# What is exactly three phase and one phase?

I live in a hostel. When I woke up today, there was no electricity. A few hours later, the fans started working, as well as the modem although they weren't too fast. A guy sitting there told some things to me -

1. Of the 220v, 100v directly comes to any equipment that we plug it in. I don't get it, why 100? Isn't it more like there's proper devices in the equipment that only take the required voltage/current in? He said that his phone is charging because of the direct 100v that he is getting when my point just was, that I wasn't sure if the socket would be getting enough voltage to charge his phone.

2. The fans were on in some rooms and not in the others because it was working in single phase. I am not even sure what he meant, as my understanding of the phase systems is very different than what he was implying.

3. Off topic - He also told me that more impedance in headphones, means a better connection and better sound quality, like, how? Isn't impedance just when a system resists current flow? I Googled it and didn't get anything like he said, this is what started to make me wonder if all the things that he said in the morning were actually true and not what he just said to sound smart.

• I think that three phase power is generally for industrial installations where industrial electric motors may use three phase power because it offers smoother power delivery.
– user93237
Aug 20, 2017 at 23:01
• It might help to add in what part of the world this was. Different regions have different systems in place. Aug 21, 2017 at 5:32

1. The 220 vac (ac-alternating current, 60 cycles per second) system in residences in the US is a single phase three wire system. One wire serves as a common connection for the other two phases, each of 110 vac. 220 vac can be obtained by connecting across the two phase connections. This allows things like air conditioners and electric dryers to be connected to the power and they use lower current because of the higher voltage. This permits usage of smaller size wiring conductors.
2. Some single phase fans were working while others were not working because some of the circuit breakers were connected to the 110 vac phase that was working and the other 110 vac circuit for the other fans connected to it was not. It has to do with how the circuit breaker distribution panel is wired. The two phase three wire power comes into the dwelling and is connected to three connections in the panel. Approximately half the 110 vac single phase circuit breakers in the panel are connected between one phase and the common connection and the other half of the circuit breakers are connected between the other 110vac phase and the common connection.
3. I think he is referring to impedance matching between the sound source and the headphones. Impedance includes resistance and inductive or capacitive reactance. Reactance changes with sound source frequency. Resistance does not. Impedance matching between source and load insures maximum power transfer from source to speakers and/or headphones.

I fail to see what three phases have to do with this. Three phase power is usually used in industrial and business applications.

• I revised this answer some. The three wire system is actually all one phase. It is possible to get two single phase 110 vac circuits by taking each phase connection to the common (=ground) and 220 vac circuit by connecting to each of the phase connections. Aug 21, 2017 at 0:59

It sounds that your electricity supply is a three wire single phase system which is also called a three wire split phase system.
It is probably not a three phase supply.

There are three conductors of which two are live (hot) and often colour coded black and red and a third called the neutral which is colour coded white.
There may well be a fourth wire with no insulation on it or with green or green and yellow insulation which is the earth.

The voltage between a live and neutral is $120 \rm V$ and the voltage between the two live wires is $240 \rm V$.

The wiring diagram for the hostel might look something like this.

You will note that each live wire is protected with a circuit breaker (CB1 and CB2) and the comparatively low power consumption $120 \rm V$ devices are connected between a live and the neutral and high power consumption $240 \rm V$ devices are connected between the two lives.

Suppose that circuit breaker 2 disconnected live 2 (L2) then fan 2 and the air conditioner would not work whilst the charger and fan 1 would continue to function.

The alternating voltages are as follows and have a frequency of 60 hertz (Hz).

In fact live 1 voltage is $180^\circ$ out of phase with live 2.
This is achieved by the use of a centre tapped transformer which you may have seen at the top of poles?

For historical reasons North America settle on a $120 \rm V$ standard but that was found to be wanting when using devices which consumed a lot of electrical power and hence demanded large currents which in turn required thick copper conductors.
Increasing the voltage to $240 \rm V$ decreased the required current and hence allowed the use of thinner (and cheaper) conductors.
The system also has the advantage of reducing the current in the neutral when $120 \rm V$ devices are connected.
This is because the current through the neutral due to a device connected to live 1 is $180^\circ$ out of phase to the current through the neutral due to a device connected to live 2.
In theory if the currents through those two devices were the same no current would flow through the neutral wire.

The three phase supply to which you refer is used to convey large amount of electrical power and the voltage graphs for the four conductor (three live and a neutral) look like this.

Again if each of the three phases draw the same current the current in the neutral will be zero.
That is generally not the case but it does mean that the neutral conductor can be considerably thinner than the live conductors.

Transmission towers (pylons) usually carry conductors in groups of three with extra earthing conductors (for lightning protection) as can be seen below.

You question about impedance is all to do with the flow of electrical power from one device to another.

If you want maximum power transfer between the source (eg iPhone) and the headphones the best that you can do is to have the output "impedance" of the device equal to the impedance of the headphones.

Making the impedance of the headphones larger will reduce the power being transferred but may alter other characteristics.

However this is rally from a different era and with modern electronics impedance matching is not always necessary and having a high impedance set of headphones can potentially improve fidelity but you need to refer to the experts for this.