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240VAC single phase from three-phase? - Page 12

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Posted by Ulysses on March 31, 2010, 2:23 am

Well, I didn't *think* I was presuming, maybe just hoping.  If I understand
it correctly then IF each heating element was powered by a different leg of
the three-phase supply then there would be no loss of power being supplied
to the elements whether they were connected to single phase or three-phase.
I'm not trying to be difficult, I'm just trying to understand it completely.
The total voltage of my three-phase power that reads 208 volts between two
legs would be 360 volts, right (208*sq/rt of 3)?  That would make each leg
produce 120 volts between hot and ground, right?  With single phase each leg
would also be 120 volts between hot and ground.  From what everyone has said
it is common for such heating elements to be connected to 208 or 240 (or
230) volts so I'm just trying to understand why.  It seems to me it would be
more versatile if the elements were powered by 120.

The motor has a 120 volt type power cord that appears to have Hot, N, and G.
I did not read the label on the motor to see if it could be wired for 240
but I'll have to do that.  The motor plugs into it's own reserved receptacle
at the rear of the machine.  I will test the voltage at that outlet once I
get the thing connected but since the power cord has a plug exactly like a
desk-top computer I'm *assuming* it's 120 volts.  I mean, they wouldn't use
the same type of power cord for 240 would they and be running two hots and
ground through it?

Yes, and thank you.

Posted by Pete C. on March 31, 2010, 1:36 pm

Ulysses wrote:

Higher voltage = the same wattage with smaller and less expensive
conductors. Double the voltage, half the amperage. The elements could be
120V, but that would be unusual unless they are quite small.

Look closely at that receptacle and see if the blades are sideways i.e.
240V receptacle. A three wire cord is entirely normal for a 240V motor
since they have no need for a neutral, and while the white conductor
should be marked black or red on the ends when used as a hot lead, it's
common to not bother in such an application.

Posted by Josepi on March 31, 2010, 1:18 pm

The machine will be fine. This is very common practice to do this.

Many utilities will crank up the voltage and call it 125 / 216 volts
(network) as this satisfied the 240 volt loads a little better and is still
acceptable for the 120 volt loads. 208 volts may be below the acceptable 240
standards (+/- 10%) for residential grid power supply specs. If you have
your own transformer feeding you place the utility may consider raising a
tap or two for you on the street transformer, provided it has them.

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

Posted by Ulysses on April 15, 2010, 4:16 pm

I finally got to the vacuum press and got it wired and everything is OAK :-D
The pump is 1/3 HP and runs from 120 VAC single phase so that was easy.  I
never did check the voltage at the 240 outlet in the unit I moved from but
there is very little doubt that the power there was also three-phase so the
press was almost certainly running from 208 volts there.  In any case  we
are not noticing any difference in the operation of the pump or heating

Thanks everyone for all the help!


Posted by Ulysses on April 15, 2010, 5:16 pm

I finally (yesterday) got the vacuum press connected and everything is AOK
:-D  The pump is 1/3 HP single phase 120 VAC so that was no problem.  The
elements seem to heat up at the same rate they did before.  I did not check
the voltage in my other unit but there is very little doubt that it was also
three-phase so it was probably running from 208 volts before.  Now for the
neon signs....

Thanks everyone for all the help!

P.S. I tried to post this from my laptop and it wouldn't send so I hope this
isn't a duplicate.

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