Delta wye grounding

[randydowns]

We have a 45 kva delta primary xfmr with a wye secondary. Input 460v output 120v. The 3 phase buss feeds the primary. Should the ground wire from the buss terminate to the x0 secondary and the transformer case ground? Also the 3 phase 120 wye output goes to a panel board. The ground then goes from case ground, x0, to the ground lug on the panel. Should we earth ground the transformer, panel or neither? The panel obviously has to be bonded to the can. I've had several different opinions on this.

 

[ryan_618]

Re: Delta wye grounding

Have you read 250.30?

 

[dereckbc]

Re: Delta wye grounding

If you are talking about the EGC ran with the primary supply conductors it terminates to the transformer case or ground terminal, not the Xo. The Xo is for a GEC connection to a GE. Although it is bonded to the case via MBJ.

You really need to read and fully understand 250.30. The secondary is required to have a GEC connection either on the transformer Xo, or the neutral buss in the first disconnect device. Do some reading and get back to us with follow up questions. If you have access to the NEC 2002 Handbook look at Exhbit's 250.13 and 250.14.

Do not go by oppinions, the NEC is very clear about how the Xo, GEC, MBJ, and bonding jumpers are configured. I have seen more transformers connected incorrectly than I have seen done properly.

 

[Ed MacLaren]

Re: Delta wye grounding

The transformer secondary circuit can be grounded (connected to an acceptable grounding electrode), and bonded to the enclosure, at the transformer X0 or at the panel, but not both.

Suggested layout -

Ed

 

[dereckbc]

Re: Delta wye grounding

Ed, again great drawing and correct. Could you provide the second method of downstream connection at disconnect similar to Exhibit 250.14?

 

[randydowns]

Re: Delta wye grounding

Thanks a lot. I have been working on machine tools for 25 years, but not from the disconect up. I was laid of from GE in Cin. Oh and now I am working at another factory. There is 1 person who is the "electrical" person and it seems that he may be in error at times. I just ordered the Mike Holt grounding book. I need to get up to speed on the terms you guys use also. GEC and others. As I say I am versed in Fanuc cnc's, GE, Allen Bradley, as well as other brands. I replaced a bad cap on a Okuma dc spindle drive last week and that saved them big bucks. They would have gotten a rebuilt drive complete for about $5000. I repaired it for 60 cents. We had facilities electricians at GE who took care of all the conduit, new installs etc. I just repaired drives, controls etc. They have went from 29000 people down to around 3500 today. They have built plants in China to manufacture the newer jet engines. We have a code book at work, I'll have to do some reading. Please be patient I am learning..

Randy