Bonding a Transformer

[jonny1982]

This might be a dumb question, but in a 112.5 KVA 3 phase Transformer, is this the proper way to wire the neutral (X0) and grounding:

1. Neutral from primary landed on X0
2. Neutral from secondary landed on X0
3. Ground lugs with ground from primary, ground from secondary, building steel ground, and bond bushing grounds
4. Proper sized bonding jumper from Ground lugs to XO terminal

Thanks

 

[iwire]

This might be a dumb question, but in a 112.5 KVA 3 phase Transformer, is this the proper way to wire the neutral (X0) and grounding:

1. Neutral from primary landed on X0
2. Neutral from secondary landed on X0
3. Ground lugs with ground from primary, ground from secondary, building steel ground, and bond bushing grounds
4. Proper sized bonding jumper from Ground lugs to XO terminal

Thanks

1) No, absolutely not. Leave the neutral from the supply back at the panel. It is not needed and would be a violation to hook up as you are suggesting. You would be 're-grounding' the primary source neutral if you did this.

2) Yes

3) No. The GEC (building steel) must land on XO. The NEC now requires a terminal bar for all the equipment grounding conductors and bonding jumper connected to the enclosure.

4) Yes, from XO to the terminal bar with the EGCs.

 

[jonny1982]

So your saying the XO should have one neutral (secondary only), one ground running directly to building steel, and one ground going to ground bus with all other grounds ( 3 wires total)? Wouldn't it be the same thing for building steel to go to terminal and one ground from terminal to XO?

Thanks

 

[iwire]

So your saying the XO should have one neutral (secondary only), one ground running directly to building steel, and one ground going to ground bus with all other grounds ( 3 wires total)?

Yes assuming that there is only one secondary neutral.

Wouldn't it be the same thing for building steel to go to terminal and one ground from terminal to XO?

Actually, when I looked into this just now I see an exception to the general rule that allows what you suggest. So either way is OK.

 

[jonny1982]

Yes assuming that there is only one secondary neutral.



Actually, when I looked into this just now I see an exception to the general rule that allows what you suggest. So either way is OK.

Thank-you

 

[Smart $]

This might be a dumb question, but in a 112.5 KVA 3 phase Transformer, is this the proper way to wire the neutral (X0) and grounding:

1. Neutral from primary landed on X0
2. Neutral from secondary landed on X0
3. Ground lugs with ground from primary, ground from secondary, building steel ground, and bond bushing grounds
4. Proper sized bonding jumper from Ground lugs to XO terminal

Thanks

1) No, absolutely not. Leave the neutral from the supply back at the panel. It is not needed and would be a violation to hook up as you are suggesting. You would be 're-grounding' the primary source neutral if you did this.

2) Yes

3) No. The GEC (building steel) must land on XO. The NEC now requires a terminal bar for all the equipment grounding conductors and bonding jumper connected to the enclosure.

4) Yes, from XO to the terminal bar with the EGCs.

FWIW...

There is no NEC prohibition from connecting the primary source neutral to a secondary X0. This would make it a non-SDS secondary system. However, to do it compliantly, there'd be no bonding jumper.

If non-SDS, a GEC to building steel would be a violation. However, it could easily be considered a bonding jumper and be completely compliant.

If SDS, there is an exception for landing a GEC on the grounding bus, as already noted.

That said, wiring a transformer non-SDS is very uncommon if not extremely rare other than service transformers. With distribution and service, neutral conductors serve as both grounded and grounding, so it ends up non-SDS under any compliant wiring method.

 

[iwire]

FWIW...

There is no NEC prohibition from connecting the primary source neutral to a secondary X0. This would make it a non-SDS secondary system. However, to do it compliantly, there'd be no bonding jumper.

Other than it is pretty much never done by electricians working under the NEC.

 

[Smart $]

Other than it is pretty much never done by electricians working under the NEC.

..and my last paragraph pretty much said so.