The connection at the transformer defines the system voltages. The system voltages define whether the system is 'single voltage' or 'multi-voltage.
While this is true for measured L-L and L-N / L-G Voltages, in the case where only the Line "A", "B", and "C" Conductors are brought from the Transformer to the first Panelboard (not including the System Conductor typically referred to as the "Neutral"), the SDS would be a Single Voltage System.
Reason: Only one Voltage is being Utilized
Circuitry and operation of Loads.
This defines a System as "Utilizing" a Single Voltage.
In the case of a Transformer with Wye connected Secondary Coils, the Star Point would be Grounded, so as to offer the lowest possible Voltage-To-Ground on that System; however the only Conductor which would extend beyond the Star Point (X0) would be an Equipment Grounding Conductor (EGC).
As an EGC is not an Active Circuit Conductor, it would not be considered a System defining Conductor (i.e.: 3 Phase 4 Wire would have 3 Ungrounded Conductors + 1 Grounded Conductor as "Active" Circuit Conductors)
If the neutral point (i.e. X0 terminal) is solidly grounded it is multi-voltage, regardless if the panelboard does or does not contain the grounded conductor.
Not necessarily... this just means the System is Solidly Grounded, as opposed to an Ungrounded System.
The reference to whether or not a System has one or two Voltages comes from the Active Circuit Conductors that Loads may be connected across.
Let's use Three separate examples:
Example #1: 1 Phase Isolation Transformer; Split-Coil Windings on Secondary, Single Coil Primary...
120V Secondary Windings setup in Series, with Jumper across X2 & X3, so as to create 240V between X1 and X4.
If we tap that jumper between X2 & X3, bond it to a Grounding Electrode System (GES), plus utilize the tapped Conductor for Active Circuitry, we have created a Grounded Neutral Circuit Conductor, and the System is 120/240V 1PH 3 Wire.
If we tap the jumper, but _ONLY_ bond it to a GES, then we have a 240V 1 PH 2 Wire Grounded System, with maximum of 120V to Ground.
Even though the L-G Potential is 120V from either of the two Ungrounded Conductors, the System is still a Single Voltage System - as the only loads connected to it will be for 240V, not 120V &/or 240V.
Example #2: Center Tap Grounded 3 Wire Delta...
Similar to the example above, to achieve a Grounded Delta System without having to "Corner Ground" it, the "X4 Center Tap" of the winding between Phase "A" and Phase "C" would be bonded to the GES _ONLY_, without a System Grounded Conductor brought to the Panelboard from X4.
Only an EGC would be brought to the Panelboard.
In this case, the max Voltage to Ground would be 200V (+/-), and the nominal Voltage to Ground would be 120V.
Lines "A", "B" and "C" would be the Active System Conductors, and the System would be designated 240V 3 Phase 3 Wire.
Example #3: Corner Grounded Delta...
Line "B" on a # Phase 3 Wire Corner Grounded Delta is an Active Circuit Conductor, which is bonded to the GES, identified by White tape, and is that System's Grounded Circuit Conductor.
Voltage between "B" and Ground (EGC) is close to zero, and raises as the distance from transformer increases.
Voltage between Lines A-B, &/or Lines B-C equal the same as measured between Lines A-C.
Also, Voltage to Ground from Line A or Line C will equal the same voltage as measured between A-B, B-C or A-C.
This is, of course, a Single Voltage System.
What defines it the most is the _Single Voltage_ Measured Between Lines A-B, B-C and A-C, and that there is no other Active Circuit Conductor being utilized which a different voltage could be measured (either higher or lower than the L-L Voltage), and which a Load would be connected across "in a correct or proper fashion".
(meaning the Load is not connected to one Line Conductor, and directly to an EGC).
To sum things up;
In the case of defining a System as Single Voltage -vs- Two Voltages, the Conductors utilized for Normal Operations Of Designed or Connected Loads
determines the scope.
Hope I did not come off sounding arrogant, I just wanted to try to clearly explain the logic behind it, and where Articles 240 + 450 would designate the SDS of having either a Single
All comments are welcome.
Please critique wherever necessary.