You're asking a really common and important question about PV system installations and the 120% rule. Here's a breakdown of how the 120% rule applies and why your initial thought process is generally correct, but with a few key caveats

Understanding the 120% Rule (NEC 705.12(B)(2))

The 120% rule is designed to prevent overloading the busbar of a panelboard when you backfeed power from a PV system. It states that the sum of the overcurrent protection device (breaker) protecting the busbar plus the overcurrent protection device for the backfed PV system cannot exceed 120% of the busbar's rating. Example

If a panelboard has a 200A busbar and a 200A main breaker, you can only backfeed up to (1.20 200A) - 200A = 40A of PV power.

Your Situation: Meter Panel Tapping to an Interior MSP

Your situation is slightly different because you're planning to tap the PV system into the conductors feeding the main service panel (MSP) inside the house, rather than directly into the meter panel's busbar.

Why Your Thinking is Mostly Correct

You're right in thinking that the meter panel's busbar might not see the full 200A from the utility plus the PV output. This is because the PV power will be directly supplying the loads within the house first. The excess power will then be sent back to the meter.

The Caveats and What You Need to Verify/Consider


Service Entrance Conductors (SE Conductors)

The critical point is whether the conductors between the meter panel and the interior MSP are rated to handle the potential combined current from the utility and the PV system.

NEC 230.42(B) Calculation

You must ensure that the ampacity of these service entrance conductors is adequate for the total calculated load of the dwelling, plus the maximum backfeed current from the PV system. Essentially, you need to demonstrate that these conductors won't be overloaded under worst-case scenarios (e.g., full sun, minimal house load, and the utility supplying the maximum demand).

Example

Let's say you have a 200A service and you want to add a PV system that can backfeed a maximum of 30A. You must calculate the service load of the house. The calculated load should be derated to 70% as required by the NEC. Your SE conductors MUST be rated to handle (the calculated load of the home/70%) + 30A.

Calculating the home load typically uses the standard method, and it could vary drastically based on the appliances used in the home. So, if your calculation is correct for the home, the home should consume 100A with a PV system capable of producing 30A. The load supplied by the utility is 70% which is 70A, and 30A is from the PV system. You must have the ampacity of the wire be greater than 100A.2. Local Utility Requirements

Even if your installation technically meets NEC requirements, your local utility may have stricter rules about the maximum size of PV systems that can be connected to a service. Check with your utility before proceeding. They may have specific limits on the inverter size or the amount of power you can backfeed.3.

Labeling

Proper labeling is crucial. You'll need to clearly label the PV system disconnect(s) and any relevant equipment to indicate the presence of interconnected power sources.4.

Overcurrent Protection

Properly sized overcurrent protection must be in place to protect the service conductors from overloads, both from the utility and the PV system.5.

Rapid Shutdown

Ensure compliance with rapid shutdown requirements (NEC 690.12), which are designed to allow emergency responders to de-energize the PV system quickly.

In summary

While the 120% rule may not directly apply to the meter panel's busbar in your scenario, you absolutely must* verify the ampacity of the service entrance conductors and comply with all relevant NEC codes, local amendments, and utility requirements.

A load calculation is essential to determine if the existing service can accommodate the PV system. Always consult with a qualified electrician and your local permitting authority to ensure a safe and compliant installation. Flag for review