Current transformers

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  #1  
Old 12-07-11, 08:25 AM
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Current transformers

I posted an earlier thread about running wire a long distance, and Furd mentioned using transformers. Question is, Are transformers a DIY thing? Also, if they are, I suppose they are mounted next to the panels at both ends, in a protective enclosure? Thanks. John
 
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  #2  
Old 12-07-11, 08:48 AM
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In a residential setting, they COULD be a do it yourself task, but not very common. Not sure your details, but there is enough knowledge on this board to get it done correctly.
I'll assume its a long run to your load on a branch circuit?

Note the use of the term "current transformer" refers to a much different item!
 
  #3  
Old 12-07-11, 09:26 AM
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I've tried searching for these before and had a lot of difficulty. All the transformers I find on the internet are large for electric companies. And these are outside "pad mount" for underground or designed to be mounted on a utility pole for above ground. They can create a lot of heat and humm. I don't know what is available for what you need or how to find that on the internet??? (50 amps as I recall.)

And when I have seen these installed inside, that is for a BIG commercial building and they are in a room by themselves.

With that said, I would ask as many people as possible about this and find out all you can. Ask your local electrical inspector about this. Options for installing inside and if he knows where you can find these. Ask if there is any limitation on voltage. Perhaps above a certain voltage and then more "safety" things are needed?

Then try a local electrical supply where electricians buy their stuff.

And also the electric company buys tons of transformers. Perhaps you could speak with an engineer or purchasing person at the electric company and they could give you a bit of guidance on what your options are and maybe where you can buy what you need.

Maybe someone here would know where to buy these?

Then once you find a product and price and this looks like the best option, then call the manufacturer and ask about mounting inside. Print out all the technical information and take that to your electrical inspector. Ask about mounting inside. He could better inform you once he saw the paperwork on a specific transformer.

Who knows? Above a certain voltage you may be required to install it in a separate fireproof room and surrounded by a chain link fence and High Voltage signs everywhere?
 
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Old 12-07-11, 10:52 AM
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You would need two at least 14-15KVA for a 60 amp service (if my math is right) and a quick look at Ebay has made me question my suggestion.

I'm thinking a generator may be cheaper. Do you already use propane?
 
  #5  
Old 12-07-11, 12:11 PM
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Nilz, refresh my mind about your project.

Bill, utility transformers are usually oil filled and yes, there are many specifications on how and where they may be installed. There are, however, hundreds, if not thousands, of sizes and ratios made in a dry configuration. Dry transformers have much lower siting requirements than do oil filled ones. Dry transformers of up to 112kVA size can generally be installed just about anywhere.

There IS the voltage issue that needs to be addressed. Since the use of transformers in what I think this post is about was to reduce voltage drop over a long distance the transformers would be used to first step up the voltage at the source, send it to the remote location and then a second transformer would step the voltage back down to standard 240/120 volts. Utilities do this all the time and use higher voltages from about 4,000 volts to 500,000 volts depending on the power (in watts) being transmitted and the distance. Where this becomes a problem for the DIYer is that the rules for wiring change drastically once yo exceed the 600 volt threshold. Without detailing the changes and requirements I'll state that working with voltages above 600 are simply not in the DIY realm.

What that leaves is the possibility of a DIYer using a pair of 480-240/120 transformers. Since the 480 is only double the voltage that is normally seen in residential installations the current flow over the transmission wires is only halved from what it would be simply using the 240 volts. In most cases this works out to paying less for the wire but adding the cost for the two transformers and associated equipment and the real savings become minimal.
 
  #6  
Old 12-07-11, 12:23 PM
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Originally Posted by Furd View Post
What that leaves is the possibility of a DIYer using a pair of 480-240/120 transformers. Since the 480 is only double the voltage that is normally seen in residential installations the current flow over the transmission wires is only halved from what it would be simply using the 240 volts. In most cases this works out to paying less for the wire but adding the cost for the two transformers and associated equipment and the real savings become minimal.
Yeah, this makes sense to me also. For really long distances, have the power company deliver the power. More moderate distances, think heavy aluminum wire. For less than a few hundred feet, upsize the copper conductor.
 
