The fifth diagram shown on this page:
http://www.allaboutcircuits.com/vol_2/chpt_10/1.html
has an error.
If you have 2 identical 120 VAC sources, perfectly in-phase referenced to the same ground, both outputting identical waveforms, if you read across the 2 hot wires in this case you should see 0 Volts AC. They are parallel identical circuits and since they are in phase, the difference between them at any instant in time is 0 Volts, not 240 V as indicated on the diagram.
http://www.allaboutcircuits.com/vol_2/chpt_10/1.html
has an error.
If you have 2 identical 120 VAC sources, perfectly in-phase referenced to the same ground, both outputting identical waveforms, if you read across the 2 hot wires in this case you should see 0 Volts AC. They are parallel identical circuits and since they are in phase, the difference between them at any instant in time is 0 Volts, not 240 V as indicated on the diagram.