You're asking about sizing fuses for a main service disconnect, specifically a 100-amp service. While breakers and fuses both protect against overcurrent, their characteristics and sizing considerations can differ slightly. Here's a breakdown

General Principles:
Overcurrent Protection

Both fuses and breakers are designed to protect conductors and equipment from damage due to overcurrent (overloads or short circuits).

Interrupt Rating

A fuse or breaker must have an interrupt rating sufficient for the available fault current at its location. This is critical and can't be overstated. Fault current analysis is often required, especially in larger installations.

Continuous Load

In most cases, overcurrent devices (fuses or breakers) protecting circuits with continuous loads (loads expected to operate for 3 hours or more) must be sized no less than 125% of the continuous load. However, for a main service disconnect, this isn't usually the primary factor in sizing.
Sizing Fuses for a 100-Amp Main Service Disconnect

In a residential situation, the main fuse (or breaker) protects against a very rare event that could happen, such as a short circuit in the meter box. In that case, a fuse is rated to protect the wire itself, not the total load of the house. As such, it's pretty similar to sizing a breaker in that situation. If the wire going into the service can handle 100 amps, the fuse would also be rated for 100 amps, typically a Class H, K, or RK5.Here's how you would approach it:1. Service Size

You stated a 100-amp service. This is the starting point.2.

Conductor Ampacity

The main service conductors must be rated to carry at least 100 amps (for a 100-amp service). Refer to NEC Table 310.16 (or relevant table in your local code) to determine the required conductor size based on the ampacity. The conductor type (e.g., copper, aluminum) and insulation temperature rating affect the ampacity.
3.

Fuse Ampacity

The fuse ampacity should match the lower of the following two ratings:

The Service Size

100 amps in your example.

The Conductor Ampacity

If, for some reason, you have conductors with a lower ampacity than 100 amps (which would be a code violation), you must protect them at their ampacity. But assuming code compliance, this won't be the limiting factor.4.

Fuse Type (Important)

The type of fuse used matters:

Time-Delay Fuses

Time-delay fuses (e.g., Class RK1, Class RK5, Class J) are generally preferred for motor circuits (as you know) because they can withstand temporary inrush currents without tripping. However, for a main service disconnect, their main advantage is increased safety and potentially improved coordination with downstream overcurrent devices.

Non-Time-Delay Fuses

Non-time-delay fuses (e.g., Class H, Class K) are faster-acting. They are less tolerant of inrush currents.

For a main service disconnect, either time-delay or non-time-delay fuses can be used if they are properly sized and have an adequate interrupt rating. Time-delay fuses are often favored due to their improved coordination characteristics.5. Interrupt Rating (AIC)

This is absolutely critical. The fuse must have an Ampere Interrupting Capacity (AIC) rating sufficient to interrupt the maximum available fault current at the service entrance. The utility company can provide information on the available fault current at the service transformer. Choose a fuse with an AIC rating higher than the available fault current. Common AIC ratings are 10kA, 100kA, or 200kA. In a residential setting, 10kA may be sufficient, but this must be verified.

In your specific 100-amp service example (assuming properly sized conductors)

You would typically use a 100-amp fuse (or fuses, if using a fused disconnect with two fuses). The type of fuse would depend on factors like desired coordination and the load characteristics. However, common options are Class H, K, RK5, or J fuses. Most importantly, the fuse must have an AIC rating higher than the available fault current at the service entrance.

Key Differences from Breaker Sizing



Fuse Type

The type of fuse (time-delay vs. non-time-delay) has a more significant impact on performance than the selection of different types of residential circuit breakers.

Coordination

Fuse coordination (selectively coordinating fuses so that the fuse closest to the fault opens first) is often easier and more precise with fuses than with breakers, especially in more complex systems.

Interrupt Rating

This is crucial for both fuses and breakers.

Important Considerations



NEC and Local Codes

Always consult the latest edition of the National Electrical Code (NEC) and your local electrical codes for specific requirements in your jurisdiction. Local codes may amend or add to the NEC.

Qualified Electrician

Electrical work can be dangerous. It is always best to consult with and have the work performed by a qualified and licensed electrician. This is especially true for service entrance work.

Coordination Studies

For larger or more complex installations, a professional engineer or qualified electrician may perform a coordination study to ensure proper protection and selective tripping of overcurrent devices.*

Manufacturer's Instructions

Always follow the manufacturer's instructions for the installation and use of fuses and disconnect switches.

Disclaimer: This information is for educational purposes only and should not be considered a substitute for professional advice. Always consult with a qualified electrician for electrical work. Improper electrical work can be dangerous and may violate electrical codes. Flag for review