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j98tt:
Assuming the neutral problem is before or within the main panel, then you can do the following:
1. Equipment a good Fluke or equivalent digital voltmeter that can resolve 0.1 v at 120 V, and a 1500 W heater or hair dryer, about 12 A.
2. Make two portable outlets that can be connected to the two phases in the main panel. Do this thru two breakers in the panel from which you have removed their normal circuit.
3. Turn off all breakers in this panel except these two and the main breaker or fuses.
4. If the power company transformer is close by run a long wire to near its ground rod and connect to a screwdriver in the ground near the ground rod, maybe 6 ft away. This will be one test lead you will use in the experiments.
Otherwise put the screwdriver in the ground in line with the direction to the transformer as far toward the transformer as feasible. One or more extension cords make a good long test lead. Severe neutral problems may produce a large voltage between this probe wire and other points to which you measure voltage. Be careful.
5. With no load on the panel you should see nearly 0 volts difference between the probe lead from the transformer to the main panel neutral, its grounding electrode, and the water pipe. I would like to see 0 volts to mean something less than 0.25 V.
6. Next place the heater load between one hot leg in the panel and the panel neutral. What are the voltages of 5 now? This will depend upon the resistance of this path and the about 12 amps load.
Also measure the difference between the long probe lead and the neutral wire into the main panel from the meter. By wire I mean touch the wire and not the input lug. This eliminates a voltage drop at this main lug.
Depending upon the result from the tests determines the next action and likely points to where the problem may be.
The results may determine different measurements to make.
Some other simple measurements:
No load. In the main panel measure phase A bus bar to neutral bus bar, then the same from B. Next A to B. Apply the load from A to neutral and make the same measurements. And repeat from B.
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