Bonding Jumpers Explained
A bonding jumper is a conductor that connects two metal parts of an electrical system together to ensure electrical continuity, most importantly to give fault current a low-impedance path back to its source so a breaker or fuse can clear the fault quickly. NEC Article 250 recognizes several distinct types of bonding jumpers, main, system, supply-side, and equipment, and while they're often confused with each other, each serves a specific role in a different part of the system. A secure connection depends on a listed ground clamp or wire connector rated for the application. This guide explains what each type of bonding jumper does, where it's installed, and how it's sized.
Why Bonding Matters
Bonding creates electrical continuity between metal parts that could otherwise become energized during a fault, ensuring any stray current has a clear, low-resistance path back to the source rather than staying trapped in a metal enclosure, conduit, or piece of equipment where it could shock someone who touches it. Without proper bonding, a fault inside a metal enclosure might not generate enough current to trip the breaker, leaving the enclosure energized indefinitely instead of clearing the fault immediately. This is the whole point of the equipment grounding and bonding system: turn every fault into a short circuit big enough to trip protection fast.
The Four Types of Bonding Jumpers
|
Type |
What It Connects |
Where It's Found |
|---|---|---|
|
Main bonding jumper (MBJ) |
The grounded (neutral) conductor to the equipment grounding conductor system |
Inside the service equipment enclosure, at the service disconnect |
|
System bonding jumper (SBJ) |
The grounded conductor to the equipment grounding conductor for a separately derived system |
At a generator, transformer, or other separately derived source |
|
Supply-side bonding jumper (SSBJ) |
Metal raceways, enclosures, and equipment on the supply (line) side of a service or system disconnect |
Between the utility meter and the service disconnect, on conduit with no upstream overcurrent device |
|
Equipment bonding jumper (EBJ) |
Normally non-current-carrying metal parts (enclosures, raceways) to the equipment grounding conductor, on the load side |
Anywhere equipment grounding continuity needs restoring, such as around a meter or flexible connection |
Main Bonding Jumper vs. System Bonding Jumper
These two are easy to confuse because they perform the identical function, connecting the grounded conductor to the equipment grounding system, just at different points. The main bonding jumper does this at the service equipment, the point where utility power enters the building. The system bonding jumper does the same job for a separately derived system, most commonly a standby generator or a transformer that creates its own new source of power downstream of the service. Both are covered by the same NEC section, 250.28, and both are typically required to be a single, unspliced connection.
Why Bonding Only Happens at One Point
A grounded conductor (neutral) and the equipment grounding conductor are only supposed to be bonded together at one point in the system, either at the main bonding jumper for the service or at the system bonding jumper for a separately derived system, never at both a main panel and a downstream subpanel simultaneously. Bonding them again downstream creates a parallel path for normal neutral current to flow through the equipment grounding system and any bonded metal parts, which is both a code violation and a genuine shock hazard, since metal enclosures and equipment could carry current under completely normal operating conditions rather than only during a fault.
Supply-Side Bonding Jumper
The supply-side bonding jumper (SSBJ) covers a specific gap: conductors and raceways on the line side of a service disconnect, such as the conduit running from a utility meter enclosure to the main panel, have no overcurrent device protecting them yet, so an equipment bonding jumper (which is sized based on a downstream overcurrent device) doesn't apply. NEC 250.102(C) governs supply-side bonding jumper sizing instead, which is based on the size of the largest ungrounded (phase) conductor in that supply-side raceway, following the same size relationship used in Table 250.66 for grounding electrode conductors.
Equipment Bonding Jumper
An equipment bonding jumper restores or maintains equipment grounding continuity on the load side of an overcurrent device, most commonly where a raceway connection alone can't be relied on for continuity, such as around a meter socket, a flexible metal conduit connection, or an expansion fitting. NEC 250.102(E) limits an external equipment bonding jumper installed outside a raceway to 6 feet in length and requires it to be routed with the raceway or enclosure it's bonding, keeping the added impedance from an external jumper low. Equipment bonding jumpers are sized per NEC Table 250.122, based on the rating of the circuit's overcurrent device rather than the supply-side conductor size.
How Bonding Jumpers Are Sized
- Main and system bonding jumpers: sized per Table 250.102(C)(1), based on the size of the largest ungrounded service conductor; for services with especially large conductors, NEC 250.28(D) requires the jumper to be at least 12.5% of the largest phase conductor's cross-sectional area.
- Supply-side bonding jumpers: sized per NEC 250.102(C), following the same conductor-size relationship as grounding electrode conductors.
- Equipment bonding jumpers: sized per NEC Table 250.122, based on the rating of the overcurrent device protecting the circuit, not the conductor size directly.
Material matters across all types: NEC 250.28(A) permits copper, aluminum, or copper-clad aluminum for main and system bonding jumpers, and a jumper connecting dissimilar materials should generally be sized assuming the same material as the conductor it's bonding to, to keep the sizing calculation consistent.
Common Mistakes
- Bonding neutral and ground at a subpanel: this duplicates the main bonding jumper's function downstream and creates a parallel neutral current path, a common and serious violation.
- Confusing equipment bonding jumper sizing with supply-side sizing: these use different tables (250.122 vs. 250.102(C)) because they apply to different parts of the system with different fault-current considerations.
- Splicing a main or system bonding jumper: these are generally required to be a single, unspliced conductor, and improvising a spliced connection doesn't meet that requirement.
- Exceeding the 6-foot limit on an external equipment bonding jumper: a jumper routed outside a raceway that runs longer than 6 feet, or that isn't routed with the raceway it bonds, doesn't meet NEC 250.102(E).
The Bottom Line
Bonding jumpers all serve the same underlying purpose, ensuring fault current has a clear path back to its source, but the four recognized types apply at different points in the system and are sized differently. Main and system bonding jumpers connect the grounded conductor to the equipment grounding system at the service or a separately derived source, sized per Table 250.102(C)(1). Supply-side bonding jumpers cover the ungrounded conductors and raceways ahead of any overcurrent device, sized per 250.102(C). Equipment bonding jumpers restore grounding continuity on the load side, sized per Table 250.122 based on the circuit's overcurrent device. Keeping these distinct in both application and sizing is what keeps a grounding and bonding system code-compliant and actually effective during a fault.