An "amp bonded" connection refers to an equipment bonding jumper (EBJ) or main bonding jumper whose wire gauge is strictly sized according to the ampere rating of the circuit's overcurrent protective device (breaker or fuse) to ensure safe fault-current clearing. In a real installation, this sizing dictates the physical cross-sectional area of the safety bond path, ensuring the wire can carry massive short-circuit currents for the milliseconds it takes the breaker to trip without melting. People commonly confuse this concept with the grounding electrode conductor (GEC), which is sized by the physical area of the service entrance wires rather than the breaker's amp rating.
The Core Principle: Why Breaker Amps Dictate Bond Wire Size
When a live, ungrounded conductor faults against a metal equipment enclosure, it creates a dead short. The overcurrent protective device (OCPD)—your circuit breaker or fuse—must detect this massive current spike and open the circuit almost instantaneously via its magnetic trip mechanism. However, the breaker needs a fraction of a second to clear the fault. During that brief window, the fault current must have a continuous, low-impedance path back to the source to trip the breaker. This is where the amp bonded equipment bonding jumper comes into play.
If the bonding jumper is undersized, the extreme thermal and magnetic stress of the fault current will cause the wire to vaporize or melt before the breaker can trip. This leaves the metal enclosure energized at line voltage, creating a lethal shock hazard for anyone who touches it. To prevent this, the National Electrical Code (NEC) mandates that bonding jumpers be sized based on the ampere rating of the OCPD protecting the circuit, as detailed in NEC Table 250.122.
The sizing table scales non-linearly because larger breakers allow proportionally higher let-through fault currents before their magnetic trips engage. A 20A breaker might let through 2,500 amps for two AC cycles, while a 400A breaker might let through 15,000 amps in the same timeframe. The amp bonded wire must possess enough thermal mass to absorb this energy without failing.
Worked Numeric Example: Sizing an Amp Bonded Jumper for a 60A Circuit
Let's walk through a real-world scenario to see how amp bonded sizing works, including a critical edge case involving voltage drop.
The Base Scenario:
You are installing a 60A circuit to feed a large motor disconnect or a workshop subpanel using rigid metal conduit. The overcurrent protective device is a 60A two-pole breaker.
- Look up 60A in NEC Table 250.122.
- The minimum size for a copper equipment bonding jumper is 10 AWG.
- The minimum size for an aluminum equipment bonding jumper is 8 AWG.
If you pull 6 AWG copper THHN for your ungrounded (hot) conductors, a 10 AWG copper bonding jumper pulled inside that same conduit is perfectly code-compliant and safely amp bonded.
The Voltage Drop Edge Case (NEC 250.122(B)):
Now, assume the motor disconnect is 250 feet away. To keep voltage drop under 3%, you calculate that you must upsize your circuit conductors from 6 AWG to 4 AWG copper. This is a two-step increase in wire size (6 AWG → 4 AWG).
Under NEC rules, if you upsize ungrounded conductors to compensate for voltage drop, you must proportionately upsize the amp bonded equipment grounding/bonding jumper. You cannot leave it at the base 10 AWG size.
- Base hot wire: 6 AWG → Upsized hot wire: 4 AWG (2 steps up).
- Base bond wire: 10 AWG → Upsized bond wire: 6 AWG (2 steps up).
By applying the proportional increase, your final amp bonded jumper must be 6 AWG copper. This ensures the impedance of the fault path remains low enough relative to the larger circuit conductors to guarantee the 60A breaker still trips instantly at the far end of the 250-foot run.
Where You Meet This in Practice
You will encounter amp bonded sizing requirements in several specific installation scenarios on the jobsite or in the workshop:
- Main Service Panels: The main bonding jumper (MBJ) connects the neutral busbar to the equipment grounding busbar and the panel enclosure. Its size is dictated by the area of the largest ungrounded service entrance conductor, but it functions on the same principle of surviving maximum available fault current from the utility transformer.
