Okay, this is a common and important question. Here's the breakdown of how to properly ground and bond the steel building and the reasoning behind it, according to the NEC (National Electrical Code)

You MUST bond the steel building to the 200 amp subpanel.

Here's the detailed explanation:1. Separately Derived System Considerations

While the 200A subpanel is fed from the 400A service, it is not considered a separately derived system. Separately derived systems (like generators or transformers that establish a new neutral point) have different grounding requirements. Your scenario is a standard subpanel, meaning you are extending the existing service.2.

Bonding is Key



Purpose of Bonding

Bonding connects all metal parts to ensure they are at the same electrical potential. This is critical for safety. In the event of a fault (e.g., a live wire contacting the steel building), the bonding creates a low-impedance path back to the source (the 200A panel's neutral/ground bar). This allows a large fault current to flow, tripping the overcurrent protective device (circuit breaker) quickly and safely.

Where to Bond

Bond the steel structure to the grounding bus (the bus bar where all the grounding conductors terminate) in the

200 amp subpanel located in the steel building.3. Grounding Electrode System (GES)

The steel building likely requires its own Grounding Electrode System (GES). This GES is what connects the building's grounding system to the earth. Common grounding electrodes include:

Concrete-Encased Electrode (Ufer Ground)

If the steel building has a concrete foundation with at least 20 feet of 1/2" rebar (or 20 feet of #4 bare copper conductor) encased in the concrete, this is the preferred electrode.

Ground Rods

If a concrete-encased electrode is not present, you'll typically need ground rods. You must install a minimum of two ground rods spaced at least 6 feet apart.

Other Allowed Electrodes

Other options, such as ground rings or metal underground water pipes (with restrictions), may be acceptable depending on local codes.

Connection to the GES

Connect the steel building's grounding system to the GES. This is typically done by running a grounding electrode conductor from the grounding bus in the 200A subpanel to the chosen grounding electrode (e.g., the ground rods).4.

Grounding Conductor Size

You'll need to consult the NEC (Table 250.66 and 250.122) to determine the proper size of the grounding electrode conductor based on the size of the ungrounded conductors (the "hot" wires) feeding the 200A subpanel. And the equipment grounding conductor (EGC).
5.

Why Not the 400 Amp Service? While everything ultimately ties back to the main service, the direct bonding should be at the subpanel. This provides the closest, lowest-impedance path for fault current within the steel building. Running a separate grounding conductor all the way back to the 400A service would introduce unnecessary length and impedance, reducing the effectiveness of the grounding system in the steel building. Bonding to the 400A service happens indirectly because the 200A subpanel is fed from it, and the grounding conductors from the subpanel will eventually connect back to the main service's grounding system.

In Summary - What to Do


Bond the Steel

Connect a properly sized equipment grounding conductor (EGC) from the steel structure to the grounding bus in the 200A subpanel. Make sure the connection is mechanically sound and electrically conductive.2.

Install Grounding Electrode System

Install a proper grounding electrode system (ideally a concrete-encased electrode if available, otherwise ground rods).3.

Connect to GES

Connect the grounding bus in the 200A subpanel to the grounding electrode system using a properly sized grounding electrode conductor.4.

Inspect Connections

Ensure all connections are tight, corrosion-resistant, and properly made using approved connectors.

Important Considerations



Local Codes

Always check with your local electrical inspector. Local codes can sometimes have variations or additional requirements beyond the NEC. They have the final say.

Qualified Electrician

This type of work requires a qualified and licensed electrician. Improper grounding and bonding can create serious safety hazards, including electric shock and fire.

Inspection

Have the installation inspected by a qualified electrical inspector to ensure it meets all applicable codes and standards.

Documentation

Keep records of the grounding and bonding work performed, including the size and type of conductors used, the location of the grounding electrodes, and the inspection results.

Example Scenario

Let's say the 200A subpanel is fed with #2/0 AWG copper conductors. According to NEC Table 250.66, the minimum size grounding electrode conductor would likely be #6 AWG copper. You would use this size conductor to connect the grounding bus in the 200A subpanel to the ground rods. (Always verify with the tables). Table 250.122 is then used for the EGC.

Disclaimer: I am an AI Chatbot and this information is for general guidance only. It is not a substitute for professional electrical advice. Consult with a qualified and licensed electrician for all electrical work. Flag for review