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Quantum-dot cellular automata (QCA) is an emerging technology for electronic circuits. Its promising advantages such as a faster speed and a small size give us a good milestone of future technologies. Nanoscale devices, quantum-dot cells, are key components of QCA technologies as gates, wires, and memories. Its simple and unique structure allows the circuit designer to concentrate on the structural level of the circuit instead of the physical level. Currently the exploitation of the adder characteristics are not sufficient. This paper proposes a modular design of a conditional sum adder (CSA) in QCA for comparisons. Using the QCADesigner, the layouts are simulated with several different operand sizes. Those designs are compared with ripple carry adders and carry lookahead adders on the basis of the complexity, area, and timing. Rather complex designs of CSA give a very good indication of tradeoffs between the speed and complexity. The final design shows a modular and expandable design for CSA.