The last two of the five bonding related terms defined in Article 100 of the NEC seem like they would be very similar:
- Bonding Jumper, Main
- Bonding Jumper, System
But things are not always as they seem. How do these two types of jumpers differ, and why does that difference matter? The main bonding jumper connects the grounded circuit conductor and the equipment grounding conductor. The place it does this is at the service.
If you have a separately derived system, it needs the same type of connection at the system. Thus, you need a “system bonding jumper.” The key here is to understand that the bonding jumper connection for a system isn’t necessarily going to be done the same way as the main bonding jumper connection.
With a separately derived system, the connection may also be, or alternatively be, to the supply-side bonding jumper. Which of these connections you actually make in a separately derived system depends upon such things as whether the source and the first disconnecting means are in a separate enclosure. You’ll find the grounding and bonding rules for these systems in 250.21, 250.28, 250.30 and 250.35.
As you may have guessed, a facility that has any sort of separately derived system, such as an emergency generator, is going to have both types of jumpers.
