Line fault at the outlet -- do I need an electrician?

Guys, way to much time has been spent on this thread about the OP measuring a difference of potential, voltage, of 4 volts, from the neutral to the equipment ground contact on the receptacle outlet.
His biggest problem is reversed AC polarity he measured at the outlet. And as he has later posted the same reversed polarity he measured at other outlets on the same branch circuit.

As for the 4 volt the OP measured was it really 4Vac? Who knows.

I just checked a duplex receptacle that is fed by a 20 amp dedicated branch that is used to feed a treadmill. The branch circuit is #12/2 with ground Romex that is about 40ft from the electrical panel at best.

The test:
Multimeter, a Fluke 87. First test, meter set to AC volts. Default range auto scale. Reading,
power switch turned on treadmill only, standby state. Belt not running. Line voltage, 121.5Vac. Neutral to equipment ground, 9.4mv. (Meter allowed to settle down). Note, mv....
(Note Motor in treadmill is DC. That means the power supply is DC. Noise?)

I changed the range setting on the multimeter to 400Vac That is the closest range available above 122Vac.
Line voltage, 121.8Vac. Neutral to equipment ground contact, (after the meter settled down after about 10 sec or so), a steady 0.3Vac.

The above is for one finite test only. A different connected load, and type of load, no doubt would yield different results. No doubt a different, longer branch circuit with different connected loads would make a difference. I could go on and with different scenarios that would, could, make a difference in the neutral to equipment ground difference of potential, voltage.

What really matters? The conductivity, integrity (for a lack of a better word), of the equipment ground to be able to safely carry a ground fault current back to the source and hopefully cause the branch circuit breaker to trip open. (Note. In the event of a bolted ground fault event the instantaneous current could very well exceed 1000 amps.)


Jim
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I would not open the panel if you dont have to. You've gotten a lot of good advice here. Like mentioned above, I would shut off the breaker to the circuit, then open up the closest receptacle to the panel that would be reversed polarity.  If you fix that one, then every other box will be fixed. Highly unlikely it is reversed inside the panel. 

2 tips. Be conscious of any switches outlets when testing, so there are no surprises.
When trying to find the first receptacle box, you can disconnect one pair of wires from the receptacle. THEN reapply power to find out if you're in the beginning or middle of the circuit. OR a more safe way is to test for continuity from one box to the next WITH THE POWER OFF. Then you'll know which is first. 
I also would not open a light fixture for starters. I say this only because of your experience.. It may very well be in a light fixture feeding the first receptacle box. So the first box may look correctly wired, but the connection from the fixture may have been reversed. This last scenario is my best guess.
Good luck
Yes, please update if you can. Curious...
Thank you

Jim: "Is the wiring in the old part of the house original wiring? Any idea what year that part of the house was built?"
Not likely. House built in 1910.
I don't know what else is on the circuit. I need to test.
Other questions will require inquiry, and I'm a novice, so may be inconclusive.

Cissado -- I will avoid opening the panel. I like your idea of trying to fix it with a receptacle.
I will watch for switches, and try the disconnect method.

Bottom line is that I'm curious up to a point. I am suspicious of the overall electrical job done in the older part of the house (DIY from previous owner). So, it won't take much to push this off onto a fully qualified electrician who will be working on other jobs (and perhaps installing a dedicated line).

The reason I'm curious now, as I think I mentioned, is that I'm trying out my audio gear and have been shoved to an inconvenient side of my room because of this polarity problem; so, a fix would be helpful in terms of audio set up.

I'll try to update soon. I have a stack of midterm exams to grade. (He says, posting on Audiogon anyway. ;-)

@ cissado

Good post except in the case of where the duplex receptacle device may have been used as a junction for the make up of neutral and Hot conductors coming in and going out of the outlet box.

We don’t know what type of wiring materials/methods were used for the branch circuit wiring.

What year NEC was in effect at the time? What were the AHJ electrical code standards/requirements back then for where he lives? Was conduit required in basements back then? Is there a chance the branch circuit is part of a multi wire branch circuit? What happens if he breaks the feed neutral at an outlet and the other circuit of the multi wire branch circuit has a connected load on it?

The OP is not an electrician. An electrician would know what to look for. Like another Hot circuit conductor passing through the box he is about to open a neutral. An open neutral on a multi wire branch circuit has killed many a electricians.

Jim