Isolation Transformer and Subpanel recommendations

we can get 240V here up to the house but given it's US, it has to be broken into 2 120V phases inside the house.

"it has to be broken into 2 120V phases inside the house."

Would you care to expand?

Zephyr24069,

I would feed the 5 Kva xfmr with 240V balance power.

If your plan was to install the xfrm by your audio equipment so you could use the existing dedicated circuit you now have, you can still use a 240V fed xfmr.

The existing dedicated branch can easily be converted to 240V. The #10 wire is rated for 600V.

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?

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.

.

.
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?
.

I currently have a 100 ft run of heavy flex conduit for my dedicated circuit;
09-11-12: Zephyr24069

If you put the new panel in the garage are you saying the new branch circuit/s run, from the new panel, would be 100' or more?

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.

"5-9s copper runs for positive, negative and neutral through that new conduit" ??? 5-9s??
2) 120V circuits + 1) equipment ground?

Not sure what you mean..... More detailed info please.

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.

What is the end result you are aiming for from the new xfmr and new panel?
.

End result is better quality power to my system; the dedicated circuit, rebalancing the house panel to make the dedicated circuit the first one off the main, all other high current circuits on the other leg/phase of the house panel, good quality wiring from the main panel to the room,

all other high current circuits on the other leg/phase of the house panel,

Sometimes that can be a bad idea. Doing so can put a strain on the secondary of the utility company's power transformer. It can also put stress on the busing and breaker connecting bus ties of the main service electrical panel.

In most residential housings areas the utility power transformer is a single phase xfmr.

The single phase secondary winding is center tapped in the middle of the winding. Thus the term split phase winding.

From the xfmr secondary to your house there are 3 power conductors.
The two hot insulated conductors connect to the outer most 240V leads of the xfmr secondary winding and the center tap conductor, the intentionally Grounded Conductor, the neutral.

To cut to the chase....

NEC code requires, as well as electricians are taught, to balance out the loads across the Lines, legs, of an electrical panel as much as possible.
Example the electrician would never put the microwave, dish washer, washing machine, sump pump, known deep freezer outlet, and the gas furnace blower motor all on the same Line, leg, in the panel.

Here is how a single phase, split phase transformer works.

Theoretically, if exactly 10 amps of load is connected across line one, L1, to neutral and exactly 10 amps of load is connected across L2 and neutral, zero amps will return on the neutral conductor back to the utility xfmr.
Only the unbalanced load will return on the neutral conductor......

The two 10 amp loads are in series with one another....
The two exactly equal 10 amp series load are actually being fed by the hot 240V legs of the panel.

Any unbalanced load will return on the neutral conductor to the source, the xfmr.

Example.

Say there are 30 amps of total connected load across L1 and neutral and only 5 amps of total connected load across L2 and neutral.

25 amps will return on the neutral conductor. The remaining 5 amps of L1 will be in series with the 5 amps of L2.... Is the balanced series load dirty or clean?
.
.
If the main panel is evenly balanced within reason and a 5 Kva iso xfmr is connected across L1 and L2, fed 240V single phase balanced power then everybody is happy. Especially the utility power transformer.

What a good iso xfrm will give you is a new separately derived grounded AC power system. The wiring method used for the installation of the xfmr, the new electrical panel, and grounding will greatly determine its power quality performance.



http://openbookproject.net/electricCircuits/AC/AC_10.html#xtocid71230

it became clear that some of my responses and my not being an electrician have caused confusion on the thread. Apologies all around for this...
09-11-12: Zephyr24069

Zephyr24069,

For a layman I thought you did fine.

Asking questions is a good thing.
It is a hell of a lot cheaper to design the wiring methods that will give you the best results you are looking for than ending up with something you are not happy with.

Good example. You would be surprised how many audio enthusiast will hire an electrician to install a 5 Kva or 7 Kva xfmr and electrical panel.

Instead of configuring, paralleling, the secondary windings for 120V out they will series the 2 windings and feed the new panel with 120/240V.

Of course the audio enthusiast told the electrician to make sure all the branch circuits are fed from the same Line, leg, in the panel. So that is exactly what the electrician does.

Problem is only one winding of the two secondary windings is being used.
In other words each of the secondary windings of a 5 kva xfmr are good for a max connected load of 2.5 Kva.. (20.8 amps @120V)

By configuring, paralleling the two windings and feeding the new panel 120V the full 5Kva rating of the xfmr is available. (41.7 amps at 120V.)

The earth connection of the secondary conductor of the xfmr that will become the grounded conductor, the neutral, can also be an important factor. NEC code says it shall connect to the main grounding electrode system of the main electrical service.

Were the connection to the main grounding electrode system is connected can make the difference in how clean, free from AC noise the ground is and that of the neutral of the new separately derived grounded AC power system.....

I have only scratched the surface....

So don't be afraid to ask questions. Choose a good state licensed electrician that will set down with you and design the electrical system you want. He will know the code for your area and what the AHJ, authority having jurisdiction, will allow.

Just remember code is bare minimum safety standards. You can exceed the bare minimum standards.
Jim