Bonding and grounding transformer

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NeilG

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Brooklyn NY
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Electrician
Hi

I have a transformer from 120/208 to 277/480 for a single piece of equipment fed by a 3pole 20amp branch circuit
1) do I need to install a disconnect on the load side of the Transformer?
If yes do I need to bond and ground the disconnect as a service switch?
 
NeilG said:
Hi

I have a transformer from 120/208 to 277/480 for a single piece of equipment fed by a 3pole 20amp branch circuit
1) do I need to install a disconnect on the load side of the Transformer?
If yes do I need to bond and ground the disconnect as a service switch?
Click to expand...
You don't exactly need a "disconnect" but what you do need is an OCPD for the transformer secondary conductors (240.21(c)), and maybe an OCPD for your transformer protection (450.3(b)), and if using a panel board, an OCPD to meet 408.36. a single OCPD can do all three.

Yes you bond the system with a system bonding jumper and also connect your gec to that point. This can be in the transformer or at the first disconnecting means. 250.30.
 
Thanks alot for your answer!

Just to clarify.. any time that I install a transformer in a building even for 1 piece of equipment and the rest of the building is on 120/208 I need to run a gec and bond the neutral on the secondary side? And do I have to bond it with the gec of the service switch of the building or it can be separately grounded to the structure?
 
NeilG said:
Thanks alot for your answer!

Just to clarify.. any time that I install a transformer in a building even for 1 piece of equipment and the rest of the building is on 120/208 I need to run a gec and bond the neutral on the secondary side?
Click to expand...

Basically yes, unless its an ungrounded system or the transformer is not considered an SDS, but 99.9% of the time yes.

And do I have to bond it with the gec of the service switch of the building or it can be separately grounded to the structure?
Click to expand...

See 250.30(A)(4), it tells you what to do.

Mike Holt has a good graphic that shows the two options for SDS grounding and bonding. I cant find it at the moment, hopefully someone can post a link to it.
 
NeilG said:
Thanks alot for your answer!

Just to clarify.. any time that I install a transformer in a building even for 1 piece of equipment and the rest of the building is on 120/208 I need to run a gec and bond the neutral on the secondary side? And do I have to bond it with the gec of the service switch of the building or it can be separately grounded to the structure?
Click to expand...
As mentioned you need a GES for this. You also need to carefully consider the type of transformer. Are you using a proper 208 delta to 480Y stepup transformer or simply using a 480 delta to 208 Y in reverse? Using a 480 delta/208Y in reverse presents a number of issues that make it a very poor choice.
 
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