Okay, let's break down your plan and address the potential issues and considerations. Your overall strategy seems sound. Here's a detailed analysis

Strengths of your plan:
Phased Approach

Gradually transitioning from off-grid to on-grid while maintaining backup power is a smart way to minimize disruption and cost.

ATS Selection (Thomson TS 910 NEMA 3)

The Thomson TS 910 is a reputable ATS, and the NEMA 3 rating provides reasonable protection against weather elements.

Heater for Cold Weather

Addressing the extreme cold is crucial for the ATS's reliable operation.

Areas for Careful Consideration & Refinement


ATS Sizing and Load Calculation



Crucial

You must accurately calculate the total load the house will draw at peak usage. This includes lights, appliances (especially heaters, AC, well pumps, etc.), and any other devices.

ATS Capacity

Ensure the TS 910 you've chosen has sufficient amperage capacity to handle this peak load from both the grid and the off-grid system. Oversizing the ATS slightly is a good practice for future needs and safety margin.

Derating for Cold

Remember to consider derating electrical equipment for extreme cold. Consult the ATS manufacturer's specifications to see if they recommend any adjustments for -40 temperatures.2.

Wiring and Conduit



Underground Wiring Condition

Since you are intercepting an existing underground line, carefully inspect its condition. Is it appropriately sized for the current and future loads? Is the insulation intact? Is it buried at a safe depth and properly marked?

Conduit Size and Type

Choose the appropriate size and type of conduit for the wiring leading to and from the ATS. PVC conduit is common for underground runs, but ensure it's suitable for the temperature range.

Wiring Type

Use appropriately rated wiring (THWN/THHN or similar) for both the underground and above-ground portions of the circuit.3.

Grounding and Bonding



Critical for Safety

Proper grounding and bonding are absolutely essential. Ensure the grounding system at the house is connected to a ground rod and properly bonded to the electrical panel.

ATS Grounding

The ATS should be properly grounded according to the manufacturer's instructions and local electrical codes.

Neutral-Ground Bond

Verify that the neutral-ground bond is only present at one location (either the grid-tied service entrance or the off-grid system, but not both simultaneously). This is essential to prevent circulating currents and potential safety hazards. This is something an electrician will need to ensure.4.

ATS Enclosure and Heater Placement



Enclosure Size

Make sure the NEMA 3 enclosure is large enough to comfortably house the ATS, wiring, heater, and any other necessary components (e.g., terminal blocks, surge protection).

Heater Placement

The Omega CREX03 is a good choice for a DIN rail heater. Position it in the enclosure to provide even heat distribution around the ATS, avoiding direct contact with sensitive components.

Thermostat Control

The Omega CREX03 has a built in thermostat. Consider a heater with adjustable thermostat, or an external thermostat for more precise temperature control. You don't want to overheat the enclosure. Set the thermostat to maintain a temperature above the minimum operating temperature of the ATS and other components within the enclosure.

Ventilation

While the enclosure needs to be sealed against the elements, consider a small amount of ventilation (e.g., a vent with a filter) to prevent condensation build-up inside the enclosure.5.

Off-Grid System as Backup



Capacity Limitations

Be aware that the off-grid system may not be able to handle the entire load of the house, especially if it was originally sized for a smaller load. Prioritize essential circuits for backup power (e.g., lights, refrigerator, heating system).

Manual Load Shedding

You may need to manually shed loads (turn off non-essential appliances) to prevent overloading the off-grid system during a grid outage.

Battery Condition

Evaluate the health of the batteries in the off-grid system. Batteries degrade over time, and their capacity may be significantly reduced. Consider replacing them if they are nearing the end of their lifespan.6.

Generator Integration



ATS Compatibility

Ensure the ATS is compatible with your generator. Some ATS models require specific generator features (e.g., voltage regulation, frequency stability).

Generator Fuel Source

Consider the fuel source for the generator and ensure an adequate supply is available, especially during prolonged outages.

Generator Maintenance

Regular generator maintenance is critical for reliable backup power.7.

Permitting and Inspections



Local Codes

Check with your local building department or electrical inspector to determine if permits are required for the grid-tie installation and the ATS.

Compliance

Ensure the entire installation complies with all applicable electrical codes (e.g., NEC in the US).

Recommendations



Consult with a Qualified Electrician

I highly recommend consulting with a licensed and experienced electrician who is familiar with both grid-tied and off-grid systems, as well as ATS installations. They can perform the load calculations, ensure proper wiring and grounding, and verify code compliance.

Detailed Design

Create a detailed electrical design that includes wiring diagrams, component specifications, and grounding details. This will help the electrician with the installation and ensure a safe and reliable system.

Manufacturer's Documentation

Carefully read and follow the manufacturer's instructions for the ATS, heater, and all other electrical components.

Thorough Testing

After the installation is complete, thoroughly test the system under various conditions to verify that it operates correctly and safely. Test the ATS switching between grid and off-grid power, and test the generator integration (if applicable).*

Consider Transfer switch location Keep in mind the ease of access for maintenance and visual inspection.By carefully addressing these considerations and working with a qualified electrician, you can create a safe, reliable, and effective system for transitioning your house to grid-tied power while maintaining off-grid backup. Good luck! Flag for review