N+1 redundancy

N+1 redundancy is a design approach in which a system has one more component than the minimum needed to carry the load: if N units are required, N+1 are installed. For example, a facility needing four chillers installs five, so any single unit can fail or be serviced without affecting operations. N+1 is the most common redundancy level in commercial data centers for UPS modules, generators, cooling units, and pumps, and it underpins Tier III concurrent maintainability when combined with dual distribution paths. It costs substantially less than full 2N duplication but protects against only one simultaneous failure per system. Variants include N+2 (two spare units) and distributed redundancy schemes like 3N/2, which share spare capacity across multiple systems.