Mike Dolt Enterprises
Mike Dolt Enterprises
User: Pinnie, Location: Ohio
Asked on: Sunday, March 2, 2025 at 10:13 am EST
Answered on: Thursday, March 6, 2025 at 9:15 am EST
Question: So I am trying to understand the different terminology for all the bonding jumpers. I understand SBJ is where you bond your secondary neutral to ground. I understand that your main bonding jumper is just connecting egc. I understand egc is just metal parts connected together. Gec is your grounding electrode conductors. I’m getting hung up on SSBJ.I understand what the 2023 NEC definition says“ a conductor installed on the supply side of a service with an service equipment enclosure, or for a separately derived system that ensures the required electrical conductivity between metal parts required to be electrically connected. “But in the diagram it’s on what I would have thought was not the supply side. Is it the supply side of the secondary because it’s a seperately derived system? Thank you in advance.
You've got a good handle on the basic bonding and grounding terminology. Let's break down the System Bonding Jumper (SSBJ) and why it might seem confusing in a separately derived system (SDS) context.
Key Idea: "Supply Side" Relative to the SDS
You are correct in thinking of "supply side" in the context of the secondary (output) of the transformer (SDS). The SSBJ is on the "supply side" of the separately derived system itself.Here's a more intuitive way to think about it, aligning with the NEC definition: SSBJ's Job
To establish a low-impedance path for fault current to return to the source of the separately derived system. This is crucial for proper overcurrent device operation (breakers tripping) when a fault occurs.
Why "Supply Side"? The SSBJ connects the SDS's neutral point (typically the X0 terminal on the transformer) to the metal parts that need to be bonded together before the service disconnect/overcurrent protection device on the SDS. Think of it as "pre-load protection" bonding.
Why the Confusion in Diagrams?
Diagrams can sometimes be misleading because they depict physical layout, not necessarily electrical functionality. The SSBJ might appear to be on the "load side" if you're looking at the transformer output and assuming that's the point of reference.
Think of it this way
Power Source
The transformer is the power source for the separately derived system.2.
SSBJ's Location
The SSBJ is located at the transformer (typically in the same enclosure or very nearby) and connects the neutral (X0) to the transformer enclosure and any other metal parts that must be bonded.3.
Fault Path
When a fault occurs on the SDS, the fault current needs a direct path back to the transformer's neutral point to trip the breaker that protects the SDS. The SSBJ provides that path, ensuring a low impedance return path for fault current.
In Summary
The SSBJ is on the "supply side" relative to the separately derived system because it's located between the source of the separately derived system (the transformer) and the service disconnect/overcurrent device that protects the system. It's about creating a low-impedance fault current path back to the transformer for proper overcurrent device operation. Flag for review