# Center tap transformer- How do we get voltages that are 180 out of phase?

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.?

• See my revised diagram better showing how the center tapped voltages are 180 out of phase. – Bob D Jan 2 at 17:13

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.
• If effective voltage is 120 V, then the maximum voltage is $120\sqrt{2}=170$ V. – Ján Lalinský Jan 2 at 20:26