Power transformer case bonding

[Isaiah]

We have several 3MVA power XFMRS on site 4160-480V , 3 ph 3 wire solidly grounded- with diagonally opposed NEMA ground pads at the base for case bonding. The inspector said it’s not required to bond the case per NEC but it was common practice to do so. Comments?


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[Dale001289]

We have several 3MVA power XFMRS on site 4160-480V , 3 ph 3 wire solidly grounded- with diagonally opposed NEMA ground pads at the base for case bonding. The inspector said it’s not required to bond the case per NEC but it was common practice to do so. Comments?


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I don’t believe the case bonding is required by NEC as long as you ground and bond per 250.30


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[ron]

If the XFMRs are solidly grounded, then there is likely a bond between the X0 and the transformer case / core in some fashion. The grounding electrode conductor also connects to the case. The ground pads may be supplementary for additional grounding and not required if the case is bonded in some other fashion.

 

[david]

We have several 3MVA power XFMRS on site 4160-480V , 3 ph 3 wire solidly grounded- with diagonally opposed NEMA ground pads at the base for case bonding. The inspector said it’s not required to bond the case per NEC but it was common practice to do so. Comments?

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250.30 (A)(2) supply side bonding jumper
From the source enclosure to the first disconnect enclosure.
If the bonding jumper is a wire type it would connect to that pad
the
The system bonding jumper will eather be at the source or the first disconnect.
I don't know what was ment by its ok to do so.

It's required eather at the source or first disconnect but not at both locations

Exterior transformers you could see it at both locations if earth is the ony other connection between them

 

[jaggedben]

The case is required to be bonded and grounded. It may come bonded from the factory.

I gather that the 'not required but common practice' part refers specifically to the diagonally opposite grounding pads. See:

Transformer Ground Pads Question

Where did grounding opposite ends of a substation transformer tank originate? Is it just a common utility requirement to ground opposite ends to a grounding electrode? The NEC requires a low-impedance path to clear a ground fault, but that can be achieved with the ground pad on the tank below...

forums.mikeholt.com

 

[Isaiah]

The case is required to be bonded and grounded. It may come bonded from the factory.

I gather that the 'not required but common practice' part refers specifically to the diagonally opposite grounding pads. See:

Transformer Ground Pads Question

Where did grounding opposite ends of a substation transformer tank originate? Is it just a common utility requirement to ground opposite ends to a grounding electrode? The NEC requires a low-impedance path to clear a ground fault, but that can be achieved with the ground pad on the tank below...

forums.mikeholt.com

Yea sir - that’s a good thread
Thanks


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[electrofelon]

And let's not forget the catch all 250.4(A) 2-5.

 

[bobby ocampo]

We have several 3MVA power XFMRS on site 4160-480V , 3 ph 3 wire solidly grounded- with diagonally opposed NEMA ground pads at tWhe base for case bonding. The inspector said it’s not required to bond the case per NEC but it was common practice to do so. Comments?


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[bobby ocampo]

What do you mean by diagonally opposed nema ground pads at the base for case bonding?