How does a center tapped transformer work?
With DC it's obvious.







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. Two 9 V batteries series connected but with various grounding options.


  How can B be at a 120V less than G, what would that mean? Can someone explain the theory behind why/how this works?


You are forgetting that your meter only measures magnitude of an AC voltage. There is no sign so saying that B is less than or greater than ground doesn't make much sense.







simulate this circuit

Figure 2. For AC we have the same situation as with DC but with no sign - just the voltage magnitudes.

When working with AC, therefore, referencing to ground doesn't tell you what the voltage is between two non-grounded points. Let's look at a high-leg delta transformer arrangement which should be familiar to you in North America (but always looks so unbalanced to us Europeans).

 

Figure 2. This transformer arrangement gives the possibility of obtaining single phases at 120 V, 240 V or 208 V plus the possibility of 3-phase 240 V. Image source: Wikimedia Commons.

If you stick one probe of your meter into the neutral (earth) terminal and measure around the circuit with the other you will get:

L1 - N:   120 V
L2 - N:   120 V
L3 - N:   208 V


These are all correct but without the knowledge of the transformer configuration there is nothing to tell you that these have three different phase angles. If we then take some more readings we get a better picture:

L1 - L2:  240 V
L2 - L3:  240 V
L3 - L1:  240 V


With three points and three same voltage measurements between any two of them the only solution is an equilateral triangle - a three-phase supply. Note again that all the voltages are "positive" as we are only reading magnitude.