How can you tell. Is it because (3) wires come off of a single phase transformer? And (4) off of a three phase transformer? Thanks!
Three phase transformers have at least three pairs of coils. Single phases will only have one pair of coils. You can have three wires come off of a three phase transformer. It simply means that you don't have a neutral, and that the transformer is in a delta configuration, instead of a wye configuration. Usually, if a system has a neutral wire, or any other conductor that is ultimately grounded somewhere in the circuit, it will be color coded as natural gray or white.
Looking at a transformer intended for mounting on a utility pole, you would see insulating bushings protruding from the metal case. The longer bushing or bushings are the high voltage bushings. The smaller bushings are the low voltage bushings. A single phase transformer would have only one or possibly two of the long high voltage bushings ( typically coming out of the top of the transformer on newer designs intended for mounting on poles typically by a utility ). A three phase transformer would have at least three or possibly four of the long high voltage bushings. All power transformers should also have a Nameplate permanently attached to the case. Primary bushings are typically identified on the nameplate with a H designation such as H1, H2 etc. Low voltage bushing are identified by an x designation such as x1, x2 etc. Make certain that the transformer is NOT energized before attempting to look at a nameplate or come near a transformer. Multiple single phase transformers may be connected into what is called a transformer bank to produce three phase power. At least two single phase transformers are necessary to make a three phase bank of transformers.
That would be a clue. If the transformers are mounted on power poles they will be exposed. For three phase you would see three separate transformers wired together.
