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I have been reading that in a center tap transformer, we get 2 voltages which are 180 out of phase. Can someone please explain how that happens.?

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  • $\begingroup$ See my revised diagram better showing how the center tapped voltages are 180 out of phase. $\endgroup$ – Bob D Jan 2 at 17:13
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A 120/240 center-tapped secondary winding of a transformer is the typical means of supplying household power in the US. The center-tap is connected to earth ground. The secondary sinusoidal voltage is 240 vac between the outer legs, +120 Vac between one leg and ground and -120 Vac between the other leg and ground, making the two 120 Vac circuits $180^\circ$ out of phase. See circuit diagram below. Applying Kirchhoff's voltage law: clockwise +120 +120 -240 = 0.

The 120 Vac circuits generally supply lighting and general branch circuit outlets while the 240 Vac circuits supply major appliance loads (electric ranges, ACs, clothes dryers, etc.). Neutral conductors of the 120 vac branch circuits are connected to ground. The center-tapped ground assures the maximum voltage to ground for the 240 Vac circuits in the household is less than 150 Vac (reducing risk of severe electric shock to ground).

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  • $\begingroup$ If effective voltage is 120 V, then the maximum voltage is $120\sqrt{2}=170$ V. $\endgroup$ – Ján Lalinský Jan 2 at 20:26
  • $\begingroup$ @JánLalinský Correct. Got a little sloppy there. will correct. Thanks. $\endgroup$ – Bob D Jan 2 at 20:44
  • $\begingroup$ Do you think 120 V causes meaningfully less harmful shock than 240 V? Numerically, yes, but in practical terms, I would think both are lethal if supplied by a hard supplier such as the power grid. $\endgroup$ – Ján Lalinský Jan 2 at 20:50
  • $\begingroup$ @JánLalinský The higher the voltage to ground, the greater the risk (potential severity) of electric shock event, all other things being equal. NFPA 70 (National Electrical Code) requires most equipment involving voltage to ground greater than 150 v to be either grounded or double insulated. $\endgroup$ – Bob D Jan 2 at 20:57
  • $\begingroup$ Well, I'm trying to understand your statement "The center-tapped ground ... reducing risk of severe electric shock to ground". It obviously reduces mathematical risk and allows manufacturer to build and sell cheaper equipment, but do electricians in US actually believe using 120 V as opposed to 240 V makes a meaningful difference in term of user safety? $\endgroup$ – Ján Lalinský Jan 2 at 21:04

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