Main Bonding Jumper [MBJ] is used to connect switchboard neutral bus and switchboard ground bus. MBJ is a very critical piece in the electrical service panel, as it helps provide stable phase-neutral voltage as well as provide low impedance path to ground fault current back to source transformer. Loose or otherwise missing MBJ can lead to unstable phase-neutral voltage if transformer X0 is not grounded. MBJ plays critical part in routing feeder side ground fault current to the neutral conductor and back to transformer X0 without which ground fault protection is compromised.
![Main bonding jumper [MBJ] and Neutral disconnect link [NDL] in a service entrance panel](https://voltage-disturbance.com/wp-content/uploads/2023/10/Figure-1.png)
The only location at which service neutral is permitted to be grounded is in the service entrance equipment by using MBJ. Exception to this rule is discussed in this article. If neutral is illegally grounded downstream of MBJ, excessive ground current can result causing nuisance ground fault trip.
Read: Open Neutral-Voltage fluctuation and Stray voltage
![Main bonding jumper [MBJ] and Neutral disconnect link [NDL]](https://voltage-disturbance.com/wp-content/uploads/2023/10/Figure-2.png)
NEC 250.30 permits connecting neutral to ground at a single point, located between source and first disconnecting means or overcurrent device. Bonding can also be done at the source transformer if there are no disconnecting means.
NEC 250.30 does allow an exception to this rule and allow neutral ground at the source and at the first disconnect if the service is fed by an outdoor transformer. The condition for this exception is that two neutral to ground connections should not result in parallel current path. Path through physical earth is not considered as parallel path. Using this exception, an outdoor padmount transformer X0 can be grounded outdoors first and secondly MBJ can be used to ground the neutral in the service panel only if equipment grounding conductors are not installed and non-metallic raceways are used. Note that bonding neutral downstream of service main is not permitted under any condition.
Neutral of downstream transformers fed from service panel are also bonded in a similar fashion, but it is called system bonding jumper [SBJ]. Secondary side of such transformers are separately derived sources. System bonding jumper perform similar function as MBJ, only difference being MBJ is in service entrance panel whereas SBJ is located on the feeder side.
Read: Neutral CT with electronic trip units
Requirements for MBJ are given in NEC 250.28. MBJ need to be un-spliced and can be made of wire, busbar or screw. MBJ can be made of copper or other corrosion resistant material. Aluminum alloys are also permitted where environment is acceptable. When MBJ is a screw, it shall be identified with a green finish that shall be visible with the screw installed.
Sizing MBJ
In listed switchboard, switchgear or MCC, main bonding jumper is provided by the manufacturer and will be sized based on UL safety standard for the equipment. If MBJ needs to be sized for any other reason, NEC table 250-66 can be used as mentioned in section 250-28 (D).
Neutral Disconnect Link
Neutral Disconnect Link [NDL] is provided in service entrance equipment to isolate neutral bus from ground bus and from transformer neutral for testing and troubleshooting purpose. Once NDL is opened, presence of any illegal neutral-ground bond on the feeder sections can be checked easily by using a continuity tester. Under normal conditions, NDL should always be connected. Neutral disconnect links are not required in switchboards that are not meant for service entrance use. See figure 2.
NDL is typically in the form of a busbar. NEC 230.75 article permits NDL to be in any section of a multi section switchboard or switchgear, provided that corresponding section is marked.