I am not sure about code, but the need for a trap will depend on design of A/C system. Condensate comes from evaporator coil. If fan is located before the coil in the system, the fan will push small amount of air and water down condensate drain. If fan is located upstream of coil, fan will pull water up drain line counter to water flowing down drain. A P-trap is needed to block air and allow condensate to drain properly.
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