Transformer--Subpanel Wiring

Ngjockey,
I've been abusing the FedEx guys for some time now. I think its only fair I spread the love.

David,
60dB is what the fan on my computer case generates from an inch away, which is fairly unobtrusive. But as you suggested I did set a boombox at the same level from a meter away and shut the furnace room door. It was quite audible--possibly my hearing is better at these frequencies. And then the burner on the water heater kicked in. I will definitely be putting some sort of sound insulation on that room, transformer or not.

Jim,
Thanks greatly for all of that. You've clarified a number of items. But there are a few things I'm not sure about yet. 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. And this will necessarily put all circuits in the isolated panel on the same phase. And the neutral bar, ground bar, and panel will all be connected and grounded back at the main service ground. Is this correct?

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

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.

As far as whether I need a transformer in the first place, I honestly don't know. How would I know whether my power is dirty? I thought that perhaps I should also be concerned with noise in my ciruits from everything else on my service. I've got fluorescent lights, dimmer switches, computers, a plasma, etc.

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?

Chris

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.

Jim,
Actually I haven't been able to hook up any of my gear. We moved into a new house and after 18 months of remodeling, we're just now to the point when I can begin to put "my space" together.

My wife will certainly appreciate your subpanel suggestions, especially the resale benefit. She doesn't understand any of this and I think a transformer might scare her--even more than all the non-integrated components. And that wouldn't do anything for the music.

I'm not going to pretend to understand everything about the transformer hookup. I find all this difficult without diagrams. I am going to look for some and review them along with your explanation. My primary reason for starting this thread is that there are so many kinds of transformers, that I didn't really know what I should be shopping for. I figured if I understood how it all needed to be hooked up, then I would be able to shop halfway intelligently.

I can relate to your guy with the high electric bill, but I'm not sure I figured the cost correctly. I did some reading today and I came across a formula for excitation current in the primary winding, which would depend on the inductance of the transformer. I=E/(2x3.14xL) The current in the primary winding would then increase only in response to a load placed on the secondary winding. So it wouldn't be pulling current full bore anytime it was hooked up. Without a draw on the secondary, only an excitation current would be drawn on the primary. Does this sound correct? What would be a more realistic cost estimate for allowing a transformer to hum away?

I've got just a few more questions regarding breaker sizes. 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?

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?

If I were to have an isolation transformer with a new main panel, what would determine the size of the new main breaker? If the rating of the isolation transformer wasn't large, realisticly wouldn't the limiting factor for the amps on the new main panel be the breaker protecting the isolation transformer?

Thanks again.
Chris

To give you an idea of the running cost.

On my 5KVA, in sequence of testing tonight...

With amps in A mode, no music playing
primary (240V) 3.3-3.5A, secondary (120V) 6.3-6.7A (approx 95% eff)

With amps in A/B mode, no music playing
primary 0.9A , secondary 0.9A

amps switched off
primary 0.6A (144W - got light bulbs more than that)

So, either I was wrong with my earlier statement or conditions were different on previous test. Your results may vary.

Bigger transformers have less of an exciting current as a % of the total KVA but will always be more than a smaller xformer.
==========

Are you ready for this? Does your electrician have a heart condition?

http://mysite.verizon.net/vze22yzp/id10.html
Some of the links are broken or misplaced but not completely dead yet.

http://www.audioasylum.com/audio/tweaks/messages/55.html

http://www.contractorsav.com/article/2006/11/07/ac-power
Maybe slightly biased

My experience has been that balanced AC has little benefit for power amps but has been occasionally dramatic with sources and preamps.

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