What is the difference between single phase and three phase electrical power services?

Answer 1

Answer for USA, Canada and countries using similar 60Hz household electricity supplies This is a very simplified answer: Single-phase has two hot "legs". Each are 120 V. for a total of 240 V. A neutral wire (white in color) will give you 120 V. When straight 240V is needed, a neutral isn't necessary. 3-Phase has 3 hot legs for a total of 480V. This voltage is generally used for motors in industrial and commercial settings. This voltage can be "stepped down" via use of a step-down transformer. This converts 480V into 120/208/277. 120V is used for common appliances. 208V can be used for dryers and ranges (if they're rated for 208). 277V is used for lighting circuits (again, lighting must be rated for 277V). 3-phase is used in commercial and industrial, including schools. A neutral is needed for 120V and 277V and is generally gray.

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Answer 2

Canada and US To best explain this visualize the three primary line that you see on pole tops in industrial areas. The voltage between these three lines is about 14,400 volts. When you see a pod of three Transformers mounted together they are combined into a three phase system. From each of the three high voltage lines the voltage goes through three separate fuses and down to a high voltage bushing in each of the transformers. Inside the transformer the voltage goes through a coil and exits the transformer through a second high voltage bushing. This bushing is tied into the ground pad completing the circuit. This connection puts 7200 volts across the transformers primary coil. The secondary voltages of these transformers are determined by the customer that wants the three phase service. It could be 600,480 or 240 volts. Each single transformer has two output bushings. These bushings can be wired into either a delta or star configuration depending on what the customer ordered. In a single phase service the name is derived from just using one of the three primary lines that are used above. It is the same primary connection, but the secondary is a bit different. The output secondary coil has three bushings coming out of the transformer. On the two outside bushings is the working voltage (house connections 240 volts). The middle bushing is a center tap of the secondary coil. This gives a voltage of (bushing left to center 120 volts) and (bushing center to bushing right 120 volts) These are the the three wires that come into a house for its 120/240 volt service.

Answer 3

For a simple description single phase = 1 hot and one neutral, three phase = 3 hots and one neutral.

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Another answer

A three phase service on its own does not have a neutral line. The voltage difference is measured between the phases. (No voltage can be measured between any of the phase lines and a neutral because there is no neutral line.) Any electrical equipment which uses a 3-phase service exactly as it is delivered by the power generator / utility - that is, without a neutral line - are connected in what is known as "delta" connection.

If you want to use just a single phase service then one way to do it is to connect the incoming three phases to a transformer whose secondary windings are connected in what is called a "star" (also known as a "Y" or "wye") configuration which has a neutral point at the center of the "star" or "Y". Then, for a single-phase service, you can use just one phase and the neutral. This results in about half the voltage - actually it's 1 / 2./'3(1 divided by the square root of three) - that you'd get across any two phases of the incoming 3-phase service.

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Answer 4

Three phase power is three sources of power, each 120 degrees out of phase with respect to each other. They work together to power a device capable of using those overlapping phase angles to maintain good performance.

Two phase power is actually one phase power. It is more correctly known as split phase power. It is produced when a one phase transformer's secondary is center tapped. That center tap is neutral. The two opposing legs, incorrectly called phases, are 180 degrees out of phase to each other, not 120 degrees as in true three phase power.

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CommentA two-phase system is an obsolete electrical system in which the two phases are displaced by 90o and (in the case of a two-phase, three-wire system) a line voltage that is 1.414 x phase voltage; it predates the three-phase system in use today.

Answer 5

A 3-phase system can generally supply more power than single phase. Most ordinary houses have a single-phase supply while larger buildings like factories have 3 phase.

In Europe a normal domestic supply has a single phase supply carried on two wires, a live and a neutral. For a large house or shop with a three-phase supply there is a 4-wire supply with three live wires and a neutral, and in principle it can supply three times the power (assuming the same wire size). So three times the power is supplied with four wires instead of two, and there is a saving in wire.

A three-phase supply has a further advantage in that the current in the neutral tends to be low, and when the live wires happen to be carrying equal currents, the neutral current is zero. Thus the power lost in the resistance of the wire is reduced, and three-phase three-wire transmission is used all over the world for power transmission ove distances.

Answer 6

Single phase means 1 live wire , 3 phase means 3 ive wires