Mike Dolt Enterprises
Mike Dolt Enterprises
User: ElecEngbw, Location: Tennessee
Asked on: Tuesday, April 26, 2011 at 2:21 pm EDT
Answered on: Tuesday, May 14, 2024 at 4:39 am EDT
Question: I am working on a project where there is a sub-building fed electrically from a main building. The sub-building has existing electrical panels located inside currently feeding the general receptacles, lights, etc. I am looking into installing a new panel exterior to the building to power HVAC equipment that will be mounted to the exterior wall of the sub-building. This new panel feed will come from the main building electrical room. I am confused about how to properly ground the new panel according to NEC 250. I am running an equipment grounding conductor with the feeder conductors to the new panel. Is it better to tie the new panel ground bus to the existing sub-building panel ground bus, to a new ground rod at the exterior panel, or simply to the building steel? Or a combination of the three?Thanks.
NEC Requirements
NEC Article 250.146(D) requires the grounding method to be selected based on the system voltage, building construction, and the presence of a metal water pipe or structural steel frame.
Options and Recommendations
1. Tie to Existing Sub-Building Panel Ground Bus
Pros
Provides a single point of grounding for the sub-building.
Cons
May not provide adequate grounding if the existing panel is not properly grounded or if there are any connection issues between the panels.
2. New Ground Rod at the Exterior Panel
Pros
Creates a dedicated grounding point for the exterior panel.
Cons
Requires additional excavation and installation of a new ground rod.
3. Building Steel
Pros
Utilizes the existing building structure for grounding, which may be more reliable than other methods.
Cons
May not be available in all cases, especially if the building is wood-framed.
Combination of Methods
Recommended Approach
Tie the new panel ground bus to the existing sub-building panel ground bus. Install a new ground rod at the exterior panel and bond it to the ground bus of the new panel.
This approach provides redundancy and ensures proper grounding of the exterior panel.
Additional Considerations: The equipment grounding conductor (EGC) from the main building must be sized according to the NEC based on the feeder ampacity. All grounding connections must be made using approved methods (e.g., compression connectors, clamps, bolts).* Ensure that the existing sub-building panel is properly grounded and that there are no loose or damaged connections. Flag for review