Sustainability has been the emphasis of intense discussion over recent decades, but mostly focused on addressing critical aspects of environmental issues. An increasing awareness of social responsibilities and ever-shifting customer requirements have led manufacturers to consider social sustainability during the design phase in tandem with addressing environmental concerns; thus, design for social sustainability has evolved as a new product design direction. Modular product design (MPD), has been widely used in both academia and industry because of its significant benefits in design engineering. Because of the potential synergy, investigating design for social sustainability in association with MPD holds promise as a field of investigation. In this paper, we introduce a novel MPD approach that uses the elements of key component specification and product impact on social sustainability. The key components carry core technologies or have the highest sustainability effects in a product (i.e., the most costly or environmentally polluting parts). Product competitiveness strongly relies on a few key components that should be a focal point during product development. However, to the best of our knowledge, key components have not been well addressed in modular product design. In this paper, we employ labor time as an indicator to measure social sustainability. A heuristic-based clustering algorithm with labor time optimization is developed to categorize components into modules. A coffee-maker case study is conducted to demonstrate the applicability of the proposed methodology.