Transformer--Subpanel Wiring

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

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

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?
01-31-11: Iabirder

Iabirder,

Sub panel?
If you are still talking about using a 5 KVA ISO transformer the panel connected to the output of the transformer will not be a sub panel.

The transformer will create a new derived AC power system. The new electrical panel becomes a main panel.

Per NEC the new derived power system must become an AC grounded system. If you configure the secondary of the transformer for 120V out only, (parallel the two secondary windings), then one leg of the parallel must be connected to earth. This grounded conductor becomes the neutral conductor. The connection to earth can be done at the transformer or in the new electrical panel. I would do it at the new panel.

What you will end up with is more than likely what your existing main electrical panel looks like.
All of the branch circuit neutral conductors and equipment grounding conductor terminated on the same neutral/ground bar. (STAR grounding)... A bonding screw will connect the neutral/ground bar to the metal electrical panel enclosure. From this neutral/ground bar the earth ground wire connects, (called the grounding electrode conductor). Per NEC this wire Shall, must, connect to the existing grounding electrode system of the main electrical service of your house.

The primary side of the ISO 5 KVA transformer and branch circuit wire feeding the transformer must be protected by an overcurrent device, breaker.

5 KVA / 240V = 20.8 FLA amps X 125% = 26 amps. Minimum primary wire size #10 awg. Breaker size, 2 pole 30 amp.
If it were me the minimum wire size I would use would be #8 awg copper, breaker size 2 pole 40 amp.

Secondary rating of a 5 KVA transformer.
5 KVA / 120V = 41.67 amps @ 120V. Main breaker size 45 amps.

Minimum wire size from transformer to new panel.
41.67 X 125% = 52 amps. #6 awg copper minimum.
I would probably use #4 awg copper.

With your ARC equipment, JMHO, a 5 KVA xfmr is too small. A 7.5 KVA would be more appropriate imo.....

The above info is just a brief not complete method of an electrical wiring installation.

Your electrician must follow the local electrical codes of your area......

Even when no load is connected to the output of the XFMR the primary still draws idle current.... The electric meter wheel still turns 24/7.....

Are you sure you want/need an ISO XFMR?

Is your house power that dirty?

How big is your existing house service. 100 amp? 200 amp? 400 amp?

Are there spare spaces in the existing panel for new dedicated branch circuits? If not you could have a sub panel installed. If you have a sub panel installed over kill the feeder wire size feeding the panel... #4awg copper minimum. # 3 or #2 copper better yet.
Jim