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Old June 11th 05, 05:06 AM
 
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Default Getting 120v Single Phase from 3 Phase

Can you get 120v single phase by picking off one line of 240v 3 phase
as long as you have a neutral to carry the current back to the box?
Thanks, Steve


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Old June 11th 05, 05:37 AM
Harold and Susan Vordos
 
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wrote in message
oups.com...
Can you get 120v single phase by picking off one line of 240v 3 phase
as long as you have a neutral to carry the current back to the box?
Thanks, Steve


Yes, but be careful if you have 3 phase delta.. The high leg to ground (B
phase if I'm not mistaken) yields 208 volts, not 120.

Harold


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Old June 11th 05, 07:23 AM
Jon Elson
 
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wrote:
Can you get 120v single phase by picking off one line of 240v 3 phase
as long as you have a neutral to carry the current back to the box?
Thanks, Steve


Only in a special circumstance, which is a 4-wire, center-tap grounded
delta service. This has one 240 V L-L circuit with a center tap, just
like everybody's home service. Then, it has another transformer that
develops the 3rd phase. In this system, there are only TWO of the three
hot wires that will give 120 V to neutral. (The third will give ~207 V,
so you will definitely be able to tell which is which with a meter.)

This service is pretty rare, at least around where I've lived.
Corner-grounded delta is more common, but you can't get 120 V
directly from that. You need a 240 - 120 step-down transformer.
Corner-grounded delta is most obvious because 2-pole circuit breakers
and disconnects are used. The 3-phase wires are hot, hot and neutral,
and you can wire a 3-phase motor up to those 3 wires. (These are also
labeled hot (A), neutral (B), and hot (C) phases, and therefore
sometimes called grounded B phase.)

If you have 208 V WYE service (sometimes called star) you have three
120 V circuits, from any line to neutral. But, in this system,
you can't get 240 V, without a transformer. Like, the building
I work in, has 208 V Wye for the office section, and we have little
autotransformers to step 208 up to 240 for the window air conditioners.

If you try this on a true 240 V Wye system, which has a neutral, you
will get a rather high voltage of about 138 V. But, 240 V Wye
is pretty rare. If you try this on a real delta system, you might kill
all the lights in the building, as it may trip the ground fault
protection. But, then, a true delta system doesn't have a NEUTRAL,
although sometimes telling the difference between a neutral and a safety
ground can be difficult. Delta transformers have a balancing
transformer in them that makes it look like they are referenced to a
neutral, but if you draw any current from line to neutral that
unbalances it, the transformer should shut down. This normally wouldn't
apply to open delta and corner or center-tap grounded deltas, as they
are expected to feed unbalanced loads.

Is that more than you wanted to know?

Jon

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Old June 11th 05, 08:17 AM
bw
 
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wrote in message
oups.com...
Can you get 120v single phase by picking off one line of 240v 3 phase
as long as you have a neutral to carry the current back to the box?
Thanks, Steve


Yes, but depending on your load requirement, get an office building
electrician to wire it. The entire USA gets 120 volt single phase from 3
phase sources.


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Old June 11th 05, 07:58 PM
Pete C.
 
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Harold and Susan Vordos wrote:

"DoN. Nichols" wrote in message
...
In article .com,
wrote:
Can you get 120v single phase by picking off one line of 240v 3 phase
as long as you have a neutral to carry the current back to the box?


Not quite. The voltage won't be right for pure three phase. I
think that with Wye connections, you will have something closer to 104V.


I'm at a loss to understand that, DoN. Care to elaborate? I have wired
three places with delta service, two of which used either the A and C phase
and the neutral for 120V. All of it was done to code. The third place
has a single phase panel along with the 3 phase, both of which are fed from
the same taps from the transformers.


If it had a neutral it wasn't a delta service.

The 104V mentioned was a typo, it's really 138V and change. Square root
of three thing for three phase power. 240V Wye service will give you
138V phase to neutral and 208V Wye service will give you 120V phase to
neutral.



And many are delta which *has* no neutral. All may be floating.


In this case, he's already suggested that there would be a neutral, so it
would be a 5 wire system.


Known as Wye.



But a frequent variation has one of the three sides center
tapped (the way the standard residential feed is supplied, 240V center
tapped with the center tap grounded and neutral connected to that.)

The breaker boxes for this have three buses, but only two of
every three positions can be used for 120V single-phase breakers. The
third phase is *way* too high.


I believe this is often referred to as the "wild" leg.


As stated above, I got around that problem in my current shop by having two
panels, one strictly 3 phase, so none of the positions are lost.


There are / were a lot of strange variations on three phase power, but
most anything new is going to be 208V Wye service. Larger industrial
stuff will get 480V.

Pete C.
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Old June 11th 05, 08:16 PM
Robert Swinney
 
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BW sez:

"...The entire USA gets 120 volt single phase from 3 phase sources."

Cute - but not quite right. See John's answer above.

Bob Swinney

"bw" wrote in message
...
wrote in message
oups.com...
Can you get 120v single phase by picking off one line of 240v 3 phase
as long as you have a neutral to carry the current back to the box?
Thanks, Steve


Yes, but depending on your load requirement, get an office building
electrician to wire it.



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Old June 11th 05, 08:32 PM
Harold and Susan Vordos
 
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"Pete C." wrote in message
...
Harold and Susan Vordos wrote:

"DoN. Nichols" wrote in message
...
In article .com,
wrote:
Can you get 120v single phase by picking off one line of 240v 3 phase
as long as you have a neutral to carry the current back to the box?

Not quite. The voltage won't be right for pure three phase. I
think that with Wye connections, you will have something closer to

104V.

I'm at a loss to understand that, DoN. Care to elaborate? I have

wired
three places with delta service, two of which used either the A and C

phase
and the neutral for 120V. All of it was done to code. The third

place
has a single phase panel along with the 3 phase, both of which are fed

from
the same taps from the transformers.


If it had a neutral it wasn't a delta service.


Wrong!! One can have a three, four or five wire delta system.

I have a 5 wire system, and it *is* a delta system. It is not a wye, which
does not have the wild leg. Mine does have. Ground is established by
tapping the center of one coil, which results in the longer path to ground
from the other two coils. 208 volts from phase to ground. It's not
conjecture, it's measured.

The 104V mentioned was a typo, it's really 138V and change. Square root
of three thing for three phase power. 240V Wye service will give you
138V phase to neutral and 208V Wye service will give you 120V phase to
neutral.



And many are delta which *has* no neutral. All may be floating.


In this case, he's already suggested that there would be a neutral, so

it
would be a 5 wire system.


Known as Wye.


Again, wrong. It *is* a delta system. He's talking about 240 volts, not
208. As far as I know, single phase service to the typical house is just
one leg of a three phase delta system. Isn't that how it comes from the
power plants, the primary service? How it's delivered to the customer
depends on the transformers that feed them.



But a frequent variation has one of the three sides center
tapped (the way the standard residential feed is supplied, 240V center
tapped with the center tap grounded and neutral connected to that.)

The breaker boxes for this have three buses, but only two of
every three positions can be used for 120V single-phase breakers. The
third phase is *way* too high.


I believe this is often referred to as the "wild" leg.


Agreed.



As stated above, I got around that problem in my current shop by having

two
panels, one strictly 3 phase, so none of the positions are lost.


There are / were a lot of strange variations on three phase power, but
most anything new is going to be 208V Wye service. Larger industrial
stuff will get 480V.


My 3 phase delta 240/120 volt service was installed just 4 years ago, at my
request. I did not want a wye service (for obvious reasons), and am
transforming to 480V for one machine.

Harold
..




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