Transformer--Subpanel Wiring

Ngjockey,

Thanks for the reply. Yeah, I'm confused and probably don't know what I'm doing. But I promise I won't begin installing until I figure it out. Safety IS first. I won't be doing any of this myself, and expect my electrician to follow code.

I really thought a 5kva xfmr would be large enough. I recall a thread where Zargon had a 4kva iso before a sub with six dedicated lines. Though I do recall that Lak had two 5kva isos before a sub, but that was to feed three different sound systems. My equipment isn't crazy big: ARC REF210s, a REF 3, and the plasma tv will draw 328 watts.

Regarding the "isolated ground"--I was going to have all the neutrals and ground wires in the dedicated lines run all the way back to the main service grounding point, per suggestion of Gs5556 in another's thread--although I don't know what this does for my sound, since they all hook together at some point.

I've collected a good bit of possible mis-information regarding grounds, and maybe you can help me sort it out: 1)I've heard of using a separate ground rod that is 6 feet from the service ground. 2)I've heard of using a separate ground rod, but bonding it to the service ground rod. 3)I've heard that the iso xfmr is a separate power supply, thus I can ground everything from the subpanel to the nearby water pipe in the utility room. 4)I've heard that isolated ground receptacles are not necessary because I have a wood frame house.

So what would be the best way to provide clean power to my gear? Thanks.

Chris

DMatt,
Thanks for the info regarding the hospital grade recepticles. I have seen a number of favorite ports mentioned in threads, most often Hubbell 5362s.

Based on what I've gleaned from other threads there is still a benefit to using recepticles with an isolated ground and running the dedicated lines in armored cable. I'm undecided on which to use here. #10 BX has been recommended. I've also seen recommendations for the Belden 8300 series. Also for twisting the wires. Do they need to be twisted if they are running through metal? You'd think someone on Audiogon would have created a cookbook for all this by now.

Ngjockey,
Thanks, I get what you mean by potential draw on the circuits. And I'll simply let the electrician worry about grounding things properly.

You brought up another point though regarding the service amperage that'll go to the sub. I'm trying to get a handle on what's possible. I guess I would like a minimum of four circuits off the subpanel: two for the monos, one analog, and one digital. 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?

Chris

Sorry I forgot to add that the transformer and subpanel would be in the furnace room, which unfortunately is right behind where I'd be listening. There is a door. It shouldn't be any more audible than the furnace, should it? Anyway, it'll have to do.

Chris

David,
I'll head to Radio Shack tonight--snowstorm be damned, and give the sound meter a try. Should be interesting.

So what sort of wire are going you going to twist? Are you aware of any shielded cables that come in this configuration already? What sort of shielding is best?

Thanks again.

Chris

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

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

Ngjockey,
It sounds like maybe having the transformer on isn't as costly as we feared. Also that formula I posted for excitation current wasn't correct--I forgot to include the frequency in Hz. so I=E/(2 x 3.14 x Hz x L) where I is the excitation current in Amps, E is the voltage, 3.14 is pi, and L is the inductance of the transformer in Henries.
Thanks for those links. I hadn't yet looked into balanced power. Looks like I'll be busy.

Jim,
I don't have any problem with two of the circuits being 15 amp. I think four 20 amps circuits just made discussing the hypotheticals easier. Thanks for the info on the plugs and outlets. That would have probably tripped me up at some point. I read the thread you linked. And I printed off the Hansen paper--can't wait to read it.

In case anyone else is interested, here is the link for it: The AC Power Line and Audio Equipment, by Charles Hansen
http://www.conceptorg.com/techlibrary/PowerTechnology/AC_and_Audio.pdf

I appreciate greatly all the help you've both provided. I'll check back in to see if anyone adds anything more. But I'm now going to retreat to my reading and my drawing board.

Chris