Think of the transformer that is supplying the service. It will be a single primary winding with a single winding secondary. The connection at the primary is not important for the question you are considering.
240V is on the secondary from one end of the winding to the other end of the winding.
Putting a centertap in the secondary winding gives 120V above and 120V below. For the instant that the Line to Line voltage is at its max, the difference from line to line will be 240V, therefore, the 120V above and below the centertap add to each other.
Now, ground the centertap.
The line to line is still 240V. The centertap to line is still 120V. One of the voltages has to negative 120V while the other is positive in order to get a total of 240V.
So, paying attention to the direction of the current, one 20A line
current must come out of the winding, while the other 20A line current is entering the winding on the other end of the winding. There is no current flowing from the centertap to or from the neutral.
Another way to get at this is to think of the current in each loop. . .half of the secondary winding, line, load, neutral, centertap; and the other half being identical. Both loops 20A currents will travel the same direction clockwise, then counter clockwise, alternating.
The neutral (common between the two loops) has two 20A currents in it going in exactly opposite directions, cancelling each other.
Hope some of that connected somehow.
Edited for Sp. & clarity.
[This message has been edited by ElectricAL (edited 07-07-2004).]