Main Bonding Jumper
A main bonding jumper (MBJ) is the intentional, single-point electrical connection between the grounded circuit conductor (neutral) and the equipment grounding conductor (EGC), or the supply-side bonding jumper, or both, at the service or supply-side disconnect. Its function is to create a low-impedance path for ground-fault current so overcurrent devices operate correctly and metallic equipment stays at the same potential as the grounded conductor.
On the shop floor, the main bonding jumper is typically hidden inside the main service disconnect or main panel. It ensures ground-fault clearing by providing a low-impedance loop for fault current to trip breakers promptly. It also maintains equipotential bonding of all metalwork, reducing shock risk. The MBJ is usually a factory-installed green bonding screw or a copper/aluminum strap connecting the neutral bar to the EGC bar or enclosure. It must be installed unspliced in the service enclosure. Downstream subpanels must have isolated neutral and EGC, with no additional MBJ, to maintain a single fault-current path. Inspectors verify its presence, adequacy, and absence of extra neutral-ground bonds.
How do I identify the main bonding jumper in an existing service panel?
Look inside the service disconnect or main breaker panel that receives utility service conductors. Identify the neutral bar (connected to the grounded service conductor) and the equipment grounding bar and metal enclosure. The MBJ will typically be a green bonding screw through the neutral bar into the metal enclosure, or a short conductor or strap connecting the neutral bar to the EGC bar or enclosure. Confirm it is unspliced and located in the service equipment enclosure, not elsewhere.
How is sizing of the main bonding jumper determined in an industrial service?
MBJ size is based on the largest ungrounded service conductor serving that equipment, per NEC 250.102 rules. It must be capable of carrying full available ground-fault current long enough for the protective device to operate. In large manufacturing plants (e.g., 2000 A services), the MBJ is often a substantial copper bar or conductor to meet required ampacity.
What is the difference between a main bonding jumper and a system bonding jumper or supply-side bonding jumper?
The main bonding jumper (MBJ) connects the grounded circuit conductor (neutral) to the EGC or supply-side bonding jumper at the service or supply-side disconnect. A system bonding jumper is a similar connection at a separately derived system (e.g., transformer secondary) between that system's grounded conductor and its equipment bonding terminal. A supply-side bonding jumper bonds service raceways and enclosures on the supply side of the service disconnect.