Let's start with Single phase. The power coming into your house is 240V. The utility company provides 3 wires to your house. For simplicity sake we can call them +120, 0 (Neutral which is bonded to Ground in residential wiring), and -120. Looking at the graph you can see that with the +120 and -120 you have 240V between them. The center wire is the 0 which provides 120V when paired with either of the other wires. If you look at your breaker box, you will notice that the 240V breakers actually span two slots. This is because you are connecting between the +120 and the -120.

The reason for higher voltage has to do with current and Ohms law where Voltage = Current * Resistance. As the voltage goes up, the current can go down. The lower the current the smaller diameter wire you need, kind of like a garden hose pumping water. Here is a practical example...

Say that you have a large light for your yard. The light has a 4,000Watt bulb to really blind the neighbors. If you run the light at 120V, you will have 33.3 amps of current (Power = Voltage * Current). If instead you run the light at 240V, the current will be half or 16.6 amps. This can have a huge cost difference when you start looking at wire cost. To carry 17amps of current, you need a wire of 12 gauge or bigger. To do 34amps, you are jumping to 8 Gauge which is considerably heavier and with all that copper will cost a lot more. To add to the problem, smaller wires and lower voltages encounter more resistance in the transmitting wires, which means you are losing a lot of power to just get the current to where you want it.

Wire Gauge/Current Reference: http://www.cerrowire.com/ampacity-charts

Now lets talk about 3-Phase. 3 Phase power is the same idea as single phase, but the voltage peaks are 120 degrees apart from each other. Looking at the 4 wires coming in they look very similar to a single phase example if you take one of the phases and pair it with the Neutral. You will see 120V and a single sine wave of voltage and current. If you pair two of the phase lines which are 120 degrees apart, the voltage ends up being 208V from phase to phase. This is very typical of Commercial wiring in the US. The higher voltage of 208V means lower current for loads, and the phase shift between the lines is very useful when running 3 phase electric motors. Think of those phase peaks firing at just the right time in a motor to keep pushing the motor around. The fact that this happens at a higher voltage than home wiring means the wire gauge can be smaller which saves weight, space, and ultimately cost.

The reason for higher voltage has to do with current and Ohms law where Voltage = Current * Resistance. As the voltage goes up, the current can go down. The lower the current the smaller diameter wire you need, kind of like a garden hose pumping water. Here is a practical example...

Say that you have a large light for your yard. The light has a 4,000Watt bulb to really blind the neighbors. If you run the light at 120V, you will have 33.3 amps of current (Power = Voltage * Current). If instead you run the light at 240V, the current will be half or 16.6 amps. This can have a huge cost difference when you start looking at wire cost. To carry 17amps of current, you need a wire of 12 gauge or bigger. To do 34amps, you are jumping to 8 Gauge which is considerably heavier and with all that copper will cost a lot more. To add to the problem, smaller wires and lower voltages encounter more resistance in the transmitting wires, which means you are losing a lot of power to just get the current to where you want it.

Wire Gauge/Current Reference: http://www.cerrowire.com/ampacity-charts

Now lets talk about 3-Phase. 3 Phase power is the same idea as single phase, but the voltage peaks are 120 degrees apart from each other. Looking at the 4 wires coming in they look very similar to a single phase example if you take one of the phases and pair it with the Neutral. You will see 120V and a single sine wave of voltage and current. If you pair two of the phase lines which are 120 degrees apart, the voltage ends up being 208V from phase to phase. This is very typical of Commercial wiring in the US. The higher voltage of 208V means lower current for loads, and the phase shift between the lines is very useful when running 3 phase electric motors. Think of those phase peaks firing at just the right time in a motor to keep pushing the motor around. The fact that this happens at a higher voltage than home wiring means the wire gauge can be smaller which saves weight, space, and ultimately cost.