Transformer--Subpanel Wiring

Any time it's going to an outlet, instead of hard-wired, you have to accommodate the POTENTIAL, not the actual load. Who's to say you won't get 600W, class A, OTL monoblocks and then plug in some 11,000 BTU portable air conditioners and a vacuum cleaner. Also have to consider the electrical service, since you're adding another 20A without taking anything else away.

The "isolated ground system" has two ground wires, one insulated and going all the way back to the bus bar. Mixed results, mixed recommendations, and not official NEC code. Romex and armored are available as 3C ... but talk about confusing... Got any green nail polish? ;) Otherwise, conduit, $$$.

1) Wrong
2) Ok, maybe, sort of. Ask Almarg.
3) Very wrong. Can't count on copper service pipe. Removed or prohibited from code decades ago. Transformer/shield has to be grounded too.
4) When are they necessary? Google.

Off to a good start. The transformer, though unusual for residential, will provide some common mode noise reduction. Multiple transformers allows isolation between components. Naturally, "the best way" is completely impractical.

Where is this transformer going to be located? They can hum.

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

Chris,

The Controlled Power Ultra-K isolation transformers show an audible noise spec in their website brochure of 45-55dBA at 1 meter.

Take a boombox and your Radio Shack sound meter out to the furnace room and play white noise at 60 dBA from three feet back to see what it will sound like (put the tuner between stations for white noise).

My guess is it will be close to the furnace in loudness, but will be making noise all the time.

David

RE twisting:

Last week I read a paper that favored twisting the hot and neutral together and leaving the ground separate. Pull the three wires through conduit. This has been touted as the way to reduce the induction effect of the H and N on the ground, reducing induced ground voltage that causes noise.

Bill Whitlock (Jensen Transformers) and Jamie Fox presented the paper at last November's AES convention that supported this approach with test data. Titled: "Ground Loops: The Rest of the Story", the twisted configuration offered lower ground voltage induction than Romex (where the ground is between the other conductors) and aluminum MC (metal clad with the three wires configured symmetrically inside).

The synopsis from the AES library website: "The mechanisms that enable so-called ground loops to cause well-known hum, buzz, and other audio system noise problems are well known. But what causes power-line related currents to flow in signal cables in the first place? This paper explains how magnetic induction in ordinary premises AC wiring creates the small voltage differences normally found among system ground connections, even if “isolated” or “technical” grounding is used. The theoretical basis is explored, experimental data shown, and an actual case history related. Little has been written about this “elephant in the room” topic in engineering literature and apparently none in the context of audio or video systems. It is shown that simply twisting L-N pairs in the premises wiring can profoundly reduce system noise problems."

Hope that helps. I am taking this approach on my dedicated lines (with isolated grounds) from my sub panel.

David