Main-lug Only and Solo Main Breakers


Detailing Main Lug-Only Panels in SolarAPP+

The Issue

Installers submitting solar projects through SolarAPP+ often struggle to identify a Main Lug-Only (MLO) panel from a single-line diagram, or don't know they can detail one in the application. This can lead to incomplete submissions and permitting delays.

The same challenge applies to solo main breakers. Better understanding these different panel types and how to properly detail them in SolarAPP+ enables smoother permitting and compliance with the National Electrical Code (NEC).

The Solution

SolarAPP+ allows you to qualify both MLO panels and solo main breakers for solar installations. Here's what you need to know about each configuration and how to identify them in your project submissions.

About MLO Panels

MLO panels, aka “hot bus panels,” are main service panels where the incoming power wires land directly on busbar terminal lugs without a main circuit breaker to disconnect power from the utility. 

Instead, MLO panels have multiple service disconnects. The code allows them to have up to six "handles"—overcurrent protective devices (OCPDs)—that serve as the disconnecting means. Making MLO panels suitable for use and allowed in the electrical busbar section of SolarAPP+.




About Solo Main Breakers

Solo main breakers are defined as a service disconnect on a dedicated enclosure. They may also be known as a meter disconnect, or can be a standalone enclosure for a single service disconnect. A solo main breaker in a separate enclosure can be mounted to an electrical pole, riser, or wall and the service feeders may land directly on the main breaker lugs. Subpanel feeders may be connected to the load side lugs of the service disconnect. These panels do not have physical space to land a solar photovoltaic (PV) OCPD.  


Entering Panel Details Into SolarAPP+

When you select "Yes" for the question "Is busbar #1 a Main Lug Only Panel?" in SolarAPP+, the system generates a diagram with an MLO main service panel (MSP) similar to the reference diagram below. Answer “Yes” to this question if you also have a solo main breaker panel.

A follow-up question appears asking, "Is busbar #1 a service disconnect on a dedicated enclosure?" If answered "Yes," the resulting diagrams will show a solo main breaker for the main service panel, similar to the reference below. Answer “No” to this question if your panel is an MLO main service panel.



Code Requirements and Evolution

The 2020 version of the NEC updated the rule for service disconnects. It didn't allow six disconnects to be housed in a single enclosure when used as the main service disconnect. Instead, each disconnect had to have its own individual enclosure or compartment to enhance safety during maintenance.  This didn’t end up being widely implemented because homeowners pushed back because the cost to replace serviceable panels was thousands of dollars.

Instead, the 2023 NEC rolled this requirement back to the pre-2020 standard with an exception: "NEC 230.71(B) Exception to (2), (3), (4), (5), and (6): Existing service equipment, installed in compliance with previous editions of this Code that permitted multiple service disconnecting means in a single enclosure, section, or compartment, shall be permitted to contain a maximum of six service disconnecting means." This means residences without an outdoor service panel would trigger NEC 230.85 Emergency disconnects, which require a disconnecting means in a readily accessible location outdoors. SolarAPP allows jurisdictions on the 2020 NEC to carry forward the exception from the 2023 NEC, allowing MLO panels to be used without penalty.

The 2026 NEC further modified the exception to apply to all existing service equipment by stating: "Exception: Existing service equipment, installed in compliance with previous editions of this code that permitted multiple service disconnects in a single enclosure, section, or compartment, shall be permitted to contain a maximum of six service disconnects." An Emergency Disconnect is still required in one- and two-family dwellings per NEC Article 225.41.

MLO Panel Compliance in SolarAPP+

NEC Article 705 covers the installation requirements for interconnected electric power production sources — essentially, it governs how systems that generate electricity (like solar panels, Energy Storage, and other distributed energy resources) connect to a home's electrical service.  MLO panels must comply with NEC 705.11 for source connection to a service and with NEC 705.12(B) for the busbar rating. The system must also have a source disconnecting means in accordance with NEC 705.20. SolarAPP+ automatically calculates NEC 705 compliance for MLO and solo main breaker panels.

Supporting Information

Visual Examples

The diagrams referenced in SolarAPP+ approval documents provide visual references to help you identify the panel type that you are working with, and ensure accurate documentation in your SolarAPP+ submissions.

  • Example Solo Main Breaker Diagrams: Show solo main breaker configurations with the Main Service Panel (MSP) and service disconnect on a dedicated enclosure or subpanel

  • Example MLO panel Diagrams: Show MLO panel configurations with multiple service disconnects and the MSP labeled

  • Actual panel photos: Images showing L1 and L2 main lugs (highlighted in yellow circles) at the top of the panel, with numbered breaker positions below



Code References

For installers seeking additional technical guidance, the following NEC sections are relevant:

  • NEC 705.11: Source connection to a service
  • NEC 705.12(B): Busbar rating requirements
  • NEC 705.20: Source connecting means requirements
  • NEC 225.41: Emergency Disconnects
  • NEC 230.85: Meter disconnect switch provisions (for solo main breakers)

What's Next?

When preparing your next solar installation submission in SolarAPP+, take a moment to identify whether you're working with a MLO panel or a solo main breaker configuration. Understanding these distinctions ensures your application includes the correct details for faster approval.

If you're unsure about your panel configuration, review your construction resources –– such as site survey photos, electrical notes, or plans –– and check for the presence of a main breaker at the top of the panel. No main breaker with multiple service disconnects? You likely have an MLO panel. A single Service disconnect on a dedicated enclosure with no room for a solar OCPD? That's a solo main breaker.

Have questions about panel configurations or other technical requirements in SolarAPP+? Our support team is here to help ensure your projects move smoothly through the permitting process.

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