Transformer--Subpanel Wiring

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

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.

If you configure the secondary of the xfmr 120/240V out and only use one leg to neutral you will only be using one of the two secondary windings of the xfmr.... Usable va rating is cut in half.

If you are only needing 120V then configure the output for 120V out only.... The two secondary windings are wired in parallel.... 2500 va x 2 = 5000 va.
Note:
Winding polarity is very important. If wired incorrectly the two windings will buck one another.
Also polarity should be observed for the paralleled leg that will be grounded.

So the two secondary windings will be paralleled and (2) lines, legs, from the xfmr will extend to the new electrical panel. One leg will terminate on the neutral/ground bar, (becomes the neutral the grounded conductor). The other leg terminates on a single pole 45 amp main breaker, (the hot ungrounded conductor). The electrical panel is 120V only.

And the neutral bar, ground bar, and panel will all be connected and grounded back at the main service ground. Is this correct?

Correct.

And if my math is right, a 7.5kva transformer would accommodate a load of 62.5 amps@120V,

Correct x 80%

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?

The main breaker on the new panel would limit the amount of load connected. You could fill the panel with 15 or 20 amp breakers. The main breaker could care less...
Add up all the "breaker handle" breakers in your main house panel. You will find they may well add up to over 200 amps.

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.

LOL, I coresponded by email with a guy that hooked up a 7.5 KVA xfmr for his audio equipment. After a couple of electric bills his wife made him shut the xfmr off when not in use.

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?

Is your audio equipment now fed from any dedicated circuits? If not I would try that first.

Have a sub panel installed in the electrical/mechanical room. That is where you were going to install the iso xfmr and new panel.

Feed the sub panel from your existing main house panel.
The electrician can move a couple of breakers near the top just below the 200 amp main breaker to accomidate a new 2 pole breaker that will feed the new sub panel.

Wire size to feed the sub panel? At least #4 copper. Feed breaker 2 pole 70 amp. Better yet #2 copper. Feed breaker could still be a 70 or up to a 90 amp.
Note: I would wire the sub panel 120/240V for future and house resale. It also allows you to use the Line, leg, (L1 or L2) that yeilds the best sound from your audio system.
No main breaker will be needed in the sub panel.

Copper busing only for the panel.... stay away from aluminum.
Example:
Square D QO is plated copper...
Square D "Home Line" is aluminum.