Final answer:
The smallest size electrical metallic tubing (EMT) that can accommodate three THW copper wires depends on the wire gauge. For 10-gauge copper wires, the smallest size EMT allowed would be one that can fit these wires.
Step-by-step explanation:
The smallest size electrical metallic tubing (EMT) allowed for three THW copper wires depends on the gauge of the wires. THW copper wire is commonly sold in gauges ranging from 14 to 10, with 14-gauge wire having a diameter of 1.628 mm and 10-gauge wire having a diameter of 2.588 mm. The electrical metallic tubing should be large enough to accommodate these wire sizes, so the smallest size EMT allowed would be one that can fit 10-gauge copper wires.