Isolation Transformer and Subpanel recommendations

my plan is to install the transformer in an adjoining attic over a garage where any humming from the transformer will not be hear and any heat will dissipate.

Not a good idea..... I doubt you will find a licensed electrician that will do it.

Why not mount the xfmr on a wall in the garage. It can be mounted up high near the ceiling. (At least 12" down from the ceiling for heat dissipation)

Is the main electrical panel for the house in the garage?

Where are you planing on mounting the new electrical panel that will be fed from the xfmr?

The main electrical panel is in the laundry room, on an inside wall but adjoining the garage....I was either planning on mounting the new subpanel there or in the attack adjoining the room that has my system.

For 240V, as an example...

http://www.ebay.ca/itm/FPE-5-KVA-1-Phase-240X480V-120-240-Volt-3R-SE2N5FS-Transformer-NEW-/310425276037?pt=BI_Circuit_Breakers_Transformers&hash=item4846c9ce85

Sola/ Hevi-Duty and Acme are usually the best bets for sheilded like this one. (the dashed line in the schematic)

So the transformer could be mounted in the garage and fed from the main panel relatively easily.

From the new transformer the load feeder could pass through the ceiling across the attic and drop down through the top plate into the wall cavity to the new recessed electrical panel. (Assuming the wall is studs covered with drywall.)

For resale you might want to install the new panel in the laundry room. Your electrician will show you your options to meet NEC code for minimum working clearances.

Example, the electrical panel cannot be mounted above the washer or dryer.

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In case you didn't know.

A 5 Kva xfmr primary FLA @ 240V is 20.8 amps when fully loaded. Per NEC FLA is used for determining wire size to feed the xfmr as well as the breaker.
Min wire size #10 awg, breaker 2 pole 30 amp min.

Secondary, 2 windings wired in parallel (120V), will handle a continuous total connected load of up to 41.67 amps
Minimum size feeder wire to new panel, #6 awg copper.

Have you given any thought to surface mounting the new electrical panel in the garage? Mount on the same common wall as the main electrical panel that faces into the laundry room.
Approx how long would the branch circuits be from the new panel to the audio room? How many branch circuits are you planning to run from the new panel? 2, 3, 4?
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Thank you very much for all of the information! I could definitely mount the new sub-panel in the garage and then go straight up into the area adjoining the audio room. I currently have a 100 ft run of heavy flex conduit for my dedicated circuit; I had an electrician pull individual 5-9s copper runs for positive, negative and neutral through that new conduit 4 years ago so that part is already decided. There is only 1 dedicated audio system 20-amp circuit in the room. The dedicated circuit feeds a Purepower APS 2000 and distributes power to my entire system.