Final answer:
To find the smallest conduit size for three No. 8 TWN 75 conductors, one must consult the NEC tables which provide the fill capacities. Without access to NEC tables, it's likely that a trade size 16 (1/2") conduit may be too small, and a minimum of trade size 21 (3/4") or trade size 27 (1") may be required.
Step-by-step explanation:
The smallest size of conduit required for three No. 8 TWN 75 conductors can be determined by referring to the National Electrical Code (NEC) which provides a table for conductor fill capacities within a conduit. No. 8 conductors have a specific area, and three of them combined must not exceed the fill capacity of the smallest available conduit suitable to accommodate them. Without the NEC table at hand, it is not possible to provide an accurate answer, as the table is intricate and must be consulted for precise conduit sizing calculations. However, it is typically expected that for three No. 8 conductors, a trade size 16 (1/2") might be too small, and you would likely need at least a trade size 21 (3/4") or trade size 27 (1") conduit, depending on the type of conduit and the exact specifications of the wire insulation.