Bonding neutral to ground at a commercial facility with many detached buildings

Status
Not open for further replies.

Jon456

Senior Member
Location
Colorado
I'm trying to ensure I understand the code because I think there are a number of problems at our installation.

We have a large number of detached buildings at our facility. Utility power (480V, 3-phase) originates for us at a large ground-mount transformer that feeds our service entrance panel which is in a stand-alone shed on the property. From there, feeders bring power to our distribution panel in a utility room attached to our main building. From there, power is distributed to several step-down transformers (480V -> 240V) spread about the property. At each transformer shed is a large breaker panel, and many of that panel's branch circuits typically feed a sub-panel for a row of detached buildings. Those "row" sub-panels, in turn, feed a very small sub-panel in each detached building (typically consisting of a 240V powered door circuit, a 120V lighting circuit, a 120V receptacle circuit, and sometimes a 240V receptacle circuit).

There are significant metallic pathways (electrical conduit and/or fire sprinkler pipes) connecting each of the detached buildings in a row. Also, virtually all of these buildings are of full metal construction. The newer metal buildings are bolted to a concrete foundation, although I've not seen any evidence that they were bonded to the rebar for a ufer ground. Older metal buildings are typically set on top of asphalt pavement and staked in place. All electrical circuits are either 4-wire or 5-wire (because of 3-phase). In other words, there is always both a neutral and a separate ground wire between each sub-panel and the upstream source.

So I want to be clear about the following:

1. Where should the neutral be bonded to ground vs where should it be isolated?
2. Where should ground rods be required?
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Simplest answer, as long as there are no existing configurations which were only legal under an older code, is:

1. The neutral should be bonded to the EGC and GES at or near the main service disconnect in the standalone shed. It should not be bonded at the other buildings.
2. Each separate building, even though there is an EGC in the feeder to that building, needs its own Ground Electrode System, which may be as simple as one or two ground rods, or if there is a qualified Concrete Encased Electrode (CEE) then just that one electrode will be sufficient. Certain other types of electrodes, such as water pipes must also be connected if they exist. These electrodes will be bonded to the EGC but not to the neutral.

Under some older code versions there could/should also be a ground neutral bond at each building.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
The neutral is bonded at the secondary of each transformer or at the first OCPD on the secondary of the transformer, but cannot be bonded at both. A supply side bonding jumper is required between the transformer and the first OCPD, no matter where the bonding connection is made. (non-flexible metallic conduit can be used as the supply side bonding jumper)
Grounding electrodes are required for each transformer and at each additional building. The feeder to the additional buildings must include an equipment grounding conductor. The grounding electrode conductor is connected to the EGC at at the additional buildings. The neutral must remain isolated at the existing buildings.
 

Jon456

Senior Member
Location
Colorado
The neutral is bonded at the secondary of each transformer or at the first OCPD on the secondary of the transformer, but cannot be bonded at both.
Just to be clear, you're referring to our four step-down transformers that are downstream of our 480V distribution panel? If so, then that would mean the neutral will be bonded to ground at five places: once at our utility service entrance and once at each of the four step-down transformers distributed about our property.

Is that correct?
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Just to be clear, you're referring to our four step-down transformers that are downstream of our 480V distribution panel? If so, then that would mean the neutral will be bonded to ground at five places: once at our utility service entrance and once at each of the four step-down transformers distributed about our property.

Is that correct?
Sort of. The neutral is not bonded at 5 locations as you don't have "a neutral". You have 5 neutrals, and each of them are bonded at a single location.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
For a Code reference take a look at 250.24(A)(5) which basically tells you the grounded-grounding connection for a service supplied system can only be made at the service.
For systems supplied from Separately Derived Systems there is a grounded-grounding connection as don notes and is reference in the Informational Note at 250.24.
 

Jon456

Senior Member
Location
Colorado
For systems supplied from Separately Derived Systems there is a grounded-grounding connection as don notes and is reference in the Informational Note at 250.24.
So are power step-down transformers always engineered to be separately derived systems? Or does it depend on the particular make/model of the transformer and its application?
 
Last edited:

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
So are service step-down transformers always engineered to be separately derived systems? Or does it depend on the particular make/model of the transformer and its application?
The service is the service and not a SDS.
All transformers, other than auto transformers, are SDS.
 
Status
Not open for further replies.
Top