130604-1005 EDT
Steelhead:
I don't really understand your first post, but the following is a means to troubleshoot the problem.
You need to establish a useful voltage reference point. Most likely this is at a main panel where earthing ground, EGC, and neutral all terminate at one reliable low resistance point. This means under load conditions that there is next to zero voltage difference between any combination of the three wires. This reference point becomes the reference for all voltage measurements. Connect a fourth wire to this point that is one test lead input to your voltmeter. You can use a #18 wire from a roll of wire, or extension cords connected together.
Use a good multimeter with millivolt resolution. I like the Fluke 27.
From this reference point to any location on any EGC there should be very little voltage. If you connect a 1500 watt 120 V heater, this is about 10 A, from a 120 V source to the any point on an EGC there should not be a large voltage difference. This will be a function of the resistance of the EGC. 100 ft of #12 copper is about 0.15 ohms, and the voltage at 10 A would be about 1.5 V. If you suspect EGC problems, then use a much lower source voltage, 6 V maybe, to generate the test current. A filament transformer with 5 to 10 A capacity and appropriate resistor can provide the test current.
Obviously you don't apply a test current if there is noticeable voltage on the EGC. You simply go looking at various points on the EGC to isolate where is the origin of the voltage.
Neutral voltages will be a function of neutral resistance and neutral current.
The voltage change on hot wires will be the wire resistance times the current change in the wire. So changing a known load here can be helpful.
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