Transformer--Subpanel Wiring

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

Jim, agreed.

In my setup with 2 Plinius SA100 and the transformer derated by half for the center tap...

(2 X 500W) / 2500 VA = 40%

Don't know the max draw on Iabirder's amps but if they're A/B, it could be much more and a different characteristic.

BTW, the xformer's idle (exciting) current drops quite a bit if there is a minimum load, like "standby", as opposed to no load.

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