An "Interesting" Question
Gentlemen, (and Cecile et al.)
While wandering over the Internet, I came across a "Posting" on a US site which is similar to this one.
The direct reference to the start of this "posting" is Running 220v Into A Bedroom, Adding 30 Amp Double Pole Breaker - Electrical - DIY Chatroom Home Improvement Forum
If you start reading these posts, I think that you quickly will gather what the OP is attempting - and all is eventually explained in Posts 41 and 42.
Along the way there are some interesting discussions concerning North American electrical practice.
I found this "discussion" to be interesting, on several levels, and it maybe that many of you will also find it "interesting".
Read the full thread at RenovateForum.com...