Transformer--Subpanel Wiring

If I understand you correctly there will be three wires running from the secondary winding to the isolated main panel. One leg will be the hot at 120V to feed the breakers. The second (center-tap?) will go to the neutral bar. The third at the other end of the winding will be attached to ground on the panel.

If you configure the secondary of the xfmr 120/240V out and only use one leg to neutral you will only be using one of the two secondary windings of the xfmr.... Usable va rating is cut in half.

If you are only needing 120V then configure the output for 120V out only.... The two secondary windings are wired in parallel.... 2500 va x 2 = 5000 va.
Note:
Winding polarity is very important. If wired incorrectly the two windings will buck one another.
Also polarity should be observed for the paralleled leg that will be grounded.

So the two secondary windings will be paralleled and (2) lines, legs, from the xfmr will extend to the new electrical panel. One leg will terminate on the neutral/ground bar, (becomes the neutral the grounded conductor). The other leg terminates on a single pole 45 amp main breaker, (the hot ungrounded conductor). The electrical panel is 120V only.

And the neutral bar, ground bar, and panel will all be connected and grounded back at the main service ground. Is this correct?

Correct.

And if my math is right, a 7.5kva transformer would accommodate a load of 62.5 amps@120V,

Correct x 80%

just enough for four 15 amp circuits. And I would need a 10kva transformer to accomodate a load of 83.3 amps@120V, enough for four 20 amp circuits. Is there any wiggle room here?

The main breaker on the new panel would limit the amount of load connected. You could fill the panel with 15 or 20 amp breakers. The main breaker could care less...
Add up all the "breaker handle" breakers in your main house panel. You will find they may well add up to over 200 amps.

You raised the issue of cost. Is having a transformer hooked up 24/7 like leaving my electric range on all the time? Well probably not that expensive. But 10kw x 24hours x 365days x 8.38cents/kwh=$7,340.88 per year to keep it hooked up. Is this the way it works? That would be prohibitive.

LOL, I coresponded by email with a guy that hooked up a 7.5 KVA xfmr for his audio equipment. After a couple of electric bills his wife made him shut the xfmr off when not in use.

I've got a 200amp service to the house and three unused spaces on the panel. Could you please clarify your last comment regarding overkilling the feeder wire size--the feeder going to the subpanel?

Is your audio equipment now fed from any dedicated circuits? If not I would try that first.

Have a sub panel installed in the electrical/mechanical room. That is where you were going to install the iso xfmr and new panel.

Feed the sub panel from your existing main house panel.
The electrician can move a couple of breakers near the top just below the 200 amp main breaker to accomidate a new 2 pole breaker that will feed the new sub panel.

Wire size to feed the sub panel? At least #4 copper. Feed breaker 2 pole 70 amp. Better yet #2 copper. Feed breaker could still be a 70 or up to a 90 amp.
Note: I would wire the sub panel 120/240V for future and house resale. It also allows you to use the Line, leg, (L1 or L2) that yeilds the best sound from your audio system.
No main breaker will be needed in the sub panel.

Copper busing only for the panel.... stay away from aluminum.
Example:
Square D QO is plated copper...
Square D "Home Line" is aluminum.

Ngjockey,

Just so Iabirder understands the max a 5 KVA xfmr can be loaded is 41.67 amps @ 120V (5000 va).
Technically the continuous connected load @ 120V = 80% of 41.67 amps = 33.3 amps (3996 va).

Most would not use the 80% rule for calculating the size of a xfmr for a connected load of a power amp.....
Jim

I've seen recommendations for a minimum 60amp breaker at the main panel to protect what is going to the sub. 1)How do I know what amperage is available to lend the sub? 2)If I can use a 60amp breaker does this mean that I can't exceed a total of 60amps in breakers on the sub?
01-31-11: Iabirder

Iabirder,

Sub panel?
If you are still talking about using a 5 KVA ISO transformer the panel connected to the output of the transformer will not be a sub panel.

The transformer will create a new derived AC power system. The new electrical panel becomes a main panel.

Per NEC the new derived power system must become an AC grounded system. If you configure the secondary of the transformer for 120V out only, (parallel the two secondary windings), then one leg of the parallel must be connected to earth. This grounded conductor becomes the neutral conductor. The connection to earth can be done at the transformer or in the new electrical panel. I would do it at the new panel.

What you will end up with is more than likely what your existing main electrical panel looks like.
All of the branch circuit neutral conductors and equipment grounding conductor terminated on the same neutral/ground bar. (STAR grounding)... A bonding screw will connect the neutral/ground bar to the metal electrical panel enclosure. From this neutral/ground bar the earth ground wire connects, (called the grounding electrode conductor). Per NEC this wire Shall, must, connect to the existing grounding electrode system of the main electrical service of your house.

The primary side of the ISO 5 KVA transformer and branch circuit wire feeding the transformer must be protected by an overcurrent device, breaker.

5 KVA / 240V = 20.8 FLA amps X 125% = 26 amps. Minimum primary wire size #10 awg. Breaker size, 2 pole 30 amp.
If it were me the minimum wire size I would use would be #8 awg copper, breaker size 2 pole 40 amp.

