Re: 120/208 Single Phase
I’m not sure I understand your question, since the “slash notation” (i.e., 120/208) necessarily refers to a three-phase system. Also, I am aware of no explicit rules that require a different meaning if the low number appears first (120/208) as opposed to second (208/120).
On a three-phase panelboard, connecting a circuit to one single-pole breaker and to the grounded bus gives you 120 volts. Connecting a circuit to one double- pole breaker without a wire attached to the grounded bus gives you 208 volts. Is this what you are referring to? I can’t be sure, because it didn’t require math.
Charles E. Beck, P.E., Seattle
Comments based on 2014 NEC unless otherwise noted.