Mastering Solar Interconnections: Strategies to Avoid Costly Panel Upgrades

Solar interconnections can be confusing, especially with the various regulations and evolving National Electrical Code (NEC) standards. One of the biggest headaches for solar installers comes when they discover that a panel upgrade is necessary, particularly when the solar system exceeds the 120% rule. Panel upgrades increase project costs for the customer and delay installation timelines, frustrating both the installer and the homeowner.

The good news is that, while panel upgrades are common, especially with meter-main combo configurations, there are several ways to avoid them. By leveraging key NEC provisions, installers can often find alternative ways to connect solar systems safely and code-compliantly without resorting to expensive upgrades.

In this article, we’ll discuss what the 120% rule is, how to apply NEC provisions to avoid panel upgrades, and how Solar Design Lab’s smart solar interconnections selector makes finding compliant options easier. Let’s dive into how to avoid panel upgrades and streamline your solar installations.

What is the 120% Rule?

The NEC 120% rule, found in Section 705.12(D)(2)(3), addresses how much solar energy you can backfeed into a breaker panel. Essentially, this solar interconnection rule rule states that the combined ratings of the main breaker and the solar backfeed breaker cannot exceed 120% of the panel’s busbar rating. For example, if you have a 200A-rated busbar, the total combined amperage between your main breaker and your solar backfeed breaker cannot exceed 240A (200A x 1.2 = 240A).

If this rule is exceeded, a panel upgrade is often required to safely accommodate the solar system. This is particularly common in homes with meter-main combo panels, where the main breaker and load center are integrated into one panel. However, several strategies can be employed to avoid these costly panel upgrades.

4 Solar Interconnections to Avoid Panel Upgrades

1. Using the 100% or Sum Rule (NEC 2020 705.12(B)(2)(3)):

When the electrical panel is lightly loaded (i.e., it doesn’t use the full capacity of the breaker panel’s busbar), you can apply the 100% or "Sum Rule" solar interconnection. This provision of the NEC allows you to add the solar system without exceeding the total capacity of the panel, provided the load on the panel is below the rated busbar capacity.

For instance, if your electrical panel is rated at 200A but only using 100A of load, you may be able to bypass the 120% rule and install a larger solar system by applying the 100% rule. This is especially useful in residential settings where the electrical load is typically far below the maximum rating of the panel.

2. Breaker Derating (NEC 705.12(B)(3)(2)):

Breaker derating is another common method to avoid panel upgrades. By reducing the size of the main breaker that protects the panel's busbar, you can free up capacity for your solar backfeed breaker. The difference between the busbar’s rating and the newly derated main breaker rating can be added to the 20% margin allowed for the solar system.

For example, if you have a 200A panel with a 200A main breaker, you could replace the main breaker with a 150A breaker, freeing up 50A of capacity. This would allow you to add a larger backfeed breaker for the solar system without exceeding the busbar's capacity.

3. Feeder Taps (NEC 705.12(B)(3)(6)):

When the main panel is fully occupied and there’s no space for a solar backfeed breaker, feeder taps can be a viable solution. According to the NEC, a feeder tap allows you to “tap” into a feeder line and connect the solar system to the electrical distribution system without upgrading the main panel. This is especially helpful when using a feedthrough panel.

A feedthrough panel is a panel that is connected directly to the lugs of the meter-main combo panel and acts as an extension of the electrical system. Feedthrough panels are often installed when the main panel is full, and they are typically used in larger residential or commercial systems. According to NEC guidelines, feeder taps can be used when the main panel is full, provided the feedthrough panel either has a main overcurrent protective device (OCPD) or the wires and busbar are rated for the full load of both the mains and the solar PV system. When installed correctly, feeder taps can help avoid panel upgrades and keep projects moving forward without the need for significant modifications.

4. Feeding the Panel off a Breaker:

If the feedthrough panel is a main lug only (MLO) panel and cannot be converted to a main breaker panel, you have another option. Instead of feeding the panel off the lugs, you can feed it off a breaker on the main panel's busbar. This effectively protects the panel and allows you to add solar to the main panel without needing additional protection for the feedthrough panel.

This method works by feeding the downstream panel through a breaker in the main panel, rather than connecting directly to the lugs. This solar interconnection medthod simplifies the system and removes the need for further protection for the feedthrough panel, thus avoiding the requirement for a panel upgrade. This is a cost-effective and simple way to maintain compliance while expanding the solar system’s capacity.

How Solar Design Lab’s Smart Solar Interconnections Selector Can Help

Navigating NEC rules can be challenging, especially when it comes to designing solar systems that comply with all local regulations. This is where Solar Design Lab’s smart solar interconnections selector becomes a valuable tool for solar designers. Our platform provides code-compliant options based on site survey information, system size, and the existing electrical service configuration.

Rather than spending hours interpreting NEC guidelines or consulting multiple resources, the smart interconnection selector automates the process. It guides you through selecting the most appropriate and compliant interconnection method, whether it's utilizing feeder taps, breaker derating, or applying the 100% rule. The result? You reduce the risk of needing panel upgrades and complete your designs faster, helping you save time and money.

While panel upgrades are often a necessary part of solar installations, there are several ways to avoid them using NEC provisions and strategic design techniques. Whether it’s applying the 100% rule, derating breakers, using feeder taps, or feeding a panel through a breaker, there are cost-effective solutions that can help you meet code requirements without upgrading the electrical system.

With the help of Solar Design Lab’s smart solar interconnections selector, solar designers can streamline their workflows, avoid common pitfalls, and ensure compliance with NEC standards. By automating the complex process of interconnection design, you can focus on what matters most—completing projects on time and within budget.

Key Takeaways:

• Use the 100% rule when the panel is lightly loaded to avoid panel upgrades.
• Breaker derating can free up capacity for larger solar systems.
• Feeder taps allow you to bypass a full panel by tapping into the electrical system.
• Feeding a panel through a breaker in the main panel busbar protects the feedthrough panel, eliminating the need for further protection.

By using these strategies, you can navigate the complexities of solar interconnections, avoid panel upgrades, and streamline your installation process.