The answer
Required when NEC Article 220 load calculations exceed 100A/125A continuous capacity. Key drivers: Level 2 EV chargers (30-60A), heat pumps/electric HVAC (30-50A), solar/battery storage (NEC 705 120% busbar rule), or replacing hazardous obsolete panels (FPE Stab-Lok, Zinsco).
Sources
This answer is drawn from the standards and guidance below. It is general information about how these systems work — not a diagnosis of your property.