Hello, everyone, and my thanks to all who replied to my OP. Although I didn't peer inside the outlet box I suspect I encountered a 208 VAC single-phase service consisting of two hots and a ground. I further discovered that a number of the higher kVA UPSs on the market are designed to operate from 208 VAC single-phase and can often use 240 VAC single-phase as an alternative. (my only previous experience had been with hefty AC-DC power supplies that required 120/208 or 277/480 VAC
3-phase on the input side) Sincerely,