Step-up transformer bonding

BackCountry

Electrician
Location
Southern California
Occupation
Licensed Electrician and General Contractor
I’m getting lost a little bit on separately derived system bonding and grounding, hoping for some help.

Scenario: single phase 120/240 service @ 400A, two 200A disconnects, all located outside on a panel board at the entrance to a ranch. One disconnect is feeding a 120/240x480v step up 50kva transformer, with a 800 foot 4/0AL feeder (two conductors plus ground), into another 120/240x480v step down transformer which feeds an automatic transfer switch with batteries/solar as the standby energy source.

No neutral needed between the transformers obviously; however, what I’m confused on is whether I bond the neutral and ground together at the transfer switch (it’s service rated) or only bond at the 400A panel and meter like I normally would, and treat everything else downstream as a sub panel along with a ground rod at each separate building.

If the neutral forming transformer in the automatic transfer switch wasn’t involved, I wouldn’t feel so confused I suppose, thanks for any help you may have.
 

hillbilly1

Senior Member
Location
Atlanta,Ga
Occupation
Field coordinator/ technical support
If the secondary is 240/480, I always bond the middle tap when using it for 480. You could bond one leg of the 480, but then you have 480 to ground one one leg. I like 240 to ground from either leg much better. This allows the ocp to work if there is a fault on the 480 volt line.
 
One disconnect is feeding a 120/240x480v step up 50kva transformer, with a 800 foot 4/0AL feeder (two conductors plus ground).

If the secondary is 240/480, I always bond the middle tap when using it for 480. You could bond one leg of the 480, but then you have 480 to ground one one leg. I like 240 to ground from either leg much better. This allows the ocp to work if there is a fault on the 480 volt line.
A better way to do it is use 250.30(A)(1) exception 2 and skip the EGC. You will use less wire and have a better fault clearing path.
 
Top