Ungrounded Transformer Secondary

[zeit]

I have a 3750 KVA Scott T connected transformer. The secondary is ungrounded. As I see it, the NEC requires a grounding electrode conductor sized in accordance with table 250.66.

This leads to my first question.

The derived phase conductors are (4) 1000 KCM per phase. Does this mean a single 3/0 grounding electrode conductor is adequate or do I need (4) 2/0 conductors. In other words does Table 250.66 "max" out at an equivalent conductor area of 1100 KCM.

My second question has to do with the equipment grounding conductor. The load end of the system is a large metal container. What section of the code governs the size of the equipment grounding conductor to this container? If I size the equipment grounding conductor with Table 250.122 I end up with a 500 KCM conductor. This is considerably larger than what I currently have. The system is ungrounded so a single fault wouldn't result in large amount of current, but I am uncertain whether or not the installation is "per code".

Please advise

 

[charlie b]

Re: Ungrounded Transformer Secondary

Originally posted by zeit:Does this mean a single 3/0 grounding electrode conductor is adequate or do I need (4) 2/0 conductors. In other words does Table 250.66 "max" out at an equivalent conductor area of 1100 KCM.

Yes, it maxes out at 1100. You have an "equivalent area for parallel conductors" of 4000. So all you need is a single 3/0

Originally posted by zeit: My second question has to do with the equipment grounding conductor.

250.4(B) talks about ungrounded systems. Even though you don't have a grounded system, the non-current carrying parts have to be connected to planet Earth, and to the supply system's grounded equipment. But this section does not tell you to size any such connections per 250.66 or per 250.122. So I don't know what to tell you here.