120729-1411 EDT
NSTech:
Your original question provides an interesting exercise for one in circuit analysis, but not much use in practical troubleshooting other than providing an understanding of what happens in the circuit.
The resistance of a ground rod or grounding electrode of some form can range all over the map from a fractional part of an ohm to hundreds of ohms. This will be an unknown to you.
You can estimate the neutral path resistance (includes both the neutral wire and ground resistance) by a simple measurement.
First, monitor simultaneously the voltages from both legs to neutral. Two 25 or 15 W incandescent light bulbs will work. Use the same wattage with one bulb on one phase, and the other on the opposite phase. Both at the main panel.
If both are about the same brightness and there is a load change and both brighten or dim together, then this is not a neutral problem.
If one brightens slightly and the other dims slightly, then this results from a 120 V load and the change in current thru the neutral from the load change. Neutral is probably OK. But the expected change is of course a function of the magnitude of the load change.
The eye can see a volt or two change in a 120 V supply to an incandescent.
Two voltmeters might be better in some ways, but not in other ways. Two recording voltmeters would be ideal.
If a small load change produces a big difference in the voltages, then you have a neutral problem.
Second, if it is not obvious that a neutral problem exists, then use a voltmeter and a test load.
Find the power company transformer. Run a long test lead from the ground rod at the transformer. You might measure the current in the transformer grounding wire. It should not be very much. This wire will be a moderately heavy wire relative to expected normal current, and whatever current is in the wire won't produce much voltage drop from the transformer center tap to the ground rod. If there is high current, then you know there is some kind of problem.
This test lead is taken to the main panel. Note: there could be a large voltage between the test lead wire and the main panel neutral. Be careful and use suitable equipment.
Measure the voltage difference between the main panel neutral bus and the transformer ground rod. This is the voltage drop across the transformer neutral to main panel neutral, assuming not much voltage drop on the transformer grounding conductor. If the first experiments with the light bulbs or meters did not indicate a large voltage change with normal load changes, then while monitoring the neutral path voltage connect a 100 W load to one phase. Note: the change in neutral voltage difference. At 8 A at my home I get about 0.2 V change. At 200 A this would extrapolate to about 5 V. If with the 100 W load you get a very small change, then try a 1500 heater, about 12 A.
Note: my calculated neutral path resistance is about 0.2/8 = 0.025 ohms.
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