Okay, let's break down your heating project enclosure needs and the code implications.

Understanding the Problem

You want to house a 24V transformer, 240V relays (likely switching the heating elements), and related wiring in a clean, organized, and safe manner. You want to separate the low-voltage (thermostat wire) and high-voltage (power) for both safety and code compliance. Finding a single enclosure with a built-in divider that's appropriately sized can be tricky.

Possible Solutions and Code Considerations

Here's a breakdown of your options, along with relevant code considerations (based on the National Electrical Code - NEC - widely used in the US and similar codes elsewhere. Always consult your local electrical code for definitive rulings.):1. Enclosure with Divider (Ideal if you can find one)

This is the best option, but you're right, they're not easy to find pre-made, especially in a specific size. Consider these potential search terms: "Control Panel Enclosure with Divider," "Dual Compartment Enclosure," "Low-Voltage/High-Voltage Enclosure." Check suppliers like:

Electrical Supply Houses

These are your best bet. They'll have the widest selection and knowledgeable staff. Think Graybar, Rexel, CED (Consolidated Electrical Distributors), etc. Tell them what you're trying to do.

Industrial Automation Suppliers

Companies that sell control systems, PLCs, etc., often have enclosures suitable for this.

Online Retailers (Amazon, etc.)

Search for the terms above, but be very careful about specifications and safety ratings. Make sure it's UL-listed or has an equivalent certification.2.

Separate Enclosures



The most straightforward and generally safest option is to use two separate enclosures

One for the 240V components and one for the 24V thermostat wiring and transformer. This avoids all mixing voltage concerns. You can mount these enclosures side-by-side. This is often the easiest solution to achieve code compliance.3.

Large Enclosure with 4-Square Box (Your Proposed Solution)

This is possible but requires careful execution to meet code. Let's analyze:

Acceptability

Generally, it's not a violation to bring the 18/2 into a 4-square box inside a larger enclosure and splice it to THHN (or other appropriate wire type) to run to the transformer, as long as you maintain proper separation and follow all relevant code rules.

Key Code Concerns (NEC based)



300.3(C)(1) Conductors of Different Systems. Conductors of ac and dc circuits, rated 1,000 volts or less, shall be permitted to occupy the same equipment wiring enclosure, cable, or raceway. Exception No. 1: Auxiliary equipment such as an enclosure, a trans- former, a relay, or an adjustable speed drive or solid-state con- troller, shall be permitted to be installed in the same equipment enclosure with associated conductors of different supply sys- tems where the conductors are connected to the auxiliary equip- ment within the enclosure. In short: the Aube RC840 relays are the "auxiliary equipment" and exception 1 would allow for different voltage systems inside the same enclosure Wiring Methods

You must use approved wiring methods. For example, if you're running THHN from the 4-square to the transformer, it needs to be within conduit (rigid, EMT, or flexible metal conduit) or be properly supported and secured. You can't just run loose THHN inside the enclosure.

Insulation Ratings

All conductors must have insulation voltage ratings appropriate for the highest voltage present in the enclosure (240V in your case).

Physical Separation

While the NEC allows mixing voltages in the same enclosure, best practice and some local codes may require physical barriers or separation between low-voltage and high-voltage wiring within the enclosure. This is often achieved with wiring ducts, raceways, or wire ties that keep the conductors neatly separated.

Transformer Mounting

Ensure the transformer is properly mounted and secured within the enclosure.

Overcurrent Protection

Make sure the transformer has appropriate overcurrent protection (fuses or circuit breakers) on both the primary (240V) and secondary (24V) sides, as required by code.

Grounding

Proper grounding of the enclosure and all metal components is essential.

Workmanship

The installation must be neat and workmanlike. This includes properly sized conductors, secure connections, and no exposed live parts.

Steps for Your Proposed Solution


Choose a Large Enclosure

Select a metal or non-metallic enclosure that's large enough to accommodate the transformer, relays, 4-square box, wiring, and any necessary DIN rail or mounting hardware. NEMA 1 or NEMA 3R rated enclosures are typical for indoor applications.2.

Mount the 4-Square

Securely mount the 4-square box inside the larger enclosure. Ensure it's accessible for wiring.3.

Bring in 18/2

Bring the 18/2 thermostat wire into the 4-square box using a cable clamp appropriate for the cable type.4.

Splice to THHN

Inside the 4-square box, splice the 18/2 conductors to appropriately sized THHN conductors (typically 14 AWG or 12 AWG, depending on the transformer current draw, but check the transformer specs). Use wire connectors (wire nuts or similar) rated for the wire sizes.5.

Run THHN to Transformer

Run the THHN conductors from the 4-square box to the 24V transformer using appropriate wiring methods (conduit, wire ties, etc.).6.

Install Transformer and Relays

Mount the transformer and relays inside the larger enclosure.7.

Wire 240V Circuit

Bring the NM cable into the larger enclosure, using a suitable cable clamp. Connect the 240V wiring to the relays and the transformer's primary side.8.

Grounding

Ground the enclosure and all metal components according to code.9.

Separation

Ensure the low-voltage (24V) wiring and the high-voltage (240V) wiring are physically separated as much as possible within the enclosure. Use wiring ducts or wire ties to keep the wiring neat and organized.10.

Labeling

Clearly label all circuits and components.

Important Considerations



Local Codes

Always check with your local building department or electrical inspector for specific requirements in your area. They may have additional rules or interpretations of the NEC.

Safety

If you are not comfortable working with electrical wiring, hire a qualified electrician.

Inspection

Your work will likely require an electrical inspection.

Recommendations



Start with Separate Enclosures

This is the easiest and safest path to compliance if you can't find a suitable enclosure with a divider.

Consult an Electrician

If you're unsure about any aspect of the installation, consult a qualified electrician. They can help you choose the right materials, ensure code compliance, and perform the work safely.*

Document Your Work: Keep a record of all materials used, wiring diagrams, and inspection results.Good luck with your project! Be safe and code-compliant! Flag for review