- Flexible Metal Conduit (FMC) Transitions: While rigid metal conduit can sometimes serve as the equipment grounding path, flexible metal conduit (like Greenfield) is limited. If the FMC is longer than 6 feet, or if the circuit is rated over 20A, the flex itself cannot be relied upon as the bond. You must pull an internal amp bonded wire alongside the circuit conductors.
- Metal Locknuts on Non-Metallic Enclosures: If you transition from metal conduit into a PVC or fiberglass junction box, the metal locknut on the outside of the box has no electrical continuity to the inside. You must install an amp bonded jumper from the metal locknut to the grounding terminal inside the box.
- Hazardous Locations: In Class I, Division 1 areas (like spray booths or fuel dispensers), standard conduit threads are not trusted for bonding due to the risk of arcing. External amp bonded jumpers with specific listed fittings are required to bridge every conduit joint.
Amp Bonded vs. Grounded: Clearing Up the Confusion
Terminology in the NEC can be dense. As industry experts frequently point out, mixing up bonding and grounding conductors leads to failed inspections and unsafe panels. Here is how the three main safety wires compare:
| Component | Sizing Basis (NEC Reference) | Primary Purpose | Typical Location |
|---|---|---|---|
| Equipment Bonding Jumper (EBJ) | OCPD Amp Rating (Table 250.122) | Ensures electrical continuity between metal parts (e.g., conduit to box) to clear faults. | Conduit transitions, flex connections, panel interiors. |
| Equipment Grounding Conductor (EGC) | OCPD Amp Rating (Table 250.122) | Carries fault current from the load back to the panel to trip the breaker. | Runs inside cable assemblies (like NM-B) or conduit with circuit wires. |
| Grounding Electrode Conductor (GEC) | Largest Service Entrance Conductor Area (Table 250.66) | Connects the electrical system to the earth (ground rods, ufer, water pipe) to stabilize voltage. | Main service panel to exterior ground rods. |
Notice that both the EBJ and the EGC are "amp bonded"—meaning their baseline sizes are derived from the breaker's ampere rating. The GEC, however, is entirely independent of the breaker size and is based purely on the physical thickness of the service entrance wires.
Frequently Asked Questions
How is an amp bonded jumper different from a standard ground wire?
Functionally, they are sized using the exact same NEC table (250.122) and serve the same fault-clearing purpose. The distinction is purely physical location and application. An Equipment Grounding Conductor (EGC) runs the entire length of the circuit from the panel to the load. An Equipment Bonding Jumper (EBJ) is a short, localized wire used to bridge a gap in metallic continuity, such as jumping across a flexible conduit fitting or bonding a metal locknut to a plastic box.
Do I need to upsize my amp bonded wire if I upsize my circuit conductors?
Yes. Under NEC 250.122(B), if you increase the size of your ungrounded (hot) conductors to mitigate voltage drop over a long distance, you must proportionately increase the size of your amp bonded equipment grounding or bonding jumper. If you increase the hot wires by two AWG steps, the bond wire must also increase by two AWG steps to maintain the correct impedance ratio for fault clearing.
Can I use the metal conduit itself instead of an amp bonded wire?
In many standard installations, properly threaded and tightened rigid metal conduit (RMC) or intermediate metal conduit (IMC) is recognized as an effective equipment grounding path, eliminating the need to pull a separate wire. However, you cannot rely on the conduit if you are using flexible metal conduit (FMC) over 6 feet long, if the circuit is over 20A, or if the installation is in a hazardous (classified) location where arcing at conduit joints poses an ignition risk.
What happens if my bonding jumper is undersized for the breaker amps?
If a ground fault occurs, the massive short-circuit current will flow through the undersized jumper. Because the wire lacks the necessary thermal mass, it will act like a fuse and melt or vaporize before the breaker's magnetic trip can physically open the contacts. This leaves the metal enclosure energized at 120V or 240V, creating a severe electrocution hazard for anyone who touches the equipment while standing on a grounded surface.
