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Refresh my memory... bonding... there's a transformer involved...

Posted by pharkus (My Page) on
Sat, Sep 21, 13 at 21:17

Hi all,

I'm pretty sure I'm staring at something that is wrong, but I don't really want to question the work of the guy before me and, I must admit, I'm quite a bit rusty in this area.

I'm in a commercial environment, a large multi-tenant office complex. We have 277/480 to our closet, through a disconnect, to a big transformer, out as 120/208 to two 200A panels, one of which contains a 100A breaker to an isolation transformer feeding a third panel.

Building maintenance, the electrician our landlord usually calls, and the town officials who came to do our recent inspection, are allowing me to do some limited things -- basically nothing more than I'd normally do in a residential environment -- and I'm not going outside of their allowances.


I'm curious about what I'm seeing in the third panel, on the secondary side of that isolation transformer... a bonding jumper is installed... well, no, not exactly... there's a bar on each side of the panel, one for neutral and one for ground... the "jumper" that bolts between these two bars is lying on the bottom of the panel, not attached... there is, seperately, a little doohickey that goes into one of the screw terminal holes on one end and has, essentially, a spade connector built into the other, which goes under a screw into the metal enclosure of the panel, thereby "grounding" it...

The puzzler? This "doohickey" is installed into the neutral bar.

The feeder to the panel is 4-wire.


MY feeling is that this is an error, and the doohickey should be tying the enclosure to the 'GROUND' bar... In fact, I'm nearly 100% certain this is the case... but I do remember reading that there were a number of permissible scenarios/arrangements for grounding/bonding when transformers get involved...


I'm right, right? This is an error?


Follow-Up Postings:

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RE: Refresh my memory... bonding... there's a transformer involve

Not enough information.

Depending on just what the intent of the "isolation" transformer and how it's windings are wired, would indicate which (if any) termianls should become the grounded one on the far side of the transformer.


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