Art. 250 - Corner Grounded Delta

[bigapple]

Does Article 250 permit a solid corner ground on a 480 Volt Delta secondary transformer winding? In this application a 750kVA, 208V Delta to 480V Delta step-up transformer is used in a high-rise commercial office building. Its purpose is to step up the system voltage to more efficiently carry power a long distance up the high-rise building, where it is then stepped back down through a 480V Delta to 208Y/120 step-down transformer. I'm having difficulty determining whether such an application is code compliant. Should the 480V system be grounded, ungrounded or impedance grounded? All opinions are welcome. Thanks.

 

[jim dungar]

Re: Art. 250 - Corner Grounded Delta

The NEC does not require any AC system, between 50 and 1000V, to be grounded if the resultant line to ground voltage is in excess of 150V.

Because you system is feeding a single dedicated load, I would not have any problems with it being ungrounded. If it was feeding multiple loads (i.e. a panelboard) I would say to ground it.

Usually, I will make recommendations, about grounding delta systems, based on the local maintenance electricians. Many times they are expecting to see nominal line voltage to ground, while in other places they would be freaked out by seeing 480V to ground. In my local area they expect to see 240V grounded and 480V ungrounded.