Grounding bonding in transformer

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ctaylo360860

ctaylo360860

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Existing work has a single phase step down transformer fed from 40amp breaker in a 400amp 3phase 277/480volt main panel. Existing electrician fed primary side of step down transformer with 2 phases, a neutral, and a ground.... everything in the 400amp pane is bonded. the secondary side transformer feeds a 100a panel where all neutrals and grounds are bonded... as I understand this is not correctly done, but I’d love to hear any input on why the existing electrician did this... Transformer should only be fed with 2 phases and a ground on the primary side. There should be a supply side bonding jumper installed inside the transformer sized on 250.102(c) that binds the neutral and the frame of transformer. There should also be an electrical grounding conductor set for the transformer and main panel. Secondary side should feed over to 100amp panel with four wires. 2 phases , neutral, and a ground. Where all neutrals and grounds should be isolated.
 
LarryFine

LarryFine

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Number one: The neutral SHOULD NOT be connected to the primary!!! Two lines ONLY!

The transformer's primary terminal(s) should be left floating. Not grounded or anything.
 
ctaylo360860

ctaylo360860

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colorado
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ME
LarryFine said:
Number one: The neutral SHOULD NOT be connected to the primary!!! Two lines ONLY!

The transformer's primary terminal(s) should be left floating. Not grounded or anything.
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The primary terminals are not grounded. they just are fed from 2 phases on a 40 amp breaker. There strange thing is the neutral on the primary, no supply side bonding jumper in the trans, and then not isolating in the 100a Panel fed from the secondary of transformer.
 
ActionDave

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ctaylo360860 said:
The primary terminals are not grounded. they just are fed from 2 phases on a 40 amp breaker. There strange thing is the neutral on the primary, no supply side bonding jumper in the trans, and then not isolating in the 100a Panel fed from the secondary of transformer.
Click to expand...
You can bond the neutral of the secondary in the first disconnect.
 
ActionDave

ActionDave

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ctaylo360860 said:
So in the transformer you can leave grounds and neutrals Unbiased, and then bond them in the 100a Panel fed from the secondary using the green bonding screw providedbwith the panel?
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Correct. 250.30 (A)(2). If you have any copy of the NEC handbook there are some good illustrations of this
 
ctaylo360860

ctaylo360860

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colorado
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ME
ActionDave said:
Can't tell what's going on in this picture
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This is the 400 amp 3 phase 277/480volt service feeding the primary of the transformer. It just shows the neutrals and grounded we’re bonded... I thought the breaker was in the picture, but didn’t catch it. I’m in the process of isolating as I’m setting a new service disconnect and automatic transfer switch before the existing 400 amp panel you see. It will now be a sub panel and have to be fed with five wires and isolate grounds and neutrals...
 
W

winnie

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Springfield, MA, USA
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Electric motor research
If I understand what the pictures show, the supply neutral conductor was connected to the secondary neutral point.

If this is the case then the system is not code compliant but probably would work.

The secondary neutral is supposed to be bonded to ground (in the transformer or the disconnect), and I don't see that bonding. But the supply neutral _is_ bonded to ground, so secondary fault current does have a path back to its source. Not a code compliant path, but one that probably works.

Jon
 
ctaylo360860

ctaylo360860

Senior Member
Location
colorado
Occupation
ME
winnie said:
If I understand what the pictures show, the supply neutral conductor was connected to the secondary neutral point.

If this is the case then the system is not code compliant but probably would work.

The secondary neutral is supposed to be bonded to ground (in the transformer or the disconnect), and I don't see that bonding. But the supply neutral _is_ bonded to ground, so secondary fault current does have a path back to its source. Not a code compliant path, but one that probably works.

Jon
Click to expand...
Yes you are correct. they ran 4 wires from main panel to the primary side of transformer... I just removed the wire and added it to my scrap pile... I added a supply side bonding jumper in the transformer and then isolated everything in the 100a panel that’s being fed on the secondary side of transformer
 
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