Household Power - Two Lines of 120V

A power transformer has one input from the higher voltage and a center taped output. Between either of the legs to the center tap (held at ground) is 120V between the two outer legs are 240V

... and the two 120 volt sources are not out of phase. They are in phase, as in there is no phase shift between one source relative to the other source.

Think it as two 6 volt batteries connected in series. Across both batteries, you have 12 volts, but from one end of a battery to the middle connection there is 6 volts. You can draw two sources of 6 volts or one source fo 12 volts or a combination.

Jerry,

"2 lines of 120V that are in opposite phasing." They are not phase shifted.

"one "hot" line and the neutral on the pole" how do they turn that into 2 lines of opposed 120v? They are opposed but not out of phase. If they were you would not see 120 + 120 = 240 Volts. They are opposite polarity with respect to neutral.

Jon

But Jerry is right, "opposed" is equivalent to a special case of phase shifting (180 deg).

brgds

Snel

Not so...

Snel,

That is not a phase shift. That is polarity reversal (or special case of phase shifting). Take two 120 Volt Batteries and connect the negative of battery A to positive of Battery B and connect a meter to that connection and measure positive of Battery A and Negative of Battery B. That is the same as measuring from neutral to each main of a household power panel. One will be negative when the other is positive. Except during Zero crossing.

A phase shift is when more than one ac source is brought in and one starts and ends at a different time than the other. One will be at maximum while the other is still climbing or falling. Out of phase. If they both are 230 Vac you will only be able to see the difference between them and wonder why it doesn't read 230 Vac.

There will never be a time when there is Zero volts between phases since there is never a zero crossing at the same time.

Jon

This still doesn't answer the question; How can you derive 2 sine waves or square waves as the case may be, with one at 120v positive and another at 120v negative at the same time from a single source wave form?

Answer: "You don't..." In a 3Ø system multiple conductors are involved (and let's not digress into the differences between 'star' and 'delta' connected equipment).

A different voltage will be measured between any of the given lines and the neutral/ground at any given instant in time. [Lest we forget: the voltage read by your meter is NOT the "Peak" voltage, but rms voltage, which is .7071 x peak]

The most direct/correct/succinct response to your inquiry is this: If all you had was ONE of these "hot" wires and a neutral (exhibiting 120v between them), then, to obtain 240v (without getting fancy-schmantzy with heavy-duty electronics involved) you would need to use a step-up transformer ~ 120v across your primary (at a 2:1 step-up ratio) would give you 240 out on your secondary...

Best Regards ~

No, no, no, no, non. See previous comments about the split phase residential system. A center tapped single phase transformer with a connected grounded conductor (neutral) is how 120 volts is pulled off. 240 pulled off at the ends of the winding.

Re-read the post , VERBATIM please...

what you state was, is, and always will be fully understood... no question about that.

My post addresses HIS inquiry ... word-for-word exactly as I interpret it to be ... without going off on tangents.

IF he has ONLY a "single hot line" and a "single neutral" (as he stated) ... then he cannot produce, obtain, or otherwise magically-extract "2 seperate sine waves out-of-phase...etc"

But, to obtain "240v" , he CAN simply use a step-up transformer.

Or , of course ...

he can get into electronic chopper / multiplier circuits and what-have-you and spend as much money as he can pull together ... the sky's the limit! But, I understood him to be seeking a simple straightforward solution ...

Regards ~

Agreed...

Though, in the case he presented, a "step down" transformer is more then likely what is needed as he is probably referring to the HV primary feeder.

"only one "hot" line and the neutral on the pole, how do they turn that into 2 lines of opposed 120v?"

Yep, a step-down transformer with center-tapped 230 Vac output.

The keyword that you must've missed in his question was "household". In the US, residences are fed by a split-phase transformer with one hot wire and one ground on the primary. THOSE are the two wires out in the boonies about what he is querying. Since he says he is an electrician, he knows what a transformer is and that it has more than one winding. I believe he was asking, how do you turn single phase 4160 on the utility pole into single phase 120/240 going into his house.

Yup...

"split-phase transformer"

That means single phase input winding and a single phase output winding that is divided (split) by a center tap and it should not be confused with any other "phase" concept.

Jon

Yup... though describing this system as "split phase" has gone out of vogue.

Hi Jerry,

The most common electrical power supplied at point of use in the USA is (Edison) configured (white neutral, black 120 VAC to neutral, red 120 VAC to neutral, with 240 VAC black to red). It is transformed down to 240VAC and center tap split as frankd20 stated to the Edison configuration at the substation feeding the power to point of use (3 lines). The two 120 VAC legs are in phase and simply add to be the 240 VAC across the black to red. If you are somewhere where you only have one leg of 120 VAC available, you would have to step it up to 240 VAC with a transformer (having a center tap if you wanted both 120 & 240 VAC available). Hope this helps!

BTW I previously lived in NH (God's Country in the '80s) - what has happened to "Live Free or Die"? Looking more & more like MA lately!

Thank you all for your time in response to my query. So let's see if I have this straight: If the 120v leg on the left side of of my box is in -, and the 120v leg on the right is in +, I have my 240v. When the left leg goes 120v + and the right leg goes 120v -, it is at null. (Just for purposes of illustration) Correct?

I know what you mean about NH; I moved here to escape the political chicanery of southern New England and it appears to have followed me here anyway.

If the left side of your box is 120 VAC to the neutral and the right side is also 120 VAC to the same neutral (which is usually grounded for safety), then you should have 240 VAC from the left side to the right side. Check it with a meter before you try to power a dryer or water heater with it. If unsure, best have an electrician wire it.

The 120 VAC sine wave from right and left are in phase so they do not cancel or null. Both the 120 VAC legs and the 240 VAC across them all hit their voltage peak at the same time. The center neutral tap on the transformer supplying your power does not change the phase of the 240 VAC, it just splits it in two. Hope this is clearer.

What a coincidence - I was originally from CT before NH (loved it there). I could go back to NH, but not CT when I retire.

Single source one sine wave 240 Vac waveform without the centertap.

With the centertap it is a voltage divider and still one sinewave. The only thing changed is the point of reference if you now measure from the center tap.

If they were both positive or both negative you would not be able to run any of your 240 volt stuff as there would be zero volts between the mains. Of course that would be done by having two generators connected in series and synchronized so they put out the same polarity with respect to neutral.

Single phase center-tapped house power cant do that.

Can't find my backup data, but I think it is referred to as split-phase.

One way to do it is to use a center tapped step up transformer that doubles the number of turns in the secondary coil. The ground will be the center tap and the ends will be in opposite phase.

Jerry clearly needs a step-up transformer.

But please check what current is needed at 240 V.

If this is, say, 20 amps, you will need 40 amps from your power line in the weeds.

Step up from what?

Jon

Samsook,

"The ground will be the center tap and the ends will be in opposite phase."

Opposite polarity. There is no opposite phase in single phase household power.

Jon

Sam,

Are you assuming that the lines on the pole are single phase 120 Vac?

Jon

As others have mentioned you seem to be referring to; one input and three outputs two center joined coils wound opposite (directions) . So the term "phase" is a misnomer if your thinking of a time aspect.

If you Google split phase electric power you will find that this used to be name given to power drops for residences. It even says that this name caused some confustion because there is no change in phase; the split part coming from the center tap on the secondary transformer winding that is connected to the grounding and grounded conductors in the drop. The proper name for this system is 3-wire, single-phase, mid-point neutral system (per Wikipedia). So, really, my first comments did not warrant two "off comment" votes.

Thank you all again. I was unaware of the center tap in the transformer, and upon thinking it through, it makes sense now.

This now gives rise to a new question....