Let's break down the situation and the code requirements

The Situation

You have a transformer, and the picture shows a section of what appears to be the Supply-Side Bonding Jumper (SSBJ) on the secondary side. You're asking if a splice in this SSBJ is permissible under the 2017 NEC.

NEC 2017 Requirements for Supply-Side Bonding Jumpers Purpose

The Supply-Side Bonding Jumper is crucial. It connects the equipment grounding conductor(s) (EGCs) to the grounded conductor (typically the neutral) at the source of the separately derived system (SDS), which in this case is your transformer's secondary. This connection creates a low-impedance path for fault current to return to the source, facilitating the operation of overcurrent protective devices (circuit breakers or fuses) during a ground fault.

NEC 250.30(A)(1): General Requirements"The supply-side bonding jumper shall be sized in accordance with 250.102(C), based on the derived phase conductors. The supply-side bonding jumper shall be permitted to be installed in the same raceway or enclosure with the derived phase conductors." NEC 250.30(A)(2): Installation."The supply-side bonding jumper shall be permitted to be run bare or insulated, and shall be installed in accordance with 250.64(A), (B), and (E)." NEC 250.64(E): Installation of Equipment Grounding Conductors"Equipment grounding conductors shall be installed in accordance with 250.64(A), (B), and (C). NEC 250.64(C): Continuous"Except as permitted in 250.30(A)(1), 250.30(A)(2), 250.68(C)(1), and 250.142(B), equipment grounding conductors shall be continuous. Splices or joints shall be made as specified in 110.14(B)." NEC 110.14(B) Splices"Conductors shall be spliced or joined with splicing devices identified for the use or by brazing, welding, or soldering with a fusible metal or alloy. Soldered splices shall first be mechanically joined such as to be electrically and mechanically secure before soldering. All splices and joints and the free ends of conductors shall be covered with an insulation equivalent to that of the conductors or with an identified insulating device."Analysis and Conclusion1. Splicing is Potentially Permitted, but Highly Discouraged

The NEC allows for splices in equipment grounding conductors, provided that the splice meets the requirements of 110.14(B). This means the splice must be made with a device identified for the use, and it must be both mechanically and electrically sound.
2.

Good Practice Dictates Avoiding Splices

While technically permitted under very specific conditions, splicing a Supply-Side Bonding Jumper is generally poor practice and should be avoided if at all possible. The reason is reliability. A failure at the splice point can significantly impede the fault current path, potentially delaying or preventing the operation of overcurrent protection and creating a dangerous situation.3.

Compliance Depends on the Quality of the Splice

If the splice in the picture is done with a properly listed connector specifically identified for grounding conductors (e.g., a compression connector approved for grounding and bonding), and it's installed correctly (properly torqued if required), then technically, it could comply with the 2017 NEC.4.

AHJ is the Final Authority

The Authority Having Jurisdiction (AHJ), typically the local electrical inspector, has the final say. Even if the splice technically meets the code, the AHJ might object if they feel it compromises safety or reliability.

Recommendations Replace the Entire Conductor

The best solution is to replace the spliced Supply-Side Bonding Jumper with a single, continuous conductor of the correct size. This eliminates any potential issues with the splice.

Consult the AHJ

If replacing the conductor isn't feasible, definitely consult with your local electrical inspector (AHJ) before doing anything. Explain the situation and ask if the proposed splicing method is acceptable. Get their approval in writing if possible.

Use a Listed Grounding Connector

If splicing is the only option and the AHJ approves, use a compression connector that is specifically listed for grounding and bonding applications. Follow the manufacturer's instructions for installation meticulously.

Document Everything

Keep records of the connector used, the manufacturer's instructions, and any communication with the AHJ.

In summary: While the NEC 2017 technically permits splicing the SSBJ if done correctly, it's strongly discouraged due to potential reliability issues. The best practice is to run a continuous conductor. If splicing is unavoidable, consult with the AHJ and use a listed grounding connector, following the manufacturer's instructions. Proper documentation is critical. Flag for review