Okay, this is a common and important project to do safely and correctly. Here's a breakdown of the considerations, options, and a likely recommendation

Understanding the Situation
Two Panels

You have a 200A main and a 100A subpanel, both individually fed from the service entrance.

Manual Transfer

You want to manually switch your 12kW generator (capable of 50A @ 240V) to your 200A main panel. You'll manage loads manually.

Existing ATS

You have a 200A ATS you're not planning to use at this time, but it adds options.

Homeowner Wiring Experience

You're comfortable with electrical work and understand permitting. This is good, but remember codes change!

Safety and Legal Considerations are Paramount! Backfeeding Dangers

The primary danger is backfeeding power onto the utility grid. This is extremely dangerous for utility workers and your neighbors. A proper transfer switch (or interlock) is absolutely essential to prevent this.

NEC Compliance

Your setup must comply with the National Electrical Code (NEC) as adopted by your local jurisdiction. Local codes may be stricter than the NEC. Permitting and inspection are essential to ensure safety and code compliance. Don't skip this step. Call your local permitting office and discuss your plans before you start any work.

Generator Sizing

A 12kW generator (50A @ 240V) is usually adequate for selectively powering essential circuits in a home, provided you manage loads carefully.



Neutral Bonding

The generator's neutral should NOT be bonded to ground if the generator is supplying power to the main panel with a neutral-ground bond. Most portable generators intended for standby power will have a switch or jumper to disconnect the neutral from ground. Otherwise, you can create a parallel neutral path that is dangerous. See below for more details.

Possible Solutions and RecommendationsBased on your requirements and existing equipment, here are a few options, from simplest to more complex (and potentially better):1.

Interlock on 200A Panel Only (Simpler, but Requires More Load Management)



How it Works

Install an interlock kit on your 200A main panel. This allows you to switch between the utility main breaker and a 50A generator breaker safely, preventing both from being on simultaneously.

The Challenge

The 100A subpanel will not have power unless you backfeed it through the 200A panel. This means you'll have to be very careful to move essential circuits from the 100A subpanel to the 200A panel if you want them powered by the generator.

This is the biggest drawback.

Wiring

Install a 50A breaker in the 200A panel. Run appropriately sized wire (6 AWG copper or 4 AWG aluminum, check local code) from the generator outlet to an inlet box mounted near your panel. Connect the wires from the inlet box to the 50A breaker. Install an interlock kit to the 200A main panel to prevent the Utility and Generator main breakers from being on at the same time.

Pros

Simple, relatively inexpensive.

Cons

Requires significant load management. You likely have to move some circuits from the 100A panel to the 200A panel. No generator power to the 100A panel without moving circuits.2.

Using the Existing ATS for the 200A Panel (Recommended, but More Work Upfront)



How it Works

Install the 200A ATS to isolate the 200A Main Panel from the grid when the generator is running. This is safer and more convenient.

Wiring

Wire the 200A ATS between the meter and the 200A Main Panel according to the manufacturer's instructions. Connect the generator to the ATS input.

Pros

Safest method No need to move circuits from 100A to 200A panel. ATS will prevent overloads Easier load management

Cons

More expensive upfront More complex to install3.

Feeding Both Panels (More Complex, Probably Not Recommended for Manual Transfer)



Why It's Less Ideal

Trying to feed both panels manually with a 50A generator is tricky and could lead to overload situations. You'd essentially be paralleling the panels, which is not recommended without careful consideration and proper overcurrent protection and load calculations.

Neutral Bonding Considerations (VERY IMPORTANT)

Multiple Neutral-Ground Bonds are Dangerous

If your generator has a bonded neutral (neutral is connected to the generator's frame/ground), and your main panel also has a bonded neutral (which it almost certainly does), you'll create a parallel neutral path. This can lead to circulating currents on the ground wire, which is a safety hazard.

Solution


Check Your Generator

Determine if your generator has a bonded neutral. Many portable generators have a switch or jumper to disconnect this bond. This is ideal. Disconnect the bond when using the generator as a standby power source.2.

If You Can't Disconnect

You might need to float the neutral in the main panel (remove the neutral-ground bond) if the generator is bonded. However, this is a complex topic and requires careful consideration and likely isn't the right solution.

Consult a qualified electrician! This might also require modifying the way some appliances are wired.

RecommendationGiven your experience and the existence of the ATS, I recommend

Option 2 (Using the ATS for the 200A Panel). It offers the best balance of safety, convenience, and utilization of your existing equipment.

Crucial Next Steps1.

Contact Your Local Permitting Office

Discuss your plans in detail with them. Ask about specific code requirements in your area, inspection procedures, and any required permits.2.

Load Calculation

Perform a load calculation to determine the actual load you'll be placing on the generator. This will help you determine if your 12kW generator is sufficient and what circuits you can safely power.3.

Consult a Qualified Electrician (Highly Recommended)

While you have experience, a licensed electrician can review your plans, ensure code compliance, and perform the final connection to the utility. They can also properly address the neutral bonding issue.4.

Follow Manufacturer's Instructions

Adhere strictly to the instructions provided with the ATS, interlock kit, and generator.

Important Reminders



Double-Check Everything

Electrical work is unforgiving. Double-check all connections and wiring before energizing anything.

Test Your Setup

Once everything is installed, thoroughly test the system with the generator running.

Safety First:* Always prioritize safety. If you're unsure about anything, consult a qualified electrician.This information is for general guidance only and should not be considered a substitute for professional electrical advice. Good luck with your project! Flag for review