Supply is from 75 kva transformer, 3 phase delta /y three wire feeding a 30 amp breaker. 100 feet down stream is a junction box that I want to tap and produce a 120 v single phase ( 20 amp CB) supply. The junction box feeds a 6 amp motor

What is your question?

Since you only have 3 phase 3 wire, you will need a minimum 2500 watt transformer to create the 120 volts, and ground the neutral of the 120 volt output.

Are you saying you have a 3 phase 120/208 4 wire y system or is a delta 240 or 480 (common but old here).. If it is 208v phase to phase.. the phase to ground reading should be 120v on each of the phases.. if it is a delta than phase to ground and phase to phase would read the same.
Please provide more info or as tk asks a question..

I have a Delta/Wye 480v/208v 3 phase THREE wire Wye circuit (no neutral). 100 feet away I want to tap the 208 3 phase and develop a 120 v single phase circuit. Since there is no neutral at this point how do I go about getting 120v neutral to phase when I haVE no neutral
Thanks

Feed a single phase 2.5 Kva transformer, 208 VAC Primary/ 120 volt Secondary, and ground one of the 120 volt legs to a grounding electrode, building steel is sufficient if steel frame.

Be sure to provide both Pri and Sec overcurrent and short circuit protection.