household wiring


I am a bit confused about 240V/120V household wiring. If there are 3 phases on the big powerlines, ie. X Y and Z, and they are 120 degrees out of phase with eachother, then how do you generate 240VAC and 120VAC from those three signals?

I assume that for 120VAC you use only one of the three phases, ie X, and then you need a common (the white wire) as the current return, but where in the circuit is the common hooked up to?

Do you always use the same phase for the 120VAC or do you try to balance the load between the different phases, ie X and Y phases each powering some of the 120VAC loads.

For 240VAC, I would have thought that the two live wires (red and black) would be 180 degrees out of phase, but they must only be 120degrees out of phase since the phases are all 120 degrees out of phase?

cheers, Jamie

Reply to
Jamie Morken
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Hi Jamie, Start here:

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Reply to
Tom Biasi

They connect a transformer between any two of those phases to produce a single phase source. The secondary of that single phase transformer has a center tapped secondary. That center tap is grounded and becomes the neutral going into your house. Each end of the secondary has 120 volts with respect to that neural. The two ends have 240 volts with respect to each other (when one end swings positive, the other end is swinging negative, see-saw fashion)

So the single phase voltage at your house may have a different phase than that at your neighbor's house, since his may be derived from a different pair of the 3 phases on the pole. Or your whole neighborhood may be fed by only a single pair of high voltage phases.

Each pole transformer usually supplies one or more nearby homes with a single phase. The power company tries to rotate which two phases are loaded so that the 3 phase load is roughly balanced at the substation level.

That is how it works.

No, the 120 and 240 volt supplies represent taps on a single winding.

Reply to
John Popelish

Not as simple or correct as it might be. What do you make of this quote from that page? "In a single phase unit the power falls to zero three times during each cycle,"

The coil diagrams showing 4 magnet poles but the first phase windings wrapped around only the two norths is also a bit strange. Just showing a north south pair (2 pole field) would have made more sense.

Reply to
John Popelish

In my area, the pole transformers are connected between one high voltage phase and ground, not between phases.

We have 3 phase HV running north, along the street at the end of my block. At each lane, one phase is tapped, and fed along the lane. If my lane gets phase A, the next lane north gets phase B, and the next, phase C, then phase A again, til we get to a commercial area, then all three phases run along the lane.

Peter Bennett VE7CEI 
email: peterbb4 (at)        
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Reply to
Peter Bennett

I haven't visited that site in some time. I was thinking that the OP would read some about 3 phase and then realize that he had 240 ct. Tom

Reply to
Tom Biasi

"Jamie Morken" wrote in message news:B%9Te.102874$Hk.40422@pd7tw1no...

Hope this isn't a troll to waste time? Jamie you are making it more complicated than it needs to be. Yes; we may have 3 phase on high power transmission lines at 166,000 volts or even higher across country. Also those three phases may be brought into a residential area or into a large building at voltages such as 15,000 or 46,000. Talking about North America now, not necessarily elsewhere ; But to your house consider just one of the phases, stepped down by means of a single phase local 'distribution transformer' either pole mounted, or not too far away on a cement pad or hidden in a manhole. Other houses and streets will be connected to other phases; in order to 'balance the load' back through the transmission system to the network/generating station. Forget about them for a moment. That single phase, into your house is alternating at 60 hertz per second. There are three wires into a typical home. Forget about ground for a moment. The middle wire (typically white) is considered neutral and to all intents is zero volts. Note 1. One of the other wires (typically) red, you can consider that to be, as it were at positive 115 volts. Note 2. The third wire is (typically) black, you can consider that to be, as it were at negative volts 115 volts. Note 2. So between the centre wire (neutral-zero volts) and either the red or the black 'hot wires' there will be 115 volts. Between the red and black there will be 115 + 115 = 230 volts. This 230 volts is used for heavier appliances such as clothes dryers, cooking stoves and water heaters. The (two supplies) of 115 volts each are both used for lights and wall outlets etc. Note 1. That neutral is grounded (only once) as it enters the house main panel. It is often/usually also grounded by the power company. Note 2. The red and black are frequently and incorrectly called 'Phases', even by electricians who should know better! A clearer designation is to call them 'Legs'; for example Leg A, Leg B. If you think about it Leg A and Leg B are the two outer ends of the single phase 230 volt supply to your house. Note 3. The whole darn thing is of course alternating, 60 times a second. That's a bit too fast for most people to see the flicker, although some people claim they can do so; so one sixtieth of a second after what was just said about 115 volt positive and 115 volts negative it will have alternated/reversed and will continue to do so ad infinitum. Note 4. The next street or another group of houses over may be fed from another phase of the power system! So if you ran a wire over and measured their alternating voltages on their single phase residential supply you would find that their service is exactly the same; BUT 120 degrees ahead or behind yours. And another group of homes could be supplied from the third phase and be 240 degrees ahead/behind. Any help or more confusing? Sorry about the long winded answer. My regards. Terry

Reply to

they take one of the single phases (at 1100V or whatever) and put it through a step-down transformer with a centre-tapped secondary. (more precisely they use a three phase step-down transformer with three cnetre tapped secondaries - one for each phase)

common is the centre tap in the secondary, also connected to ground.

they use X and -X phases (opposite ends of the secondary),

that's in the americas anyway

in much of the rest of the world where the domestic supply is (about) 230V they don't bother with X and -X phases - everything runs of a single phase, or if it uses more than one they are 120 degrees out of phase. and the voltage between phases is (about) 381 volts

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Reply to
Jasen Betts

Take your dog for a walk, finding a road with pole-routed 3-phase power. You'll soon pass a power line pole that has a transformer, or two or three. Note that each transformer takes a tap off only one of the phases, and the secondary output will be onto two insulated wires, and one bare. This is the 240Vrms that supplies residential power -

240V between the insulated secondary outputs, and 120V between each of these lines and the bare-wire center-tap, which will be grounded.

Following my first paragraph, you can see that indeed, typical residential power uses only one of the three phases. Your confusion lies in the fact that you don't realize that the typical residential feed is 240Vrms, not 120Vrms. The 120V results from splitting the phase with the grounded center-tap at the transformer. In your residence, you end up with two separate 120V supplies, one 180 degrees out of phase with the other, along with a single 240Vrms supply.

The power generation, transmission, and supply companies work together to keep the loads balanced. You need not worry about balancing phases in your residence, as you have only one phase. Industrial power feeds typically use all three phases, and the industries' electricians sort it out from there.

The 240Vrms is in phase with itself - the red and black wires in your residence correspond to the two different sides of the power supplied by the pole transformer. (It is possible that your 240Vrms might be

120 degrees out of phase with a near or distant neighbor's residence, which is fed with a transformer on a different phase.) Part of your 120V is 180 degrees out of phase with the other part, as the 120V service is actually the 240V service split to equal sides of ground. Make sense?
Reply to
Kitchen Man

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