Breaker Information



Fuse Data

Obtain the manufacturer's time-current characteristic (TCC) curve for the specific Class T fuse you are using. This is a graph showing how long it takes the fuse to blow at different levels of overcurrent. You can usually find this information on the fuse manufacturer's website. Note the fuse's amp rating.

Breaker Data

Obtain the TCC curves for the 100A residential breakers. You'll need the manufacturer and model number of the breakers. Again, the manufacturer's website is usually the best source.

Software: Use software such as SKM, EasyPower, or ETAP. All have a cost associated to them.3.

Coordination Study (Time-Current Coordination Analysis)



Plot the TCC Curves

Create a graph (either manually or using electrical design software) that plots the TCC curves for: The Class T fuse in your main disconnect. The 100A breakers in the residential panels.

Analyze the Curves

The goal is to ensure that the 100A breaker curve is below and to the left of the Class T fuse curve over a range of fault currents. This means that for a given fault current, the 100A breaker will trip faster than the Class T fuse. You need to maintain a coordination margin (typically a time gap) between the curves to account for manufacturing tolerances and other variables.

Coordination Margin

The National Electrical Code (NEC) doesn't specify a precise coordination margin, but a generally accepted rule of thumb is to maintain a separation of at least 0.1 seconds (100 milliseconds) between the curves in the region of expected fault currents. This ensures selective coordination, where the downstream device clears the fault before the upstream device.
4.

If Coordination is Not Achieved



Change Breaker Type

Consider using a different type of 100A breaker with a faster tripping characteristic (e.g., an inverse-time breaker) if possible. However, this may not always be practical or cost-effective. Consult with the breaker manufacturer for recommendations.

Change Fuse Size (Carefully)

You might be able to slightly reduce the size of the Class T fuse to improve coordination.

However, this is a very delicate decision! You must ensure that the smaller fuse can still handle the normal load current of the meter bank without nuisance tripping. A load study is essential. Current Limiting Devices

Consider adding current-limiting reactors or fuses upstream of the residential panels to reduce the fault current seen by the 100A breakers. This will help the breakers trip faster and improve coordination with the main fuse. This is often a more expensive solution.

Zone Selective Interlocking (ZSI)

In more complex systems, ZSI can be used to coordinate breakers. This involves communication between the breakers to allow the closest breaker to the fault to trip first. This is typically used in larger commercial or industrial installations and is probably not practical for residential panels.

Consult a Professional

If you're unsure how to proceed, hire a qualified electrical engineer or a coordination study specialist. They have the expertise and software to perform a thorough analysis and recommend the best solution.5.

Documentation



Document all calculations, TCC curves, and decisions made. This is important for future reference and for demonstrating compliance with electrical codes.

Important Considerations



NEC Compliance

Make sure your design complies with all applicable National Electrical Code (NEC) requirements, especially those related to overcurrent protection and coordination.

Qualified Person

This type of analysis should only be performed by a qualified and experienced electrician or electrical engineer.

Safety First

Always prioritize safety. Improper coordination can lead to equipment damage, fires, and electrical hazards.

Load Study: Complete a load study to determine the actual load on each of the ten residences. This can impact the size of the main fuse and whether or not selective coordination can be achieved. Utility Requirements: Determine if there are any requirements or standards required by the local utility.

In summary: Start by verifying the interrupting rating of your Class T fuses. Gather the TCC curves for both the fuses and the breakers. Perform a coordination study, either manually or with software. If coordination isn't achieved, explore the options outlined above. Document everything. When in doubt, consult a qualified professional. Flag for review