The wires themselves don't have polarity, that only exists within an outlet based on it being wired correctly, or reversed (hot/neutral reversed).
So essentially you just need to identify the hot wires and the neutral wires, and if you maintain this correctly throughout all of the wiring and correctly wire up your outlets, then polarity will be correct.
Non-contact testers are good for determining if there is any power on, but not perfect for finding the hot wire - sometimes neutral wires will give false positives if there is a hot wire in close proximity.
The best tool to use is a multi-meter. The neutral and ground should read 0 (or a very low number), and hot to neutral or hot to ground should give you 120V (assuming your are in North America).
A standard outlet tester will tell you whether the polarity is correct or reversed. If it's reversed, then you just swap the hot/neutral wires.
It sounds like you have or had knob and tube wiring that might have been converted,
which consists on a Hot and a Neutral With no Ground (which is needed for GFCI, but
sometimes use the Neutral pigtail to mickey mouse false ground,).
*You can add a ground with a wire to copper pipe from sink’s water supply or run to basement)
14-2 wire is rated at 15 amps MAX
12-2 wire is rated 20 amps MAX
If you splice or run off a 14 Gauge to a 12 Gauge you must treat it as a 14 gauge
The higher the gauge the smaller the wire weakest link being 14 while using 14 to 12
If you are rewiring the whole bath and rooms then 12 gauge is the sanest answer
12 Gauge handles 15 or 20 amperes 14 should not be more than 15 for safety reasons
(When using 12 on 20A breaker use screw terminal as opposed to tension insert)
Testing for a hot wire when there is no labeling on wire casing (sheath) use a meter on
~ AC
And use a ground from ground cold water pipe and then touch pos lead on each wire (they
do sell polarity test tools also don’t always work with conversion wiring)
The 110 – 120V reading wire is the hot
Hope that answers your question.
Sincerely BEAR
Best Answer
My house had a fair bit of that wiring. It was apparently available both with and without ground but the ungrounded type is all I have ever seen.
If you arent doing any work on it its probably fine to just leave it alone. If you start doing significant rewiring, adding outlets, or any of the cloth is damaged, its probably time to run new cable back to the electrical panel. In my case, the cloth on most of it had become very brittle and started to flake off if it was moved too much, so if rewiring things I would replace it.
I have slowly replaced most of it in my house when convenient - when we were renovating and had the walls opened up anyways, mostly to get proper grounds everywhere.