one more question- how would you suggest to install 2, or 3 duplexes on the same line in the same location?
The wiring method depends on the construction of the house. Without knowing the construction of the house there is no easy answer to your question. The electrician you hire will look over the construction of the house and will determine the options of the wiring methods he will have. He then should explain the options to you.
Things to consider:
Wood stick framing? Other?
Basement? Unfinished or finished ceiling in area below the audio room?
Foundation crawl space?
House/audio room built on a concrete slab?
Above ceiling attic accessible crawl space? Other?
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i don’t know the feeder size, bu the one I have in my current home is pretty big
you can see it in my system pics
what size do you think I’d need?
Well if you are going to spend the money to have a sub panel installed the bare minimum size in my opinion would 70 amp. Wire size #4 copper. Or bump up to #3 which is good for 85 amps.
What is the aprox total length of the feeder? Include up down and all around in the calculation. Does the garage have an accessible attic crawl space to install the feeder? If so you might want to consider NM-B sheathed cable or MC instead of conduit with loose random conductors pulled inside. Depends on the total run, length, of the conduit and installed feeder conductors.
http://centralindianaaes.files.wordpress.com/2012/09/indy-aes-2012-seminar-w-notes-v1-0.pdf
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And what is your opinion on twisting the wires?
Twisting the wires? As in wires, conductors, that are pulled in an empty conduit? Not sure that meets NEC (National Electrical Code). Note: Dedicated branch circuits should never share the same conduit or raceway with other branch circuits. That includes multi conductor cables.
For the branch circuit wiring from the sub panel to the wall receptacle outlet boxes I would use NM-B cable, Romex. (If local code permits. And or the wiring method that must be used will meet local code for NM-B cable.) If the wiring must be in conduit ask the electrician if aluminum armor MC cable, 2 wire with ground, meets local code. Note: MC, not AC cable.
(MC comes in both solid core and strand copper conductors. Specify solid core only. Beings you will be using #10 wire the electrician will try to talk you into using stranded. He will tell you they are both rated for 30 amps. Stand your ground, Solid core only.
Example of 10-2 with ground MC cable with aluminum armor.
http://www.lowes.com/pd/250-ft-10-2-Solid-Aluminum-MC-Cable/3637694