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Transformer--Subpanel Wiring

I'm trying to get a handle on exactly what to ask of my electrician. I've been searching past threads for awhile now and I might know just enough to ask questions. I'm going to need power for the usual array of AV toys: monos, a pre-amp, CDP, TT, tuner, and plasma TV, BluRay, etc., and I hope to do things right. I plan to install a 5kva transformer and a subpanel for six dedicated lines as well as whole house surge suppression and a isolated ground.

1) Which is more desirable--to pull a 240V feeder from the service before it goes to the main panel or from a double breaker at the main panel? If I use the double breaker, does it make a difference where it is located within the main panel?

2) Should I use the isolation transformer to stepdown to 120V or should I take 240V to the sub?

3) Which audio/video components need to be on the same phase?

4) I've only seen diagrams on wiring a main service panel. What is the sub going to look like if I stepdown to 120V? Will it involve taking only one hot leg off the transformer secondary with a center-tap neutral, resulting in something that looks like one side of the main panel? I would think that this would put everything in the sub on the same phase. Correct? Is this the way to go?

5) Anything else I should be mindful of?

I greatly appreciate the help.
iabirder

Showing 5 responses by ngjockey

ngjockey
1,042 posts
 
Subpanel after the 5KVA transformer gives option of two 20A circuit protection, maybe three 15A circuits with a little stretch of imagination. More than that is not advised, even with a 20A 240V breaker from the main panel to the transformer. Safety first.

Generally, everything connected by analog interconnects should be on the same phase. That's not a law.

Plasma can be a large draw in itself, sometimes over 6A for the big ones. With that kind of a SMPS, probably better off not being on the same transformer. Good example of what you're isolating from.

I'm sure the electrician can figure out the subpanel.

Isolated ground? Bad idea and probably not code.
ngjockey
1,042 posts
 
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.
ngjockey
1,042 posts
 
Jim did the math, but if you want four circuits...

4 X 15A X 120V = 7200 VA
or
(2 X 20A X 120V) + (2 X 15A X 120V) = 8400 VA

You never know how noisy a transformer will be till it's installed and the same one could be quiet in one nearly identical setup to where it was noisy elsewhere or vice versa. I have a 5KVA and a 2KVA and both are inaudible to within inches. Work with and around a lot of xformers and, except for massive (15 KVA+) three-phase, unusual noise seems to be exception, not the rule. I've had open and regulated xformers that have been noisy. Both mine at home are shielded , encapsulated (NEMA 3R) and potted in epoxy, as typical for the type. Both cost $100 off Ebay (industrial recyclers) and cost more to ship to Canada. Both times the mounting brackets got bent in shipping but casing remained solid as a rock. A 7.5KVA of this type can weigh 140 lbs and put you on a UPS hate list.
ngjockey
1,042 posts
 
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
1,042 posts
 
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.
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