.........Meanwhile I have a question - when a 12-2 cable meet old style 3 wire cable (red white and black) - how do you connect these 2 cable together?
My idea is from black of new 12-2 create 2 pigtails - one will go to red (of 3 wire cable) and another will go to black (of 3 wire cable)
I agree with Wayne, the way you are asking questions we do not want for you to get hurt, cause a short, or start an electrical fire.
I found this that shows what you are dealing with. Realize that the BLACK and RED are separate circuits with a separate breaker in the breaker pane. They are 180º out of phase of each other. Placing a voltmeter between the RED and BLACK you'll read 240 volts. This type of circuit uses a "common" neutral. In the picture, your junction box is where the wire nuts are.
If you want to add an outlet on the right outlet pictured, you can use a 2 wire cable, BLK to RED, WHT to WHT and GRN to GRN. Be sure that if the 3 wire is12 gauge (12=3, 20 amp breaker), you must use a 12 gauge cabe (12-2 with ground). If the breaker is 15 amp, then 14 gauge can be used.
I would highly recommend for you to buy this book. I bought one similar back in 1974 and referred to it for many years. My first choice as a career was to be an electrician, but I ended up doing telephone work for 40 years.
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