All right, I don't get it. I tried and tried, but I just don't understand. I have a dozen questions about this so somebody just pick one out all of these, I don't care.

What's inside a North American residential 240V pole transformer?

Is it (secondary) double iron core windings with 120V on each core and each end of the coil has one hot and one neutral wire that get tied in series to add up to 240V?


Is it a single iron core winding with two hot wires on each end and a center wire tapped at the 120th winding to make a 120V wire?

Are there two sine waves in a (secondary) double iron core transformer or one?


Are there two sine waves in a (secondary) single iron core center tapped transformer or one?

If either of the two types of transformers do truly have two sine waves, are they 180° opposite from each other at peak?


Are they 90 degrees out of step from each other at peak?


Are they on top of each other?

Please don't use to many equations or symbols, because I've exhausted my explanation abilities as it is with these questions and might not understand much more than the simple words that were already used here.


The power feed to a North American residence is normally provided by a center-tapped transformer, with the center tap of the transformer secondary grounded. the full secondary winding produces 240 Volts.

the connection is like so:


simulate this circuit – Schematic created using CircuitLab

Arghhh! The shcematic editor won't let me draw what I want!

The two ends or the transformer winding (120 V A and 120 V B) are each 120 Volts from Neutral, but 180 degrees out-of-phase, so you get 240 volts between them.

Most outlets in a home will connect between 120 V A and Neutral, or between 120 V B and neutral, so will get 120 volts.

Certain heavy loads (electric stove, electric water heater, electric clothes dryer) will connect between 120 V A and 120 V B to get 240 volts.

  • 1
    \$\begingroup\$ Yeah, that's why I hand-draw stuff relating to mains. Positioning and cable grouping is also important there. It probably helps to not relate neutral to backplane GND, which it isn't, or draw the N-G equipotential bond as a specific component. \$\endgroup\$ – Harper - Reinstate Monica Mar 4 at 14:06

I designed electrical systems for buildings since 1983. I think I can make sense of this for you!

Forget the 180 degrees out of phase that you are being told. You have a single phase sine wave on the primary side of the transformer. One sine wave. Let's assume for a minute that it is 1 volt per turn on the secondary of the transformer. You have three terminal connection points L1, L2, and N. You have 240 total windings with a conductor also connected at 120 turns. As this is AC, let's assume first half of secondary sine wave travels from L1 to L2. At the same exact time, same direction it travels from L1 to N and N to L2. So two sines at 120 windings and volts and one at 240 windings and volts.

The second half of sine wave, again at exactly the same point in time, goes from L2 to L1. etc. This is called split phase because there is more than one voltage from a single transformer. This is not what is taught in engineering classes or looks like on an oscilloscope because an oscilloscope has one common lead and reads from L1 to L2, from L1 to N, and from L2 to N. This is what actually happens inside the coil of a transformer. Please ask those saying 180 degrees out of phase to explain how the sine waves can be out of phase! All the secondary sine waves MUST happen at exactly the same time as the primary and the rules of electricity say in the same direction through the coil.

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    \$\begingroup\$ As one of those who say that L2 is 180° out of phase with L1 please allow me to point out that we measure the voltage and phase from the centre tap which is neutralised by the Earth link. If you check this with an oscilloscope you will get two AC sinusoids 180° out of phase. We don't use one end of the secondary coil as reference as you seem to think. Welcome to EE.SE. \$\endgroup\$ – Transistor Mar 22 at 23:34
  • \$\begingroup\$ Yes that is what you will get on the scope. But there is no good explanation for the electrons being as termed "out of phase" . The system is in phase, the test is out of phase. I just think it confuses people. \$\endgroup\$ – Kevin Cooper Mar 23 at 1:35
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    \$\begingroup\$ Nobody else is talking about electrons. Everybody else is talking about voltage and current. "The system is in phase, the test is out of phase." No, the test measurement shows what is really happening to the voltages with respect to ground. Add a phasor diagram into your answer showing voltages with respect to neutral / ground. \$\endgroup\$ – Transistor Mar 23 at 7:42

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