Electrical – 3 way switch bathroom wiring to code

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

Answering your questions in turn:

1. Yes, you would have to bring Circuit #2 up to Code if you altered it. The applicable provision is actually E3401.4 of the International Residential Code as your jurisdiction doesn't use the Annex H model legislation to adopt the NEC.

2. The primary hazard the current wiring poses is that tripping Bathroom 1's GFCI turns the lights out on the unsuspecting occupant of Bathroom 2, which could lead to a minor disaster (say involving a hot curling iron, or stumbling and falling in the shower in the dark).

3. Adding the fans to circuit #1 isn't a problem, by Code -- bathroom outlets other than the required receptacles serving the countertops can be on any general-use branch circuit. With a fair bit of rewiring, though, it's possible to juggle things so that the bathroom lights and non-bathroom receptacles move to circuit #1 while the bathroom receptacles stay on circuit #2, which would make things almost fully Code-compliant (bathroom #3 would need to be addressed separately, but that's simply a matter of putting a new breaker in the panel and moving bathroom #3's homerun to it, based on your diagrams).

4. Last but not least, you don't need a divider to put two different circuits in the same wallbox as the voltage between adjacent devices can't exceed 300V on a residential 240/120V service. The requirement for dividers between circuits in mains-only boxes only kicks in for commercial folks and their 277V lighting circuits, see NEC 404.8(B) for details.