A residential automatic washing machine shall have a _______ trap.

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In residential plumbing, an automatic washing machine typically requires a 2-inch trap. This size is standardized to ensure that the washing machine can effectively drain wastewater without risk of clogging. A 2-inch trap provides adequate capacity to handle the flow and volume of water expelled during a washing cycle, which can be quite significant.

Using a trap that is too small, such as a 1-inch trap, could lead to drainage issues, including slow drainage or backups, because it would not be able to accommodate the larger volume of water that a washing machine discharges. On the other hand, while larger traps like 3 inches or 4 inches could theoretically work, they are unnecessary for this application and may complicate the plumbing design without providing any additional benefit for a standard residential washing machine.

Overall, the 2-inch trap strikes the right balance between functionality and compliance with plumbing codes, ensuring that the installation is effective and reliable.