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 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.
This article may help clarify. See option 4 under the busbar section of the 705.12(D)(2) portion of the article, which notes the following...
"Option 4: Subsection (d) applies specifically to multiple-ampacity busbars or center-fed panelboards, which have long been a source of confusion as previous methodologies did not adequately address them. This new method allows load-side connections in these types of panels, provided that the interconnection is designed under 'engineering supervision,' which must include fault studies and busbar load calculations. This method should alleviate concerns that some AHJs have had about whether to allow load-side connections in center-fed panels, which are very common in some areas of the country. These connections are clearly allowed, provided that the proper due diligence is performed."