How to tell if the electrical panel is single-phase or three-phase?

Question

I have a question in my school. Goes like this. 1 line diamgram and the panel C is the electrical panel. How is it possible for you to determine from the diamgram (not shown) if panel C is a single phase or three-phase panel. Here are the answers they give you. A) because the conductors feeding panel C are in 2.5 inch conduit, B) the letter C is a standard code that identifies it as single phase, C) the four #3/0 conductors feeding it most likely identify it as three-phase. but looking at a three-line diamgram of the same system would tell me for sure if it's a single-phase or three-phase or D) All time clocks are single-phase, so it must be single-phase since it has a time clock connected to it. Sorry I do not have the single-line diamgram uploaded. But could someone try to figure it out, by what I said. I think th answer is C. Thanks!

Answer 1

I would pick C) while 3 wire single phase circuits are common (at least in the US, e.g 120/240V)  and there may well be a fourth equipment grounding conductor, the grounding conductor is usually smaller than the current carrying conductors, so 4 conductors the same size suggests three phase (and grounding via the conduit)

Answer 2

The best answer is C.

If all four wires are 3/0, then you most likely have a 3-phase, 4-wire service.  For a typical school in the US, this would likely be 120/208 V.  The voltage between any one phase conductor and the neutral is 120 V.  Phase to phase is 208 V.

If the service voltage is 120/240 V 3-phase, 4-wire, then all three phase conductors will measure 240 V phase-to-phase.  Two of the phase conductors will each measure 120 V phase-to-neutral.  The remaining phase conductor will measure 208 V phase-to-neutral.  Electricians refer to this conductor as the "high phase" or "wild phase".

It is not D because 1Ø equipment can be connected phase-to-neutral on a 3Ø service.

Answers A and B don't make any sense.