I'm not entirely clear on the proper terminology for polyphase AC systems. I hear it used informally, but I'm not sure what the proper formal names are, if there are any. How I've got it in my head is this:

  • A leg is a current-carrying conductor
  • A phase is the voltage difference between two of these conductors

Is this accurate? Also, is there standard nomenclature for the names of the legs vs. the names of the phases? (A B C, 1 2 3, AB BC CA, that sort of thing.)


3 Answers 3


"A phase" is usually one of the three 'live' conductors - and a 'leg' is presumably just slang for that. A 'line' would also be the same thing. So no, I don't think you do have it right.

In the UK they've often been referred to by colour ("Red phase", "Blue phase", "Yellow phase"), though now our cable colours have changed, I guess they're "Brown Phase", "Black Phase" and "Grey Phase". They're also often labelled L1,L2 and L3. Custom and practice this sort of stuff does rather vary by country, so if you want to blend in with local electricians you'll probably need local advice.

'Phase voltage' is the voltage between one of the phases and neutral. 'Line voltage' is the voltage between two of the phases - usually sqrt(3) * phase voltage.

The word 'phase' comes about because the three live conductors carry the same waveform but shifted by a phase angle of 120 degrees between each conductor - in normal 3ph systems, one almost never talks/cares about the phase angle, because it's always 120 degrees and it's irrelevant to everything which electricians do in normal work.


I use this: -

  1. Line voltage is the voltage difference between two conductors (this doesn't include earth or neutral)
  2. Phase voltage I use to denote the voltage difference between a "line" and neutral

Phase current is exactly the same as line current when talking about the cable but when it comes to loads (star versus delta on 3-phase systems) the distinction gets a bit cloudy because with a star load, the line current is the phase current into a limb of the load whereas line current into a delta load splits into two phase currents because the load makes two connections to each line.

Clear as mud!


Terminology varies by country.

In Australia, we have:

  • A phase is one of the three active conductors. I.e. 'red phase', 'white phase', 'blue phase'. The word 'phase' is used in the same sense as your 'leg'.

    In general usage the phase is the physical conductor or something associated with the physical conductor. We speak of 'connecting a load to red phase', or 'the red phase cable was faulty', or 'the red and blue phase CT's were swapped'.

    Note, the red/white/blue colour system is standardised in our national electrical code, AS3000. This used to be 'red', 'yellow', 'blue', after the British system. Green/yellow stripes is earth, black is neutral.

    You can also talk about A phase, B phase, C phase, or L1/L2/L3 after the European system, or Brown/Black/Gray after European colours... as Will Dean notes, there is a great deal of variance between countries. However Australian electricians and electrical engineers refer to the phases by colour.

  • Line-to-neutral voltage is the volts between a phase and neutral.

  • Line-to-line voltage is the volts between two phases.

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