Transformer--Subpanel Wiring

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.

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