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.
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
Assuming a single 3 phase transformer rated 25,000VA
25,000/3 = 8,333 VA per phase
8,333/120 = 69A per phase
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.
"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:
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 !!!
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
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
Chris Lewis, Una confibula non set est
It\'s not just anyone who gets a Starship Cruiser class named after them.
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).
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
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
it just went on forever, with him claiming 208 was fine and my seeing
as low as 200 depending on other building loads.
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.
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