Final answer:
The appropriate size of THWN conductors for a line current of 50 amps in a typical three-wire system can be determined by consulting the NEC ampacity chart, considering ampacity derating for multiple conductors in a conduit.
Step-by-step explanation:
The question pertains to electrical engineering principles, specifically conductor sizing for a given current load in a three-wire system with a 208/120V source and fluorescent lighting loads. The sizing of THWN conductors depends on various factors including the electrical current capacity, temperature rating, and environmental conditions. For 50 amps line current, the National Electrical Code (NEC) provides guidelines on the appropriate conductor size. The standard ampacity chart in the NEC would be consulted to determine the minimum conductor size required to safely carry 50 amps of current.
It is important to note that for three current-carrying conductors in a conduit, the ampacity of the conductors should be derated according to NEC guidelines. Assuming the conduit is installed in an environment with typical conditions and without any additional factors that require further derating, the NEC table provides a direct reference for the minimum conductor size, which would typically be a size 8 or 6 AWG for THWN wire rated for 50 amps under 'normal' conditions.