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240VAC single phase from three-phase? - Page 11

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Posted by amdx on April 5, 2010, 7:58 pm



That was a bad URL, this ones correct.

Posted by m II on March 30, 2010, 6:49 am

Ulysses wrote:

The 240 heating element is fed by the two coloured wires ONLY. The 120
pump is fed by ONE of the coloured wires and the WHITE. you should have
a four prong plug on the cord now. (two hots, a white and ground)

You will need one white, two coloured wires and a ground attached to the
receptacle. Either coloured wire will read 120 volts to ground and 208
volts between them. The third coloured wire should be capped off and not

Make sure your new 3 phase outlet has a white in it. Most 3 phase motors
only use the three hots and a ground, so a lot of outlets don't have white.

Your heater will NOT work if there is no white in the box, as you can
NOT get 120 volts without it.

Generally, but not always, at 120/208 volts three phase, the hot colours
are red, black and blue. The white is a white (neutral) by code and can
not be changed.

You will keep your present cord/plug and wire the new receptacle for 2
hots, a white and a ground. The connection will look exactly as it does
now, but with one of the three hots not connected. Get the number off
the face of your existing 20 amp plug in and get another like it.

There may be a slight extension in heating time, as 208 isn't 240 volts
and the heat produced varies as the square of the voltage difference.

Get the store to show you how the screws where the wires go in are
marked and make sure you aren't using a larger breaker than previously.

Please let us know how it works out.

As always, make sure you are working in a safe and informed manner. No
one else is responsible for your safety. If, after your research, you
have any concerns, get an electrician.

Remember, any internet advice may well be worth exactly what you paid
for it.


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For solutions which may work for you, please check:

Posted by Ulysses on March 30, 2010, 5:21 pm

I found the installation instructions (finally) but there is no wiring
diagram.  You seem to be sure about how the heating elements are wired so
perhaps you are familiar with the type of press I'm talking about.  It turns
out the 208 volts is "acceptable" as per the instructions so that should not
be a problem.  I guess my question was not clear (or too many questions at
once) but IF each heat element was connected to a different leg and neutral
THEN each would be supplied with 120 volts and the 208 volts would be
irrelevant.  Is this correct?  The only three-phase I ever worked on before
was with small alternators and I always used all three legs and was only
concerned with the total output.  In any case I've worked with 240 volts so
I'm sure I can do the wiring without any major mishaps.  The vacuum press
has controls for setting the desired temperature and shows the present
temperature so it should work, just take a little longer to heat up as was
mentioned.  There is a card included with the instructions that tells what
the voltage readings should be at an accessory outlet (120 volt) on the
front of the machine so that should tell me if everything is wired
correctly.  I does, however, leave out one hot lead.....    The hot leads
that I indend to use are each connected to 20 amp breakers and nothing else
is being used on those circuits so I think I'm good-to-go.  Thanks everyone
for the help :-D

That's what I have.

As of this moment I have no doubts, but if any arise I'll get the

Posted by Pete C. on March 30, 2010, 6:10 pm

Ulysses wrote:

I'm not sure why you keep presuming that the heating elements are 120V.
Unless you find ratings marked on them it is far more likely that they
are 240V heaters and they are all wired in parallel across the 240V (or
208V) feed. The only odd thing would be if the motor is actually on 120V
as it would be more likely to be a 120/240V motor and wired for 240V.
Only the controls would have much probability of running on 120V only.

At any rate, since you indicated the press documentation indicates 208V
is acceptable, any two of your three hot legs and the neutral (120/208V
3ph Wye config) is sufficiently equivalent to 120/240V 1ph 3wire.

Posted by BobG on March 30, 2010, 7:37 pm

No discussion of three phase would be complete without comparing and
contrasting 120V three phase Y (4 wires..... A B C and neutral, 120V
any line to neutral, 208V line to line) and Delta (A B and C, but no
newtral! 208V line to line). And there is a Scott -T transformer that
somehow magically converts one leg of the 208 to 240. I'll leave the
explanation of that to a real electrician. I just took a course in
machines and transformers in EE school.... back in '71....

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