Mike Dolt Enterprises
Mike Dolt Enterprises
User: 10Driver, Location: California
Asked on: Sunday, January 19, 2025 at 3:58 am EST
Answered on: Thursday, March 6, 2025 at 7:30 am EST
Question: OK...today's dilemma. Have a single-family dwelling with a 400A MCP with a single GEC installed. Also have a hangar with a MLO panel with a GEC installed there (being fed by a 100A CB in an MLO in the garage which in turn is being fed by a 100A CB on the MCP). Understand that one and maybe two GEC's may be required per 250.56 but the question is, should both GEC's be connected through the MLO's (as they currently are) and what wire size should be used (assuming it needs to be one continous run from the GEC in the hangar to the GEC located at the MCP)?
Okay, let's break down this grounding scenario and address your questions based on the NEC (National Electrical Code).
Understanding the Requirements
Multiple Buildings/Structures
You have a single-family dwelling and a separate hangar. The hangar is fed from the dwelling, making it a separately derived system at the hangar location.
250.32 (Buildings or Structures Supplied by a Feeder or Branch Circuit)
This is the key section of the NEC governing grounding at the hangar.
250.32(A) Grounding Electrode System
A grounding electrode system is required at the detached building or structure (hangar).
250.32(B) Grounded Conductor
The grounded conductor (neutral) is NOT permitted to be used for grounding at the detached building (hangar).
250.56 (Supplemental Electrode)
The code section being referenced may be for specific types of grounding electrode systems which are NOT related to the installation being considered.
Analysis and Recommendations
Current Configuration Assessment
You have a 400A MCP at the house with a GEC. This is good. You have a 100A breaker in the MCP feeding an MLO panel in the garage. You have a 100A breaker in the garage MLO feeding an MLO panel in the hangar with a GEC. This configuration is partially compliant, but needs some changes to ensure it meets the code requirements.2.
GEC Connection Point
Hangar: Because of the nature of being a detached building, the GEC should be connected directly at the hangar to it's own grounding electrode system (Ground rod or other appropriate electrode per 250.52). Dwelling: The GEC at the dwelling should be connected at the service equipment (MCP) as it is currently.3.
GEC Sizing
The GEC size for the hangar should be based on Table 250.66 (if connected to a ground rod) based on the size of the feeder conductors supplying the hangar. Since you have a 100A feeder the GEC will most likely be a #8 CU or #6 AL, but verify using the table. The GEC size for the dwelling's MCP should be based on Table 250.66 based on the size of the service entrance conductors (400A service). You'll need to know the conductor size to determine the proper GEC size.
Corrected/Improved Configuration (Recommended)
Existing Configuration
Keep the existing configuration but add the following: Establish a proper grounding electrode system at the hangar (ground rod(s) as per 250.52). Connect the equipment grounding conductors to the ground busbar.
Important Considerations
Local Codes
Always check with your local electrical inspector or building department. They may have additional requirements or interpretations of the NEC that apply to your specific location.*
Safety
Working with electrical systems can be dangerous. If you are not a qualified electrician, consult with a professional.
In summary, to comply with the NEC, the best approach is to create an independent grounding electrode system at the hangar. This provides a low-impedance path to ground for fault currents at both the main service and the detached structure, enhancing safety. Flag for review