What does the line to line voltage mean?
For WYE connection is Vna line to line or Van?
How about Delta connection?
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\$\begingroup\$ See also electronics.stackexchange.com/questions/92678/… . \$\endgroup\$– Li-aung YipDec 19, 2013 at 9:07
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2 Answers
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In a wye or star connected system there are 4 wires available as shown below: 
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Source:
The phase voltage is the voltage between any of the phases ABC and the neutral. Therefore Van is the phase voltage and Vna is the same thing, just reversed. The line to line voltage is the voltage between any two of the three phase wires ABC, e.g. Vab.
With delta, there is no neutral wire available and so the voltage between any two of the three available phase wires is the line voltage by default.
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Most three-phase power service is provided through a "wye" connection. In a wye connection, the line to line voltage is equal to the rated voltage, that is to say, a 120/208V wye connection will have 208 volts (RMS) between any two phases. If you call the three phases a, b, and c, then Vab = Vac = Vbc = 208V.
The voltage between any phase and neutral (for example Van) is the line-to-line voltage divided by 1.73 (the square root of 3), for a 208 volt system this would be 120V.
In a "delta" connection, which is less common and now unobtainable from power utilities, the voltage from phase-to-neutral varies: For a 120/240V system, the line-to-line voltage between any two phases is 240V, but Van and Vbn = 120V while Vcn = (Vab/2) * 1.73 or 208V.
See http://www.phaseconverterinfo.com/phaseconverter_deltawye.htm http://www.ccontrolsys.com/w/Electrical_Service_Types_and_Voltages
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\$\begingroup\$ Delta connection is only unobtainable in 208V. Medium voltage connections (i.e. 11kV in my country) are routinely provided as delta. \$\endgroup\$ Dec 19, 2013 at 9:09