Let the voltage source , transformers(generally) , be called 'T'.

Let the electric device (drawing current from the transformer) be called 'D'.

Now the domestic circuit is a closed circuit consisting of Live, neutral and earth wires.

Live wire ---> Carries current from T to D ---> around 200V(Different for different countries but close to 200 in many of them)

Neutral wire ----> carries current back from D to T ----> has to be ideally 0V but in reality there is a small voltage.

But when we begin at 'T' and algebraically add all potential drops till we get back to 'T', the sum is not 0 (it is around 200). The voltage is different when calculated from different paths -- 200 from 'T' to 'D' and 0 from 'D' to 'T'.

Isn't this against KVL? Besides the above statements would mean the breaking of law of conservation of energy. Does the earth wire have some role to play here? Please help me find the point I am missing.

  • 1
    $\begingroup$ Please include a schematic diagram, including the reference direction for each component, and show what loop you summed the voltages around and how you calculated that the sum came to "around 200". $\endgroup$
    – The Photon
    Aug 13, 2020 at 2:48

1 Answer 1


You have left something out. In most circuits you might (a) start at the neutral of the transformer

(b) go up to the active , thats +200v difference ,

(c) go over to the load active that's 0v difference ,

(d) go down the load that's -200v difference,

(e) go back on the neutral, thats 0v difference.

Total = +200 + 0 + -200 + 0 = 0

With Kirchoff it's the potential differences that matter, not actual potentials. If active were at +300v and neutral at +100v , the above steps would be identical. By labelling the Active as "200v" you have probably confused yourself.

  • 1
    $\begingroup$ I'm well aware of the Kirchhoff's laws, I know its the potential difference that matters. However I missed the point that the potential drops at the load or device that uses current (if I am not wrong.) So that 200V which was created at live or active wire was used up by devices for spending energy. Then we go back to neutral. Thanks!! $\endgroup$
    – user248823
    Aug 14, 2020 at 2:09
  • $\begingroup$ It's a bit like pumping water out of a lake 200ft up a hill, then using a turbine on the other side of the hill to extract the energy before the water drains back to the lake. The water starts at lake level and finishes at lake level, no matter how pipes or devices it passes through, it still ends up in the lake, total height change for the whole loop = 0. $\endgroup$
    – BobT
    Aug 14, 2020 at 13:30
  • $\begingroup$ I've been messing about with electricity for 45 years, and only really used "Kirchoffs laws" in the first couple of years, after that it's so bleedingly obvious you don't think about it anymore. It's just like taking algebra of A + B =A + B turning it into A + B - A - B =0. Now Thevenin equivalent circuits, that's something that is very useful. $\endgroup$
    – BobT
    Aug 14, 2020 at 13:38

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