# With a transformer center tap, why are the two resulting circuits out of phase by 180*?

I'm trying to understand how the power in my home works, and the part I'm having trouble with specifically is why in the step-down service transformer, the resulting out-of-phase 120V "hot" lines are out of phase by exactly 180*.

I understand in a step-down transformer a center tap halfway in the middle would result in half the voltage because there are fewer coils, but why would each of these circuits be out of phase by 180*?

What about if there is more than one "center tap?" Perhaps 3 instead of one? Would the resulting circuits be out of phase by 90*?

• How can there be more than one center? Jul 12 '14 at 19:53
• Taps at 1/4, 1/2, 3/4 Jul 12 '14 at 20:12
• only 1/2 is centre Jul 12 '14 at 20:23
• @ensnare: If you had taps at 1/4, 1/2 and 3/4 positions of a 240 volt secondary, the voltages at each tap, measured from one end, would be 60 volts, 120 volts, and 180 volts, with 240 volts at the far end. since I'm measuring from one end, all these voltages would be in phase. If I took one of the taps as my reference, voltages on one side of the reference point would be 180 degrees out of phase with the voltages on the other side. Jul 12 '14 at 21:46

If you had two 9 volt batteries wired in series to produce 18 volts you could connect the lower battery negative to ground and you would have ground aka 0V and +18 volts.

If, instead, you connected the centre tap of the two batteries to ground you'd have -9 volts, 0V and +9 volts - the +9 volts is, in effect, 180 degrees different to the -9 volts.

So, a centre tap on a transformer does the same - if you reference all voltages to the centre tap (let's call it 0V) then one end of the winding would reach maximum positive when the other end of the winding was reaching maximum negative.

• +1 from me, as this is more intuitive than Wouter's answer (which is also correct.) Jul 13 '14 at 15:09

180 degrees means 'exactly the opposite'. Seen from the center, the direction of the windings of the two coil parts is opposite, hence the same magnetic flux change through both coils results in opposite voltages ate the end of the coils, hence the two hot connections are 180 out of phase (when one is positive the other is negative, etc).

• Thank you. The coils in opposite directions was the missing link for me. Jul 12 '14 at 20:28
• @ensnare: Actually (and I hope this doesn't re-confuse you) the transformer secondary is a single continuous coil with an extra connection on the center turn. You will get 240 volts between the ends of the coil, and 120 V between the center tap and either end connection. Jul 12 '14 at 21:37

The opposite polarities are obtained because the center tap is used as the reference. If you use one end or he other of the secondary winding, then all the voltages relative to that on any other taps, and on the opposite end, have the same polarity.

Look, the Earth spins in one direction, yet we have time zones like +800 and -700. Why is that? Because they are referenced to UTC, and UTC is somewhere in the middle; it is not the most Eastern nor the most Western time zone.