Okay, let's break down this tap rule scenario and address your concerns. You've correctly identified the relevant code section and the key considerations.

Understanding 240.21(B)(4): Feeder Taps Not Over 10 Feet Long

This rule allows for short feeder taps with specific conditions: The tap conductor's ampacity is not less than

The calculated load of the circuit. The rating of the overcurrent protective device at the termination of the tap conductors.

1/10th the rating of the overcurrent device protecting the feeder. The tap conductors must be enclosed in a raceway or otherwise protected from physical damage. The tap conductors cannot extend beyond the equipment they supply.

Your Situation Analysis Feeder

Protected by 100A RK5 fuses.

Tap 1

#4 THHN to 60A fused disconnect.

Tap 2 & 3

#10 THHN to 30A fused disconnects.

Tap Length

Less than 10 feet.

Tap Location

Leaving the enclosure in which the tap was made

Addressing Your Concerns1. 1/10th Requirement

You've correctly observed that the #10 THHN has an ampacity of 40A (per Table 310.16), which is greater than 1/10th of the 100A feeder protection (10A). Therefore, strictly applying 240.21(B)(4), this appears to meet the letter of the code.2.

Short-Circuit Protection and ICEA Table

This is where your concern is valid and important. While the 30A disconnects will protect the load-side wiring, the tap conductors themselves are vulnerable to high-level short-circuit faults before the 30A protection operates. You've highlighted a potential weak point. The ICEA table reinforces this concern that an 8ga conductor is the minimum sized conductor for a 100A fuse.

The Problem: Low-Level Faults vs. High-Level Faults Overload Protection

The 30A fuses provide overload protection for the #10 THHN. If the load on those circuits consistently exceeds 30A, the fuses will trip.

High-Level Fault Protection

A dead short (e.g., phase-to-phase or phase-to-ground) creates a massive fault current. The 100A fuses on the feeder are intended to clear this fault. However, depending on the impedance of the circuit, a fault on the #10 could see very high currents that melt the conductor before the 100A fuse clears. The 100A fuse may be too slow to protect those smaller #10 conductors in the event of a bolted fault right at the tap.

The Dilemma: Code Compliance vs. Best Practices

You are correct. While arguably meeting the minimum requirements of 240.21(B)(4), the installation poses a potential hazard.

Recommendations

Here's how to approach this:1. Consider the specific application and load

Is the 30A load ever close to 30A? If the actual load is small, the risk is lower.2.

Calculate Available Fault Current

This is critical. Use a fault current calculator (many are available online) or consult an engineer to determine the available fault current at the tap point. This will tell you how quickly and intensely a fault will escalate.3.

Evaluate Selective Coordination

Ideally, the 30A fuses should clear a fault before the 100A fuses. This minimizes disruption. Check the time-current curves of both fuses to see if they provide any degree of selective coordination.4.

Documentation

Document your findings, calculations, and concerns. This is crucial for liability protection.

Options for Improvement (In order of best practice)



Re-design the Tap

The best solution is to eliminate the #10 taps and use conductors sized appropriately for the 100A protection and the applied load. Perhaps 8ga would be a better minimum. You may need to re-configure the system with larger conductors all the way back to the load-side of the 100 amp fuses.

Change Fuses

Depending on the fault current calculations and the load characteristics, you might be able to use fast-acting fuses that match the ICEA rating, in order to increase short circuit protection for the tap conductors. (This is more complicated and requires careful evaluation).

Oversized Conductors

If it's not feasible to increase conductor size, consider other protection devices, such as using circuit breakers instead of fuses.

Addressing Photo Uploads

Unfortunately, this forum's native functionality doesn't typically allow direct image uploads. You would need to host the images on a third-party image hosting site (like Imgur, Google Photos, etc.) and then provide the links to the images in your post.

In Summary

You've correctly identified a potentially problematic installation. While it may technically comply with 240.21(B)(4), a high-level short circuit could damage the #10 conductors before the 100A fuses clear the fault. Further investigation and documentation are essential, and implementing one of the improvement options is highly recommended to ensure safety and reliability. Consult with a licensed electrical engineer if you need assistance with fault current calculations or system re-design.Always prioritize safety and follow the intent of the code, not just the letter! Flag for review