Help with a new transformer and service enterance installation

• posted on February 16, 2006, 10:24 pm
hi I'm getting a new transformer installed. (25KVA what ever that stand for) I want to have the biggest breaker size on my service panel that is safe for the transformer. the installation is supposed to be a three phase panel with a 125AMP breaker. I don't undersand the three phase part of for that matter much of anything about this. thanks-Jim
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<%-name%>
• posted on February 16, 2006, 10:38 pm

P = I V
25,000 VA = Current * 208V (208 for typical 3 phase)
25000/208 = 120 Amps
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<%-name%>
• posted on February 16, 2006, 11:20 pm
PipeDown wrote:

Hi, P is not equla to Volt-Amp rating in AC circuit. It will be very difficult to have a power factor of one in real life.
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<%-name%>
• posted on February 17, 2006, 12:13 am

For AC VA is the correct unit as the division proves. Do the math without numbers. Now if the transformer were rated in Watts, that would be different. Power factor correction is done after you know what the load is. In this case, I ignored any loads completely.
Volts * Amps = VA
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<%-name%>
• posted on February 17, 2006, 7:38 am
PipeDown wrote:

Assuming a single 3 phase transformer rated 25,000VA
For 208Y/120: 25,000/3 = 8,333 VA per phase 8,333/120 = 69A per phase
For 240delta power = 240 x (squre root of 3) x current current = 25,000/(240x1.732) = 60A per phase A high leg delta, which is what a BigAl post probably refers to, could have 2 equal 120V leg fuses with a smaller 208V high leg fuse.
bud--
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<%-name%>
• posted on February 16, 2006, 10:49 pm
"What is 3 phase power? Should I use it? Can I get it in my house?
Three phase power has three "hot" wires, 120 degrees out of     phase with each other. These are usually used for large motors     because it is more "efficient", provides a bit more starting torque,     and because the motors are simpler and hence cheaper.
You're most likely to encounter a 3 phase circuit that shows     110 volts between any hot and ground, and 208 volts between     any two hots. The latter shows the difference between a normal     220V/110V common neutral circuit, which is 240 volts between the     two hots. There are 3 phase circuits with different voltages.
Bringing in a 3 phase feed to your house is usually     ridiculously expensive, or impossible. If the equipment you     want to run has a standard motor mount, it is *MUCH* cheaper to     buy a new 110V or 220V motor for it. In some cases it is     possible to run 3 phase equipment on ordinary power if you have     a "capacitor start" unit, or use a larger motor as a     (auto-)generator. These are tricky, but are a good solution if     the motor is non-standard size, or too expensive or too big to     replace. The Taunton Press book ``The Small Shop'' has an     article on how to do this if you must.
Note that you lose any possible electrical efficiency by using     such a converter. The laws of thermodynamics guarantee that." so says and see more links at: http://www.faqs.org/faqs/electrical-wiring/part2/section-10.html
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<%-name%>
• posted on February 17, 2006, 2:45 pm
wrote:

I never understood this. How can you get 110V to ground when the hot is 208V. Half of 208V is 104V. While half of 220 is the actual 110. Then there are these people that call it 240, which is wrong, unless they are referring to the peak or maximum limit, knowing voltage is never precise. The voltage in my house varies from 114 to 117 the times I measured it.
Another reply said that 3 phase has 3 transformers. I have seen this quite often on poles, but never understood how the things is wired. I'd like to see a schematic of how 3phase is distributed to a building and how the 110/220 is derived from it, without a 4th transformer. Is there such a schematic on the web? Of all the wiring I have done, and even hooked up a few 3 phase motors, I still have never comprehended how the 3phase really works, especially how the 220 and 110 come from these transformers. I thought there were special taps on the transformers, but there dont seem to be enough wires entering the buildings. I know all about transformers used in electronics and how taps provide different voltages. When I was in high school I used to play with these old tv power transformers from old tube sets and they would commonly have a 5V tap, 6.3V tap (filaments) Often a 12V center tapped for more filaments of both the 6 and 12 V tubes, and then there was the "fun stuff". The high voltage section. I had one transformer that had 1500V on the HV side and that thing kicked butt. I found out the hard way once.... OUCH !!!

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<%-name%>
• posted on February 17, 2006, 3:47 pm

If you draw the 3 phases in a Y or delta (triangle) and use a little bit of trig or geometry you will be able to understand the voltage relationships...
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<%-name%>
• posted on February 17, 2006, 4:02 pm
snipped-for-privacy@dont.send.any.com wrote:
<SNIP>

This is probably far more than you wanted to know <g>, but have a look: http://www.faqs.org/docs/electric/AC/AC_10.html
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<%-name%>
• posted on February 17, 2006, 5:52 pm
Regardless of cost or availability or what is normal is a 3 phase or a single phase system better to have for normal multiple house/shop use?
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<%-name%>
• posted on February 17, 2006, 10:10 pm

The primary difference between single phase and 3 phase is that electric motors are simpler, cheaper (in \$ per HP) and more reliable and that you get a bit more electrical power per pound of copper circuit wiring at a given voltage.
Neither of which is of significant benefit in a house or with the tools you'd be likely to have in a hobby workshop.
The only time where a 3 phase feed to a home workshop would be useful (if you could get one for a reasonable cost) is where you were buying old surplus industrial equipment with 3 phase motors (usually 5HP+).
It's advantageous for feeding reasonably sized multiple dwelling units like apartment buildings where there's a common feed. In smaller units (2- and 4-), the electrical utilities like the feeds being totally seperate.
--
Chris Lewis, Una confibula non set est
It\'s not just anyone who gets a Starship Cruiser class named after them.
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<%-name%>
• posted on February 16, 2006, 11:22 pm
snipped-for-privacy@hotmail.com wrote:

Hi, 3 phase into residential dwelling? Not usual.
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<%-name%>
• posted on February 17, 2006, 12:18 am
wrote:

Nor is is normal for a residential installation to need any sort of transformer that big. BUT
The OP never said anything about his home. I assumed he is a manager of an industrial site. It is typical to have single and 3-phase power in the same building in an industrial area (at least around here).
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<%-name%>
• posted on February 17, 2006, 1:38 am
Thanks for the info this three phase is what the guy was going to install
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<%-name%>
• posted on February 17, 2006, 6:03 am
wrote:

an
same
And, most small 3 phase power feeds use three separate transformers. The OP said he is getting a new transformer installed. My house has 200 Amp 3 phase power, and three transformers feeding it. One is a tad larger for the single phase loads.
Al
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<%-name%>
• posted on February 17, 2006, 12:49 pm
perhaps single 3 phase transformers are cheaper and thats why its all in one box.
had a #!@\$#^%&^ micheals store once, they had 3 phase power but the vacxuum press ( used for mounting photos) needed 220V
The electrician was a idiot, and I dont know if they ever got it fixed.......
it just went on forever, with him claiming 208 was fine and my seeing as low as 200 depending on other building loads.
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<%-name%>
• posted on February 18, 2006, 6:36 am
micheals store once, they had 3 phase power but the

It depends if the circuit is a wye or delta. My service is 240 volt leg to leg. Went out and looked at the pole today. All three transformers are 25 KVA. One is just different looking than the other two.
Al