Transformer--Subpanel Wiring

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

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,

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.