R736 Error Tutorial 2: Breaker Sandwich IX on Subpanel Feeders

Understanding the Issue

Often, projects involving a Backup Initiation Device (BID) use Terminal Lugs to connect to a subpanel feeder that supplies a backup load panel.  The subpanel feeder sees both the utility supply and the additional solar and storage output from the system while supplying power to the backup loads panel.  The electrical code has a provision to protect the subpanel feeder from overload, which is sometimes referred to as a “Breaker Sandwich.” 

Here’s how it looks in SolarAPP+....

Resolution Steps:

1. Busbar Inputs: The image below shows an example submission; your project details may vary. Ensure that the subpanel connections are correctly configured without stacking breakers in a manner that violates the 120% rule.

   

2. Confirm Panel Ratings: Verify that the subpanel and feeder configurations do not exceed panel and breaker specifications.

   
   
   

3. Review OCPD Sizing: Make sure the Overcurrent Protective Device (OCPD) ratings align with manufacturer guidelines and do not exceed the subpanel's capacity.

   
   
   

Additional Tips:

• SolarAPP+ can qualify projects using the “Large Feeder” method, where the backup loads panel does not have a dedicated service breaker. However, the conductor used must support the combined output of the utility and connected sources.  SolarAPP+ can qualify feeders up to 3/0 AWG.

  
  
  

Reach out to Support@solarappfoundation.org for more.