This is a common scenario in HVAC retrofits, and you're thinking through it logically. Here's a breakdown of the code considerations and best practices for your proposed solution

Code Considerations and Safety:
Separation of High and Low Voltage

The National Electrical Code (NEC) generally requires separation of high and low voltage wiring to prevent hazards. However, there are exceptions, particularly within listed equipment (like the Honeywell transformer/contactor).

Class 1 vs. Class 2 Circuits

Your new 24VAC control circuit will likely be a Class 2 circuit. Class 2 circuits are limited in power and voltage to reduce fire and shock hazards. The transformer is likely designed to be a Class 2 transformer (check its markings).

Wiring Methods

Since you're using 16/4 cable, ensure it's rated appropriately for the voltage and conditions (e.g., plenum-rated if running in a plenum). Consider using CL2 or CL3 rated cable, which is specifically designed for Class 2 or Class 3 circuits.

Overcurrent Protection

Verify that the transformer has built-in overcurrent protection, or add a 24VAC-rated fuse or circuit breaker at the transformer's secondary. This is vital for Class 2 circuits.

Insulation

All conductors should be properly insulated for the highest voltage present in the enclosure.

Grounding

Ensure that all metal enclosures are properly grounded.

Best Practices and Recommendations

Given the considerations above, here's the recommended approach and why:1.

Use the External Screw Terminals

Connecting to the external screw terminals on the 24VAC side of the transformer is the preferred and safer method. Here's why:

Minimizes Internal Congestion

Avoids adding more low-voltage wiring inside the crowded 4" box.

Clarity and Serviceability

Keeps the low-voltage connections visible and accessible for future maintenance.

Code Compliance

The terminals are specifically provided by the manufacturer for external connections. This inherently indicates that using them is within the UL listing of the transformer.

Reduces Risk of Damage

Less manipulation inside the box reduces the risk of accidentally damaging existing wiring.2.

Running the 16/4 Cable



Secure the Cable

Properly secure the 16/4 cable to the 4" box using a suitable cable clamp or connector designed for flexible metal conduit (FMC) knockouts. This prevents strain on the connections inside the box.

Identify Conductors

Clearly label the conductors in the 16/4 cable at both ends (inside the 4" box and in the new enclosure) to avoid confusion.

Color Coding

Maintain consistent color coding for your wiring (e.g., red for 24VAC+, blue for 24VAC-, and specific colors for switched outputs).

Strain Relief

Provide adequate strain relief for the 16/4 cable at both the 4" box and the new enclosure.3.

Connecting to the Contactor



Use Appropriate Wire Connectors

Use appropriate wire connectors (wire nuts, crimp connectors) to ensure secure and reliable connections to the contactor coil terminals.

Wire Gauge

Ensure that the wire gauge of the 16/4 cable is sufficient for the current draw of the contactor coil. It's likely adequate, but check the contactor specifications.

Routing

Route the wires neatly inside the box to avoid contact with high-voltage components.4.

New Enclosure Considerations



NEMA Rating

Select an enclosure with an appropriate NEMA rating for the environment (e.g., NEMA 1 for indoor use, NEMA 4 for damp locations).

Layout

Plan the layout of components (relays, terminal blocks, power supply) in the enclosure for easy access and maintenance.

Terminal Blocks

Use terminal blocks for all connections within the enclosure. This provides a clean, organized, and maintainable wiring solution.

Enclosure Grounding

Ensure the new enclosure is properly grounded.

Step-by-Step Guide1. Power Off

Turn off the power to the HVAC system at the breaker. Lock out/Tag out the breaker to prevent accidental re-energization.2.

Inspect Existing Wiring

Carefully inspect the wiring in the 4" box for any signs of damage or deterioration.3.

Install Connector

Install a proper connector for the 16/4 cable into the knockout of the 4" box.4.

Run the 16/4 Cable

Run the 16/4 cable from the new enclosure to the 4" box.5.

Connect 24VAC

Inside the 4" box, identify the existing 24VAC wires connected to the external screw terminals. Disconnect these wires, one at a time, and immediately connect one of the conductors from the 16/4 cable to the screw terminal. Repeat for the other conductor. In the new enclosure, connect these 24VAC wires to the appropriate terminals (e.g., a terminal block or directly to the transformer).6.

Connect to Contactor Coil

Run the other two wires from the 16/4 cable into the 4" box. Disconnect the existing wires connected to the contactor coil terminals. Connect the wires from the 16/4 cable to these terminals, ensuring secure connections with wire nuts or crimp connectors.7.

Secure and Label

Secure the 16/4 cable with the connector in the 4" box and with a similar connector in the new enclosure.8.

Install New Enclosure

Mount the new enclosure in a suitable location.9.

Connect Thermostat and Valves

Wire the thermostat and zone valves to the new enclosure, following the wiring diagram.10.

Test

Before energizing the system, double-check all wiring connections. Then, turn on the power and test the operation of the thermostat, zone valves, and fan motor.

Important Notes



Local Codes

Always consult with your local electrical codes and regulations, as they may have specific requirements for HVAC control wiring.

Qualified Electrician

If you're not comfortable working with electrical wiring, hire a qualified and licensed electrician to perform the retrofit. This is particularly important if you are unsure about code compliance or safety regulations.*

Documentation: Create a clear wiring diagram of the new system for future reference.By following these guidelines, you can safely and effectively retrofit your HVAC system with modern controls while ensuring code compliance and long-term reliability. Flag for review