Secondary rating of a 5 KVA transformer.
5 KVA / 120V = 41.67 amps @ 120V. Main breaker size 45 amps.

Minimum wire size from transformer to new panel.
41.67 X 125% = 52 amps. #6 awg copper minimum.
I would probably use #4 awg copper.

With your ARC equipment, JMHO, a 5 KVA xfmr is too small. A 7.5 KVA would be more appropriate imo.....

The above info is just a brief not complete method of an electrical wiring installation.

Your electrician must follow the local electrical codes of your area......

Even when no load is connected to the output of the XFMR the primary still draws idle current.... The electric meter wheel still turns 24/7.....

Are you sure you want/need an ISO XFMR?

Is your house power that dirty?

How big is your existing house service. 100 amp? 200 amp? 400 amp?

Are there spare spaces in the existing panel for new dedicated branch circuits? If not you could have a sub panel installed. If you have a sub panel installed over kill the feeder wire size feeding the panel... #4awg copper minimum. # 3 or #2 copper better yet.
Jim

My understanding is that if I'm needing 20 amps on a circuit, then I will need a 20 amp breaker at the subpanel, because a 15 amp breaker would trip. And a circuit protected by a 20 amp breaker would require #12AWG or larger, because 20 amps would overheat anything smaller. Is this correct?
02-01-11: Iabirder

More than likely your ARC power amp/s requires it to be connected to a 20 amp branch circuit. Not because it draws anywhere near 2400 va (20 amps x 120V = 2400 va), but rather because of the amount of inrush current the amp draws when it is first turned on. Read the owners manual for the Amp.

So why use 20 amp branch circuits for other equipment instead of 15 amp? 15 amp would be more than enough. If VD, (voltage drop), is an issue #12 or #10 wire could be used and terminated a on 15 amp breaker. The breaker determines the size of the branch circuit.

First a little back ground info. In most cases, if not all today, the guts inside a good quality 15 amp receptacle are the same as a 20 amp receptacle. The only difference is the face plate. A 15 amp recept will only accept a 15 amp male plug. It's a safety thing... Most 15 amp branch circuits use #14 awg wire, bare minimum per NEC, which is rated for 15 amps. NEMA /UL /CSA requires manufactures that use a cord and plug for an appliance/equipment cannot exceed 12 amps FLA continuous load if a 15 amp plug is used.

Per NEC code 2 or more 15 amp receptacles can be installed on a 20 amp branch circuit. ( a duplex is two recepts.)

Per NEC code a 20 amp receptacle can only be installed on a 20 amp branch circuit.

So if you want to install Oyaide R1 recepts, per NEC code, the branch circuit must be a 20 amp.... Per code it cannot be installed on a 15 amp branch circuit.

If a manufacture builds a piece of equipment with a cord and plug that draws more than 12 amps continuous FLA then he must use a NEMA 5-20P plug. A 20 amp plug will not plug into a 15 amp recept. The max allowable continuous FLA for a 5-20P plug is 16 amps, 1920 va.

If I have 80 amps worth of breakers on the subpanel, would I need to have at least an 80 amp breaker protecting the feeder on the main panel? And then the feeder size would be based on whatever wire can handle 80 amps? Can I have an even larger breaker on the main panel as long as I increase the size of my feeder appropriately?

You could have 200 amps worth of breakers in a panel. The main breaker a head of the branch circuit breakers is the gate keeper.... If the main breaker has a handle rating of 60 amps any continuous load of 60 amps and over will/should trip the breaker open. Technically, per NEC, the max continuous load placed on the breaker should not exceed 80%. Same for a branch circuit breaker, 80%.

In a residential environment most convenience outlet branch circuits have very little load if any on them. In a bedroom maybe a clock radio, table lamps, TV.... A 15 amp branch circuit with maybe an amp or two of load at any given time. Family room, living room, den, 15 amp branch circuit/s with hardly any load. Several 15 amp branch circuits breakers in the panel but hardly any load at any given time of the day.
Night time add a few lights.

20 amp branch circuits? Kitchen has a few required by NEC.
Microwave
dish washer
refrigerator
garbage disposal
GFI recepts above the counter tops for what ever....
Most of the time during the day they just sit idle. But when needed the branch circuit must be able to handle the load placed upon it.

Laundry room.
20 amp branch circuit for the washer
30 amp 240V branch circuit for the cloths dryer.

The list goes on and on. Lots of branch circuits and breakers. At any given time of the day hardly any load if any.

I believe you were talking about installing 4 new dedicated circuits for your audio/video equipment. Other than the inrush current of the ARC power amp/s a 15 amp circuit would more than likely handle all the connected load. More than likely a 20 amp branch circuit would handle the inrush current of the ARC amp/s plus the rest of the equipment.

So why do you want 4 dedicated circuits? Why do some guys have 10?

Here is just one post on the subject. There are many out there...
http://www.audioasylum.com/audio/tweaks/messages/16/167770.html