Okay, this is a common scenario, and you're right to be thinking carefully about the service size, transfer switch ratings, and potential code issues. Let's break it down

The Problem:
Service Upgrade

Tapping directly into the service with 200A breakers for the transfer switches will likely trigger a service upgrade requirement to 320A, which is costly and may involve lengthy permitting.

Transfer Switch Over-Protection

Feeding a 200A transfer switch with a 100A breaker seems counterintuitive and potentially problematic from an inspection standpoint.

Analysis of Your Proposed Solution



100A Feed to 200A Transfer Switch

You are correct; there is likely no NEC code violation in feeding a 200A rated transfer switch with a 100A breaker. The transfer switch is rated to handle up to 200A, but the breaker protects the conductors feeding it at 100A. However, it may raise eyebrows with the inspector. They may ask why you're using a transfer switch rated for twice the breaker size.

Load Control on Backup

You're planning to limit the loads on backup power to 48A per inverter, which is essential given the inverter's capacity. This is a critical part of the design.

Potential Issues and Considerations


Inspector Concerns

Even if code-compliant, an inspector might question the logic of using a 200A transfer switch when it's protected by a 100A breaker. They might argue it's inefficient or ask about future upgrade plans. Be prepared to explain your reasoning. Have documentation on the inverter's backup output capacity.2.

Load Calculation

Crucially, you need to perform a proper load calculation for the entire house. This includes:

Existing Loads

Include all existing fixed loads (lights, outlets, etc.) based on square footage.

Appliance Loads

Include nameplate ratings for oven, dryer, AC. Remember that AC loads are motor loads and require special calculations.

Inverter Capacity

Take into account that, based on the 48A output and 240V (assuming split-phase), each inverter can handle approximately 11,520 Watts of load on backup.This load calculation will justify the 100A breaker to the transfer switch.3.

Load Management Strategy



Automatic vs. Manual

How will you control the loads during a power outage? Will it be automatic load shedding, or will the homeowner need to manually switch circuits? This is essential. The Growatt documentation may have suggestions for how to implement this, but you will probably need external components such as contactors.

Prioritization

Which circuits are critical and should be backed up first (lights, refrigerator, well pump, etc.)? Make sure the homeowner is very clear on which circuits will be backed up.4.

Neutral/Ground Bonding

Carefully review the manufacturer's instructions regarding neutral-ground bonding in the transfer switch. It should only be bonded in one location (typically the main service panel). Improper bonding can create hazardous conditions and interfere with GFCI protection.5.

Permitting

This type of installation always requires a permit. The POCO will likely want to inspect it, even if they don't require a service upgrade immediately.6.

Future Expandability

Consider whether the homeowner might want to add more batteries or inverters in the future. If so, you may need to plan for a larger service panel eventually.7.

Wire Sizing and Conduit

Ensure your wire sizes are appropriate for the 100A breakers (typically #3 AWG copper, THHN/THWN insulation). Make sure the conduit size is correct for the number and size of conductors you're pulling.8.

Overcurrent Protection

Double-check the overcurrent protection requirements for the inverters and batteries as outlined in the manufacturer's documentation and relevant codes.

Recommended Steps


Detailed Load Calculation

Perform a thorough load calculation before proceeding. This is the foundation of your design.2.

Consult with the Inspector

Before starting the work, discuss your proposed plan with the local electrical inspector. Explain your reasoning for the 100A feed to the 200A transfer switch and your load management strategy. Get their feedback and ensure they are comfortable with your approach.3.

Develop a Load Management Plan

Create a detailed plan for how you will manage loads during a power outage. This should include a circuit prioritization list and a clear explanation of the load shedding process (automatic or manual).4.

Neutral/Ground Bonding

Verify and implement proper neutral/ground bonding.5.

Documentation

Keep all documentation, including load calculations, manufacturer's instructions, and permits, organized and readily available for inspection.6.

Clear Communication with the Homeowner

Explain the limitations of the backup system, the load management plan, and any potential future upgrades.

Alternative Solution (Potentially More Expensive)



Sub-Panel for Critical Loads

Install a new sub-panel specifically for critical loads that you want to back up. Feed this sub-panel from the transfer switch. This keeps the existing panel untouched and allows you to size the new sub-panel and its feeder appropriately for the inverter capacity. This may be easier to get approved by the inspector and makes load management cleaner.

In summary: Your proposed solution of using 100A breakers to feed the 200A transfer switches is likely code-compliant, but you need to be prepared to justify it to the inspector and have a solid load management plan in place. Consulting with the inspector and performing a thorough load calculation are crucial before starting any work. Flag for review