Okay, let's break down your plan for the 20kW generator and the future 800A service upgrade, addressing the NEC 2017 concerns and the future-proofing aspects.

Regarding the 20kW Generator Installation (NEC 2017 Compliance)



Generator Placement & Mounting

Check any restrictions for placing a generator on a garage roof (local permitting, structural load, fuel type proximity, etc.).

ATS Location and Wiring

Putting the ATS in the detached garage next to the 600A service disconnect is generally a good idea.

ATS Compatibility

Ensure the ATS is properly sized for the generator output and compatible with the existing service.

Bonding and Grounding

Properly bond and ground the ATS and generator per NEC Article 250. Pay close attention to the neutral connections.

Feeder to the House (100A MCB Panel)

This is where things get tricky and your understanding of 225.30(A)(4) is critical.

225.30(A)(4) Exception

This exception allows for an additional feeder for standby systems if the standby source (the generator) is legally required. This is most often for emergency systems in commercial applications (fire pumps, etc.). If the generator is strictly for convenience and power outage backup, this exception likely does NOT apply under the 2017 NEC. If the generator is required by any other code (ADA, etc.), then this exception applies.

Code Interpretation

I strongly recommend getting clarification from your local electrical inspector on whether they will accept the generator setup as a legally required standby system. Their interpretation is what matters. If they don't accept it, this feeder becomes a violation of 225.30's single-feeder rule.

Alternatives (if 225.30(A)(4) doesn't apply)

If the exception doesn't apply, you have a few options (none of which are ideal in terms of cost or convenience):1.

Interlock/Transfer Switch on the Main House Panel

Move the ATS to the main house and use it to switch the entire main house panel between the utility and the generator. This is the most straightforward approach for code compliance.2.

Load Management

Install load management devices (shedding circuits) to ensure the generator can handle the load transferred to it. This may still require significant modifications to the main house panel.3.

Larger Generator/ATS

Install a larger generator capable of handling a much larger portion of the house load, potentially the entire house. This is expensive.

Conduit Run to the House



Sizing

Size the conduit adequately for the 100A feeder (minimum 1", likely 1.25" or 1.5" depending on conductor type and fill calculations) and ensure it's suitable for direct burial if any portion is underground.

Conductor Selection

Choose conductors rated for the application (THWN, XHHW, etc.) and sized appropriately for the 100A overcurrent protection and any voltage drop considerations.

Regarding the Future 800A Service Upgrade & "Future-Proofing"



Conduit for Future Feeders

There is nothing in the 2017 NEC to prevent you from installing an empty conduit for future use, as long as it isn't currently being used for an illegal purpose. The concern arises if that conduit is used to bypass a current code restriction.

Labeling

I would strongly advise labeling the conduit clearly with something like "FUTURE SERVICE FEEDER - DO NOT USE UNTIL SERVICE UPGRADE PER NEC 2020 OR LATER." This helps prevent accidental or intentional misuse before the code allows it.

Conduit Sizing

Size the conduit for the future 800A service conductors based on the conductor size you anticipate using. Remember to factor in derating and conduit fill requirements. Err on the side of larger, as conduit is relatively inexpensive compared to the cost of re-doing the work later.

Physical Protection

Ensure the conduit is adequately protected from physical damage.

Understanding NEC 2020 (and later)



225.30(B) - Multiple Feeders Supplied by a Single Service

The 2020 NEC (and later versions) does allow multiple feeders to a single building from a common service. This significantly simplifies your future upgrade. However, until your jurisdiction adopts the newer code, you are bound by the 2017 NEC.

Pull Boxes

Install properly sized pull boxes at each end of the conduit run to facilitate future conductor installation.

In Summary & Recommendations


Consult with the Inspector

Crucially, discuss your plan with your local electrical inspector before starting any work. Get their interpretation of 225.30(A)(4) regarding the generator as a legally required standby system. Their approval is essential for the feeder to the house.
2.

Alternative Designs if 225.30(A)(4) Doesn't Apply

If the inspector doesn't allow the exception, consider moving the ATS to the main house panel or exploring load management options.3.

Future Conduit

Installing the empty conduit for the future 800A service upgrade is a good idea, but label it clearly and ensure it is sized appropriately.4.

Documentation

Keep detailed documentation of your installation, including permits, inspections, and any deviations from the original plan. This will be helpful for future upgrades.5.

Consider the Cost/Benefit: The cost of potentially re-doing some of the wiring if the inspector rejects the 225.30(A)(4) interpretation might make moving the ATS now more economically sensible.This is a complex situation with significant code implications. Thorough planning, communication with your local inspector, and careful execution are essential for a safe and compliant installation. Flag for review