The National Electrical Code (NEC) addresses this in several sections. Here's the breakdown of where the Main Bonding Jumper (MBJ) should be installed when you have multiple service disconnects in a common wireway

Key Principles from the NEC:
NEC 250.28 (Main Bonding Jumper and System Bonding Jumper)

This section dictates the requirements for the MBJ.

NEC 250.24(C) (Grounding at Services)

This section, in particular, explains the need for a MBJ, which can be either wire or bus.

NEC 230.71 and 230.72 (Maximum Number of Disconnects and Grouping of Disconnects)

These articles permit up to six service disconnects per service and detail how those disconnects must be grouped.

NEC 250.24(C)(1) (Location of Main Bonding Jumper)

The main bonding jumper shall be connected at the service equipment.

Where to Install the MBJ

The MBJ must be installed within each individual service disconnect enclosure. Here's why:1.

Bonding at Each Disconnect

Each service disconnect needs to be bonded individually to the grounded conductor at that point. The MBJ is the connection that makes that happen.2.

Fault Current Path

In the event of a ground fault, the MBJ provides the low-impedance path for fault current to return to the source, enabling the overcurrent protective device (circuit breaker or fuse) to trip and clear the fault. If the MBJ were only in the wireway, fault current would have to travel through the wireway itself to get back to the grounded conductor, which is not a desirable or safe path.

The Wireway

The wireway itself must also be bonded. That being said the wireway is not a substitute for an MBJ. The wireway should be bonded by standard bonding methods per NEC 250.92 and NEC 250.96 (for metallic and nonmetallic wireways, respectively).

In Summary



MBJ: Install a main bonding jumper inside each individual service disconnect enclosure. Wireway: Bond the wireway itself to the grounding system, separately from the MBJs in each disconnect.Example Scenario

Imagine you have three service disconnects grouped together in a wireway. You would:1. Install a MBJ inside the first disconnect, connecting the grounded conductor (neutral) to the enclosure and grounding electrode conductor.
2. Install a MBJ inside the second disconnect, connecting the grounded conductor (neutral) to the enclosure and grounding electrode conductor.3. Install a MBJ inside the third disconnect, connecting the grounded conductor (neutral) to the enclosure and grounding electrode conductor.
4. Bond the metal wireway itself to the grounding system using a bonding jumper (or other approved method) connected to a grounding electrode conductor or the grounding bus in one of the disconnects (or a separate grounding electrode).

Important Considerations



Local Codes

Always check with your local electrical inspector and adopt the most stringent interpretation of the code. Local amendments may exist.

Manufacturer's Instructions

Follow the manufacturer's instructions for the wireway and disconnects regarding bonding and grounding.

Qualified Electrician

This work should only be performed by a qualified and licensed electrician who is familiar with the NEC and local codes.
Disclaimer: I am an AI Chatbot and not a qualified electrician. This information is for general guidance only and should not be considered professional electrical advice. Always consult with a licensed electrician for any electrical work. Flag for review