Is the mains power supply in Delta or Wye configuration before it is stepped down to supply power to homes

Question

( I am not sure if this is a location issue or more about principle of electricity. But just in case, make that Washington State, United States)

This question has recently been on my mind and when I googled it I didn't get anywhere except the explanation between the wye and delta configuration.

I see three wires on the main pole by my house and two of them goes in to the transformer. I subconsciouly used to think that the three wires were 3-phase supply which of course it would have meant a delta supply since there is no 4th wire.

But one fine day I came to realize that could not be only because I began to think about the phase difference. If the power supply is stepped down from the the two delta legs being at 120 degree apart, what would that translate into the phase of the step down voltage supply to my house? I somehow have been having a hard time getting a picture in my mind.

I seem to feel comfortable if the main supply was from a wye configuration with two hot legs and one from its neutral.

Could the three wires also be delta? If so, what would that translate into the power supplied to the home?

And could supply to the home be made from the two hot legs of wye instead of one hot and the neutral? Assuming I have all this right in the first place?

Furthermore would I be right to assume that at any junction from the Origin of power source onward be it 3 phase delta or 3 phase wye, each can be transitioned or converted from one to the other?

Answer 1

One more point that nobody mentioned in the answers. If you put an AC voltmeter (or oscilloscope with isolated high voltage probe) across any two conductors in three-phase power, you will get a sine wave. So don't be confused by the 120 degree phase shift issue. The voltage across any two conductors is a perfectly good sine wave and can be connected to a transformer to adjust the voltage to the correct level for home service.

This is exactly what I have at my house, too. Three phase power on the pole, and a single phase transformer with center tap secondary to drop the voltage down to 240 V. The center tap is neutral. Different homes pull from different phases to hopefully keep the load balanced from the utility company's perspective.

Answer 2

In my area (Vancouver BC) the final High Voltage (13200 V) distribution is three phase Wye. The single phase step-down transformers to 120/240 Volt are connected between one HV phase and Ground/Neutral. The Ground/Neutral wire is connected to an Earth ground rod at each pole carrying a transformer and also runs between poles, presumably all the way back to the substation.

The power distribution in this area is all overhead, so I can see the connections.

The end user gets single phase power whether the stepdown transformer is connected between one phase and Ground, or between two phases.

Answer 3

it would have meant a delta supply since there is no 4th wire.

You appear to be talking about the three phase output from some remote transformer and, it doesn't matter whether the remote transformer output was configured in wye or delta. Without delving into what the remote transformer's secondary configuration is, you cannot know and shouldn't care and it doesn't matter. The local transformer on your pole will have its secondary side grounded and connected to a neutral wire. That's the important thing.

If the power supply is stepped down from the the two delta legs being at 120 degree apart, what would that translate into the phase of the step down voltage supply to my house?

You cannot know this without measuring but clearly, if single phase transformers are being used then the phase is not shifted.

Could the three wires also be delta?

It's irrelevant; the output of the remote transformer feeding your street might by wye or it might be delta and, importantly, it makes no difference.

What I can say is that any local three phase transformer will likely be configured with a delta primary to ensure that magnetic imbalances inside the transformer do not produce a significant secondary imbalance voltage due to light secondary loading.

would I be right to assume that at any junction from the Origin of power source onward be it 3 phase delta or 3 phase wye, each can be transitioned or converted from one to the other?

Sure, transformers that are delta-to-wye are commonplace but, as I had hoped to point out above, it is irrelevant given what seems to be your motivation as stated here: -

I seem to feel comfortable if the main supply was from a wye configuration

And, I will add, that the final secondary voltage of three single phase transformers or one three phase transformer is likely to be wye configured so that a neutral and earth can be made.

Answer 4

In the U.S., wye is commonly used for distribution, delta for commercial services (building wiring) <reference, pg. 41-42>. My power distributor uses either 7.2kV or 14.4kV for local distribution. Transmission lines are 69kV or 138kV. Voltages in your neighborhood may be different.

Answer 5

Could the three wires also be delta? If so, what would that translate into the power supplied to the home?

It certainly can be. The standard in the UK for supplies of 11kV and above is three phase delta, no neutral and no ground. So the transformer is delta wound on the primary, and wye (or "star") on the secondary.

Answer 6

Since there are only 3 wires feeding all the poles, this implies a delta feed.

From the three wires only two wires feed the tranformer, it implies a single phase feeding each block, however many homes are there.

In each pole there are one or more transformers. The transformers are different sizes depending on how many homes are connected to any one transformer.

Minimum size of transformer has to provide 240 volts per household. I happen to have my own transformer that feeds my house from the look of it.

Since U.S. households operate on both 120V and 240V, there is also what is called a neutral wire that comes out of the transformer. This wire is center tapped on the transformer.
It basically splits the single phase 240V and therefore provides 120V as well.

So we have split phase; 120V between each leg and the neutral and 240 between the two main hot legs.

The two hot legs are for such things as stove, water heater, dryer, HVAC and other high voltage devices. And the 120V is all other things.

I think that sums it up.