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Where does the neutral come from when turning 3 phase into single phase say in a street sub station?
Wiki User
∙ 11y ago
In a split phase service, which is the standard for residences and very small businesses in the US, Canada, and some other areas, one transformer is connected to one phase of the three phase primary distribution mains. The secondary winding is centertapped, with the centertap grounded and called neutral. The other two phases are not involved at all.
Do not call the hot legs of the service phases. They are not phases. They are opposite legs of only one phase.
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Canada and US transformer connections
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 12,460 volts. Eight feet below these top lines that is a single conductor. This conductor is grounded every three poles with ground rods and is used as a ground (neutral) on wye connections. 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 grounded conductor (neutral) and then down to a grounding pad completing the circuit. This connection puts 7200 volts across the transformers primary coil (12460/1.73 = 7200. 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) The center tap of this transformer is grounded and connected to the single grounded eight foot lower conductor. The single phase service neutral is established at this point. These three secondary points are connected by triplex to the homes weather head where another connection is made to the homes service distribution. This is classed as a 120/240 volt service.
Wiki User
∙ 11y ago
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What function does the neutral serve when the supply is single phase in a wye connection?
In a single-phase circuit like a house fed from 4-wire three phase in the street, the current flows in the live and neutral wires. The neutral currents from a street of houses are expected to cancel each other so that the overall current in the neutral leaving the transformer is small.
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