210.9/215.11 Autotransformer question

[Smart $]

The title of the chart I linked to is "Autotransformer - 240 Primary Volts - 120/240 Secondary Volts - 1Ø". Their isolation transformers are on a different page, and they weigh about twice as much for a given kva:

https://www.hubbell-acmeelectric.com/p-133-240x480-120240

Cheers, Wayne

Okay, I see that now. I didn't go to the degree of viewing the schematic. Shame on me. :ashamed1:

Yep, the link you gave is definitely for autotransformers. If you dig further through their website you can find schematic showing that they have only one winding.
http://aspdotnetstorefront.hubbell-acmeelectric.com/Images/drawings/T253062.pdf



Also would like to know answer to this question.

Well since Code requires a system that can be grounded such that it limits voltage to not more than 150 volts to ground [250.20(B)(1)], the only way these could be used compliantly is if the 240V source is ungrounded prior to grounding the center tap of one of these autotransformers... that's assuming it is used to derive 120V from 240V.


I didn't read the instructions but wiring it to derive 240V from a 120V source shouldn't be a problem for compliance with the Code sections being discussed.

 

[wwhitney]

Well since Code requires a system that can be grounded such that it limits voltage to not more than 150 volts to ground [250.20(B)(1)]

OK, that's an interesting section. What is the meaning of "can be grounded"? Does it mean that the only compliant way to get a 240V delta is to derive it from a 240Y/138 system with the neutral grounded?

the only way these could be used compliantly is if the 240V source is ungrounded prior to grounding the center tap of one of these autotransformers... that's assuming it is used to derive 120V from 240V.

How is that compliant with 210.9/215.11? I read those as requiring both sides of the autotransformer to have a grounded conductor, which are to be electrically connected. So one of the 240V conductors would have to be grounded.

Cheers, Wayne

 

[Smart $]

OK, that's an interesting section. What is the meaning of "can be grounded"? Does it mean that the only compliant way to get a 240V delta is to derive it from a 240Y/138 system with the neutral grounded?

No. A 240/120 delta high leg is grounded and limits two legs to less than 150 volts. By the nature of it's physical configuration it is not possible to ground this sytem and limit the third leg to not more than 150 volts to ground.

How is that compliant with 210.9/215.11? I read those as requiring both sides of the autotransformer to have a grounded conductor, which are to be electrically connected. So one of the 240V conductors would have to be grounded.

Because the supplied grounded conductor is the grounded conductor of the system concurrently. In fact, it is the only grounded conductor. The center tap of the autotransformer winding is grounded such that it limits the other connected conductors to not more than 150 volts to ground, as there is no grounded conductor on the primary side.