Installing a Sub Panel for a Detached Building

Do I need to run a ground wire for a 200 amp sub panel that is 200 feet away from the main panel? Grounding a Remote Sub Panel at a Detached Building. © By: Dave Rongey

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Grounding a Sub Panel at a Detached Building
Electrical Project #1: Do I need to run a ground wire for a 200 amp sub panel that is 200 feet away from the main panel?

• My question is about Grounding a Sub-Panel: I am installing a 200 amp sub panel to an existing structure that is 200 plus feet away from the main panel.
• The structure has a ufer ground rod installed, (20 foot rebar tied to the main slab / footing rebar) and I can install secondary grounded rod as well outside.
• Question: can the structure stand alone with its own grounding system? or do I need to run a ground wire back to the main panel.
• I am Using 2/0 copper wire in PVC conduit buried to get from the main panel to the structure.

This electrical wiring question came from Dennis, in Placereville, California.

Electrical Project #2:

What is the proper way to run an electrical supply to a sub panel in a barn or outbuilding?

This electrical wiring question came from Chris, a Handyman in Johns Creek, Georgia.

Dave’s Reply:
Thank you for your electrical wiring questions.

Grounding a Remote Sub Panel at a Detached Building

Application: Installing a Sub Panel for a Separate Building.
Skill Level: Advanced – Licensed Electrical Contractor – Not Recommended for Homeowners.
Tools Required: Electricians pouch of hand tools and the various power tools necessary for the specific method of electrical service installation.
Estimated Time: Depends on the type of panel, the location, and available access to the project area.
Precaution: This is an extensive project. Working in an electric panel has possible arc flash hazards and should be performed by a Licensed Electrical Contractor.
Notice: Installing an Electrical Service Panel must be done according to local and national electrical codes with a permit and be inspected.

Grounding a Sub Panel that is 200 Feet from the Main Panel

Electrical codes and information that should be considered when installing an electrical sub panel at a remote building.

Electrical Code Terminology

The following terms are essential for this topic.
EGC – Equipment Grounding Conductor
This is the wire or conductor that serves as the connection to the ground.

Detached Building
Specific grounding and bonding rules apply to separate buildings (a detached building) or structures supplied by feeders or branch circuits.

NEC
National Electrical Code – The electrical codes that are generally adopted and applied to electrical wiring projects in the U.S.A.
Part III of the National Electrical Code (NEC) Article 250
This information applies to the requirements that are located in Part III of the National Electrical Code (NEC) Article 250, specifically article 250.32.

Conditions that affect Grounding a Sub Panel
The following are conditions that must be taken into consideration:

• CONDITION:
  A Grounding Electrode Does Exist at the Building or Structure.
  If any of the grounding electrodes in 250.52(A) exist at the building or structure served, they must be bonded together to form a grounding electrode system. This includes water pipe electrodes, in-ground metal support structures that qualify as electrodes, ­concrete encased electrodes, or any other grounding electrode system that qualify as an electrode.
• CONDITION:
  A Grounding Electrode Does Not Exist at the Building or Structure.
  If no grounding electrode is present, one must be installed.
  The exception to the grounding electrode requirement in 250.32(A) applies to a building or structure that is supplied by a single branch circuit, either an individual or multiwire, that includes an equipment grounding conductor.

SPECIFIC CONSIDERATIONS

• In accordance with NEC Article 250.32(B)(1), a feeder or branch circuit supplying a separate building or structure generally must include an equipment grounding conductor (EGC).
  The EGC can be a wire or any wiring method in provided in Section 250.118 that qualifies as an EGC.
  If the EGC is a wire type, it must be sized in accordance with 250.122.
• Exception No. 1 allows the grounded conductor (usually the neutral) of feeders or branch circuits to be used for grounding at separate buildings or structures under specific conditions.
• First, an EGC is not included with the circuit supplying the separate building or structure.
• Second, there are no continuous metallic paths between the feeder source and the destination at the building or structure served.

IMPORTANT:
Specific grounding and bonding rules apply to separate buildings or structures supplied by a feeder(s) or branch circuit(s).
A grounding electrode is generally required at such buildings or structures, unless the building or structure is supplied by a single branch circuit and meets the requirements in the exception to 250.32(A).

CONDITIONAL SUMMARY:

• The Grounding System for a Separate Structure
  • A ground wire is not required in the conduit that is feeding power to the panel at the separate structure.
  • A GES, Grounding Electrode System must be provided at the separate structure for the sub panel.
  • Grounding electrode conductors for the remote panel must be sized per NEC Table 250.66
  • Because a ground system is installed at the detached building the ground and neutral must be bonded together at the panel.
  • NEC Article 225.31 – A main breaker must be installed if there are more than six circuit breakers in the sub panel.

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