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| Subject - 208 VOLT VS 220VOLT | |
| David Hyatt |
In a 120/208 volt three phase system, you read phase to neutral 120 volt and phase to phase 208. I have never work on one but if it is required by drawings to have a 120/240 volt 3 phase system I was told that it did not have a neutral and you would have to set a transformer? Why is it not 240 volt phase to phase and phase to neutral 120 volt just like the 120/208volt? Help? |
| JimmyDee | 120/208 is a Y system. I you look at the windings, they are connected in a Y configuration. HERE is a drawing of that system. The 240/120 is a delta system. The Greek letter Delta is a triangle. The windings are wired in this triangle configuration. Here is a drawing of an open delta system. A closed delta system is no different except there is an additional transformer hooked between A and C. This gives the system about 40% more capacity but the voltages are the same, phase to phase and phase to ground. Jim |
| David Hyatt | Can you have a delta center tap neutral? And be the same as above? |
| JimmyDee | Yes you can. If you remove one of the transformers from the delta system, it is no different than a normal single phase, 120/240 transformer. Remember that the center tap is on one transformer and only one and would be tied to X3 and X2 leads of that transformer. The 208 neutral tap would not be on any one transformer with a center tap on it, but all the transformers are ties together and the neutral is tied to the point they all tie together. Jim |
| David Hyatt | would I have to set the transformer or would that come that way from the power company. On a 120/240 delta, how many wires come in from the weather head, 3 0r 4? From the info you gave I would say 3 hot legs only. |
| JimmyDee | quote: If you have the system as a service, usually the power co provides the transformers. I have seen primary metered systems where the electrician would provide the transformers. There are 4 wires, 3 hots and 1 neutral. If it was just a 240 only three phase, you would have only the 3 hots. This system is also called the wild leg because the voltage between the neutral and the 3rd leg is 208 volts. Study the drawing and it should clear up as to why. Jim |
| Talvez | Please read Article 215.8 and 230.56 and keep in mind that one bus bar (high leg) to ground is not available for 120 VAC loads. Be very careful in your load distribution as this is very easy to unbalance all but three phase loads as well. |