• We will be performing upgrades on the forums and server over the weekend. The forums may be unavailable multiple times for up to an hour each. Thank you for your patience and understanding as we work to make the forums even better.

Separately Derived System versus Service

Merry Christmas

KthOtto

Member
Location
Spokane WA
Occupation
Electrician
I work on a hospital campus with a single "Service Point" which I assume to be the load side of the MV meter to the 13.2MV main utility breaker in our hospital owned primary switchgear. From there we use 13.2KV loops around the campus. 13.2KV/208/120Y stepdown transformers sit outside facilities. As designed 10-15 years ago, we treat these transformers like they are utility owned and do not pull a ground into the facilities. We establish a ground at the main panelboard, bond the neutral, etc. We have new projects in the works and the A/E wants a ground pulled for the transformers to the building main. I assume he's treating these transformers like Separately Derived Systems downstream from the "Service". If we do this, I'm thinking I will need to pull grounds into any other facilities powered from the same transformer and float the neutral so I don't have two paths for fault current. A bit cloudy on this as is the Electrical Contractor.
 

Elect117

Senior Member
Location
California
Occupation
Engineer E.E. P.E.
I think the code section you are looking for is 250.30(A)(1) Exception 2. That is the section for installing a transformer and service similar to that of a utility, where you bond the neutral to ground at both locations and only run a neutral from the transformer to the switchgear inside.
 

KthOtto

Member
Location
Spokane WA
Occupation
Electrician
Thanks for the reply. So...if I am reading this correctly, they are giving permission to treat this similar to a standard building service supplied by a utility....that is not run a ground from the xfmr. I would interpret this to mean that if I ground/bond at both locations as the Exception allows then I would not be allowed to run a ground wire (parallel path), but if I did run a ground wire I would not be allowed to bond the ground/neutral at the building first means of disconnect.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
Thanks for the reply. So...if I am reading this correctly, they are giving permission to treat this similar to a standard building service supplied by a utility....that is not run a ground from the xfmr. I would interpret this to mean that if I ground/bond at both locations as the Exception allows then I would not be allowed to run a ground wire (parallel path), but if I did run a ground wire I would not be allowed to bond the ground/neutral at the building first means of disconnect.
Somewhat difficult to answer using the word "ground"
This is one of those areas where terminology is critical.
"Ground" won't suffice as "Ground" in the Code is "The Earth" ie: dirt.
The Section referenced allows you to omit the SSBJ from an SDS installation if all the perimeters are in place.
You would still need an appropriately size GroundED conductor.
 
Top