You're on the right track in considering the tap rules, but let's break down the scenario to determine the most accurate interpretation of the NEC and the best approach.

Analysis of Your Diagram and Potential Code Issues



VFD as the Source

The VFD output is your "source" for the motors.

HVAC Disconnect

This serves as the required disconnect for the HVAC equipment.

Terminal Block Splitting

The wiring after the terminal block is where the "tap conductor" question arises.

Motors

The actual loads.

Tap Conductor Analysis (and Why it Might Not Apply Directly)

While it looks like a tap conductor situation, the NEC considers a factory-assembled piece of equipment (like your HVAC unit) differently than field-installed wiring.Here's why you might not be strictly under the tap conductor rules in Article 240.21:1.

Factory Assembly

The HVAC unit is being listed or field evaluated. The components within the unit are considered part of the product listing if the HVAC manufacturer has designed and tested the wiring configuration within the unit. A recognized testing agency (like UL) will have evaluated the whole assembly. This means the wiring configuration is covered under the listing.2.

Manufacturer's Responsibility

The HVAC manufacturer is responsible for ensuring that the internal wiring and components are appropriately sized and protected for the intended use. They likely have internal OCPD (Overcurrent Protective Devices) or thermal protection built into the motors themselves.

However, there are important considerations



HVAC Unit's Instructions

You MUST strictly follow the HVAC manufacturer's instructions for connecting the VFD output to the unit. The instructions will specify conductor sizes, any required external OCPD, and any other specific wiring requirements. This is the most important thing. Deviating from these instructions voids the unit's listing and creates liability.

OCPD Selection

If the HVAC unit's instructions require an external OCPD for each motor circuit, before the terminal block, then you must provide that. This is the most common scenario in my opinion. If it is a requirement, you would most likely have to install a small fuse or breaker block inside the HVAC unit near the terminal block.

Conductor Sizing

Even if the tap rules don't strictly apply, you must still ensure that the conductors are sized appropriately for the motor's full-load amps (FLA) and the VFD's output current. This is not just about overcurrent protection, but also about voltage drop and conductor heating.

Recommendations and Best Practices


Consult the HVAC Manufacturer's Documentation

Thoroughly review the wiring diagrams and installation instructions for the HVAC unit. This document is the definitive guide for proper connection. Look for specific requirements related to VFD connections and motor protection. Pay close attention to any sections that cover field wiring to the unit.2.

Confirm Listing Requirements

Verify that the HVAC unit is properly listed (e.g., UL, ETL). The listing mark indicates that the unit has been tested and certified to meet applicable safety standards.3.

Consider an External Motor Starter (If Necessary)

If the HVAC unit's instructions require additional motor protection, you might consider installing a small motor starter (with overload protection) for each motor before the terminal block. This provides a more robust solution than simply relying on fuses.4.

Professional Review

Have a qualified electrical engineer or licensed electrician review your design and confirm that it meets all applicable NEC requirements and the HVAC manufacturer's specifications.

In summary

While the tap rules might not apply directly due to the HVAC unit being a listed assembly, you must adhere to the manufacturer's instructions and ensure adequate overcurrent protection for each motor circuit. Always prioritize safety and code compliance.

Next Steps


Provide the Make and Model of the HVAC unit. This will allow me to give you a more specific answer.2. Find and Review the Installation Manual and Wiring Diagrams. Pay close attention to the sections that cover VFD connections and motor protection.

Example Scenario to Illustrate

Let's say your HVAC unit is a packaged rooftop unit. You connect the VFD output to the HVAC disconnect. Inside the unit, at the terminal block, the HVAC manufacturer requires that each motor circuit (before connecting to the motor) be protected by a fuse rated at 125% of the motor's FLA. You would then install fuse holders and appropriately sized fuses before each motor's connection to the terminal block.This example demonstrates how the manufacturer's instructions dictate the specific requirements for overcurrent protection.

Disclaimer: This information is for educational purposes only and should not be considered a substitute for professional electrical engineering advice. Always consult with a qualified professional before making any electrical installations. Flag for review