There are a number of reasons why our tool might derate a breaker on a user's project, but the most common reason typically has to do with how the Busbar Point of Connection is set up.
Center-Fed Busbars on Load-Side Interconnected Systems
Whenever a user specifies that a system is center-fed, instead of top-fed or bottom-fed (on a load-side interconnected system) it creates the risk of an overloaded busbar that could result in a fire safety issue. The 2014 NEC code requires "engineering supervision" to ensure the safety of the interconnection, which our app does not provide.
However, if a user can demonstrate compliance with 2014 NEC Article 705.12(D)(2)(d), they can edit the single-line or three-line diagram's PDF or DWG file. While the majority of AHJs do not allow overloading a busbar, some AHJs do. In general, overloading a busbar is a code violation, and we are obligated to make the derated recommendation.