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3-Phase Corner-Grounded Delta & GSM

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Tom

#85373

3-Phase Corner-Grounded Delta & GSM | 3 August, 2020

We are looking at moving into a building that has drops from a three-phase 240V delta, corner grounded transformer. They currently have a lot of 230V Bridgeport milling machines and other motor operated equipment on this transformer. We are coming from a 208Y service in our existing building.

Does anyone have any experience connecting a GSM machine or other 230V equipment to a corner grounded system? I presume it is much the same as the 208Y since the machines do not have a neutral requirement. The only thing throwing me is the corner ground where one phase is 0V with respect to ground (it is grounded.)

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#85381

3-Phase Corner-Grounded Delta & GSM | 4 August, 2020

Are you sure it is not a 240V transformer with a center tap? That is how residences are wired. If you use the center tap you get 120V. The other tap to center tap is 120V that is 180 degrees out of phase. So if you use the two outside taps you get 240V. The center tap is grounded where it comes in the building.

You get 208V from a three phase system because the phases are 120 degrees out of phase with each other. Each leg would be 120V to ground.

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Tom

#85383

3-Phase Corner-Grounded Delta & GSM | 4 August, 2020

Yes, I am certain it is 240 corner grounded delta. It confuses many electricians and it took me some time to understand. It is fed from a 480:240 delta 200A transformer. As I mentioned, they have a lot of Bridgeport 3ph machines and cnc machines connected to it. I am familiar with residential split phase service and 208Y and this is definitely corner grounded three phase.

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