Originally Posted By: Charles Palmieri
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Kevin it would help if you could be a bit more discript regarding the circuit configuration.
That being said there are several types of three phase services but typically your concern seems on target. Most services for other than industrial applications (SOMETIMES) will consist of A four wire installation. That is, phase A, B, and C (UNGROUNDED OR HOT) with a fourth grounded conductor that has equal potential from it to the other three ungrounded conductors (refered to as the neutral) this fourth (neutral ) conductor provides common lighting voltages generally 120V or 277V depending upon the premisis requirements.
This does not exclude three wire installations with out a lighting voltage, or neutral conductor. There are times where a straight three phase (so to say) is brought to the premises without a neutral. This is common as a three phase delta supply and in many cases one corner will be grounded (yes it is a grounded phase leg).
If you are looking at a HIGH (above 600V) voltage entrance in the diagrams you may be viewing an installation that does not have a utility supplied grounded conductor and in that case only three wires would enter the premisis. In these cases you ususally have a customer owned distribution transformer and the end user will benifit (usually) by having primary metering. What is the voltage of the conductors that you are questioning
Hope this helps