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Old 12-07-11, 01:44 PM
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It was a post about running 240V about 400 feet, for a 100 amp service, in a pole building. Here is your response:

At four hundred feet I would be thinking about something different than feeding the barn from the house. Having the utility bring service to a pole about halfway between the two structures may be something to investigate. The pole would have the meter and a service panel and then feeders to the house and barn.

If that isn't practical then I would seriously look at getting a pair of transformers and boosting the voltage to 480 for the run to the barn and then dropping it again. Doubling the voltage halves the current and reduces the voltage drop to one-fourth. For 100 ampere/240 volt service to the barn (24kW) you would only have to use wire sufficient to carry 50 amperes plus voltage drop. The cost savings on wire would go a long way to paying for the transformers.

Also, you would likely NOT need the same size of equipment grounding conductor as the current carrying conductors. Normally for a maximum of 100 amperes a #8 copper or #6 aluminum conductor is sufficient but when the current carrying conductors are upsized for voltage drop consideration the equipment grounding conductor must also be upsized proportionally.

Last point, do you REALLY need 100 amperes with a maximum of 3% voltage drop?

And on the solar...24 kilowatts of solar panels is probably gonna cost upwards of forty grand, not counting any possible governmental assistance.
 
  #8  
Old 12-07-11, 05:24 PM
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Bingo! (For being able to search for these...)

The terms "15kva" and "dry" are the trick.

Search for: 15kva transformer dry 1ph
 
  #9  
Old 12-07-11, 05:50 PM
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Here is Q and A all about these dry transformers including using them in reverse to step up...

http://www.acmepowerdist.com/pdf/Page_6-16.pdf

The above company main page...
Acme Electric
 
  #10  
Old 12-07-11, 06:40 PM
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Nilz, yes, the transformers would be mounted near to the supply and load panels. Most transformers of the necessary size will be complete with enclosure (ventilated) so an additional enclosure is not a concern. Under some circumstances they may be mounted outside.

Sizing is important! an undersized transformer will have poor voltage regulation, meaning the output voltage will vary widely with changes in load. An undersized transformer will also develop excessive heat leading to premature failure if continuously overloaded. On the other hand, an oversized transformer will have higher "standby losses" which are defined as power used in magnetizing the transformer but which does not contribute to the output power. These losses are released as heat from the transformer.

Smaller transformers (meaning of less than 30kVA size) are often not as efficient when connected "backwards", that is, using the secondary as a primary and vice versa. Finding a transformer with a 240 primary and a 480 secondary will be difficult, if not impossible and this unit would have a high price tag. Using a standard 480 volt primary to 240 volt secondary connected backwards will work but with overall reduced efficiency and performance.

Read carefully the links that Bill posted.
 
  #11  
Old 12-08-11, 01:10 AM
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Hi nilzl, almost everything runs fine on a range of voltages. Even new homes can go as low as 110/220 and up to 128/256. It's not perfect but that's what I've seen utilities provide. If your utility is providing at the high end, a little extra voltage drop won't make any difference. For finicky electronics, a point of use Uninteruptable Power Supply should take sub par voltage and correct it at it's output.
Another solution might be a single Buck-Boost Transformer (self adjusting or active type) at the barn. It is smaller and cheaper ( usually) than a two transformer set up. It passes most of the voltage straight through and only boosts a portion to "add" on top. You bring a four wire feed to the barn and boost the voltage drop back up by 5, 10, or 15%.
FYI, with a two transformer set up, if you want 120v and 240v at the barn the step down transformer needs a center tap (neutral).
 
  #12  
Old 12-08-11, 03:19 AM
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Thanks guys for the feed back. Its Probably a lot less of a headache, and close to the same money, to go with large aluminun. 2/0 can get me there with a little more than 5% voltage drop. Plus, that's at 100 amps draw. My actual voltage drop will not be anywhere near that.
 
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