Here is a great example of how a split phase secondary winding of a utility power single phase 240V/120v transformer works.
https://www.youtube.com/watch?v=eVamt9IdQd8
Note: Only the unbalanced load of one Hot leg, Line, through a load(s) to neutral , and the other hot Leg, Line, through a load(s) to neutral returns to the neutral center tap of the 240V/120V secondary winding of the Utility Power Transformer.
If both connected 120V loads are exactly the same, zero current (amps) will return on the service neutral conductor to the neutral leg of the transformer. The two identical loads are in series with one another and are fed by 240V.
If one 120V leg to neutral has a load of 20 amps and the other 120V leg to neutral load is 10 amps then 10 amps, the unbalance load (current), will return on the electrical service neutral conductor to the utility power transformer neutral leg.
Pay close attention in the video when the two loads are not equal and the electrical service neutral conductor connection to the neutral bus bar in the electrical panel to the connection of the neutral leg of the utility transformer is open/broken. (knife switch on the transformer neutral wire in the video.)
More common, instead of the connection being open/broken the connection is loose and or corroded and adds a series resistance in the loose and or corroded connection. (Heat in the poor connection caused by current adds more resistance) That will cause an uneven voltage at the electrical service panel from one Hot Leg to neutral and the other Hot Leg to neutral. The Hot leg to neutral with the greater of the two connected loads, the voltage will be lower than 120V nominal. The lesser loaded Hot leg to neutral voltage will be higher than 120V nominal. The voltage differences between the two depends on the connected loads, and constantly changing 120V loads connected to circuit breakers in the panel.
Example the higher loaded Hot leg to poor connection of the electrical service neutral conductor at the main electrical panel neutral bar might measure 110V. The lesser loaded Hot leg will measure 120V nominal + 10V = 130V.
FWIW I have seen this happen more often with multi wire branch circuits where a neutral conductor is shared with two hot conductors. The 3 wire multi wire branch circuit neutral conductor has a poor/bad joint connection.
(The two Ungrounded Hot conductors must be connected to circuit breakers on opposite legs. One to L1 and the other to L2.) ( Starting, 2008 NEC required a 2 pole breaker or two single pole breakers with a Listed handle tie to connect the two handles together)
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