# How to know if questions in an exam mean line, phase, or RMS voltage?

What is the difference between line, phase and RMS voltage?

We get many questions where it is not mentioned whether the question is specifying line, phase or RMS voltage. I will attach some example questions to be more specific:

In all the three questions, how do you know if it means line, phase or RMS voltage?

• ALL voltages are ALWAYS RMS in such problems unless otherwise specified. You do not appear to know what RMS means. You MUST understand it. Your course notes, your text books and the internet are dripping with RMS references - find out now. | In a 3 phase system that is delta connected the winding voltages in eg a transformer are the line to line voltage. | There are std AC 3 phase voltages. If they say 400V or 415V this is almost certainly phase-phase. A 100V (Q2) system is more obscure but as they say delta connected odds are they mean phase=phase as no winding has phase to ground applied. – Russell McMahon Aug 19 '14 at 15:13
• They told you it's 3 wire. That means there's no neutral from the supply. It has to be line-line. And no, RMS vs peak has nothing to do with this. – SomeoneSomewhereSupportsMonica May 11 '18 at 6:18

On AC power questions, unless otherwise specified the default measurement is RMS.

On 3ph power questions, unless otherwise specified the default voltage is always line-voltage$^1$.

Phase voltage is between any of the 3 power-conductors and a neutral wire (as per star connected loads. Line voltage is between 2 of the 3 power-conductors and, for a balanced load, this will be $\sqrt3$ times higher than phase voltage.

RMS is the preferred method of measurement.

$^1$ Except in aerospace where phase voltage is always the default.

• not always, Aerospace is ALWAYS phase voltage. – JonRB Aug 19 '14 at 15:15
• @Naib After I wrote my answer I thought that there must be exceptions to the rule and so I'll list this in the answer. Thanks – Andy aka Aug 19 '14 at 15:22