Bonding the pri. neu. and the sec. neu. together & to earth gr.

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Miketuc81

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Just recently on a job I am working on I noticed a possible mistake in the drawings. They show 4-4/0 & #2gr off of a sw. gear that is 600A 277/480v
fed off of the seconday side of the power co. utulity xfmr. the secondary V is 277/480v 4 wire out of the utility xfmr so their must be a bonded neutral in this xfmr. So back to the 4 4/0 & #2gr out of the gear, this goes into an ATS sw. that gets generator pwr from an existing 277/480v genset and backup pwr sw. gear. The load of the ATS then feeds a 150KVA xfmr with what is drawn as 4-4/0 & #2gr again. Yet the xfmr is a 480v delta to 120/208v Y.
I brought this to my boss's attn. and he said that we need to bond the line side neutral (277/480v-N), the load side neutral (120/208v-N) & then bond them to ground also. This just doesn't seem right at all... would there be any damaging effects as a result of this? I would like to know for my own knowledge so I don't make the same mistake in the future if that was the case.
 
Mike, what you have is a violation of article 250.24(A)5.If the transformer is the only load from the ATS, you could have saved the expense of running the grounded conductor from each of the main gear feeding the ats.

A(5) Load-Side Grounding Connections. A grounded conductor shall not be connected to normally non?current-carrying metal parts of equipment, to equipment grounding conductor(s), or be reconnected to ground on the load side of the service disconnecting means except as otherwise permitted in this article.

Rick
 
I know I'm dense but I'm having a hard time picturing your installation.


Do you want to know if you can bond the neutrals in the generator, the 150 KVA transformer secondary, and the 4 wire commercial 480 together?

Or what you're going to do with the neutrals from the generator and the commercial power, at the 150 KVA transformer.
 
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realolman, my boss and the owner of the company I work for have come up with this idea that because the drawing shows the 4-4/0 & #2gr from the sw. gear to the ATS, then from the ATS to the "only" load off of the ATS, the 150KVA xfmr (which is a 480v 3PH delta primary/ 120/208V Y secondary,
that provides ups pwr to some computer equip inside the building). The emergency power to the ATS comes from an existing genset-ATS-SW. board, that we have installed a 225 A breaker into. The way I see it, having the grounded conductor is useless after the 600A sw. gear that is fed from the utility. Because we only need 3ph 480v to the primary of the 150kva xfmr, em. pwr or normal power. Bonding the "utility 277/480" Neutral again in our 150kva xfmr would be a no no. My boss tried to explain that it is because the generator is an additional service... and we need to bond the neutral for that, but if that was the case that nuetral is already bonded in the existing em. pwr. sw. board... He couldn't really explain the theory behind their decision, and I wish I knew more about the technical side of neutrals and harmonics in xfmrs so I could foresee a possible problem, along with the code violations. I wanted to ask the on-site design team about this, but my boss didn't want to. But I am sure that they would look at it further and decide that the grounded conductor would not be necesary after the 600A gear, and the co. I work for would save alot of $ on wire. Thanks.
 
realolman, I want to know what the electrical efects of bonding all of these together in the 150kva xfmr on the Xo terminal (which I don't see how the 4 hole mech lug is going to fit....) are. IE Harmonics, possible higher volages if bonding fails upstream, and so forth. thanks
 
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