Why So Many Bonding Jumpers?

Bonding Jumpers in General

In this article we will discuss the different type of bonding jumpers and what they are used. NEC article 250 covers the application of bonding jumpers and all the different type. There are essentially 4 types of bonding jumpers discussed in article 250:

  1. Main Bonding Jumper
  2. System Bonding Jumper
  3. Supply side Bonding Jumper
  4. Equipment bonding Jumper

Let’s start off with some definitions first:

Bonded (Bonding): Connected to establish electrical continuity and conductivity. Bonding establishes an effective path for fault current so that overcurrent protection devices can operate properly to clear a fault condition.

Bonding Conductor or Jumper: A reliable conductor to ensure the required electrical conductivity between metal parts required to be electrically connected.

Bonding Jumpers, Equipment: The connection between two or more portions of the equipment grounding conductor.

Bonding Jumpers, Main: The Connection between the grounded circuit conductor (Neutral) and the equipment grounding conductor, or the supply-side bonding jumper, or both at the service.

Bonding Jumper, Supply-Side: A conductor installed on the supply side of a service or within a service equipment enclosure(s), or for a separately derived system, that ensures the required electrical conductivity between metal parts required to be electrically connected.

Bonding Jumper, System: The connection between the grounded circuit conductor (neutral) and the supply-side bonding jumper or the equipment grounding conductor, or both at a separately derived system.

Main Bonding Jumper

NEC 250.24 tells us that all wiring systems supplied by a grounded AC service shall have a grounding electrode conductor connected to the grounded service conductor or neutral at each service. It is important to note that a grounded conductor shall not be connected to normally non-current carrying metal parts of equipment, to equipment grounding conductor(s) or be reconnected to the ground on the load side of the service disconnecting means as per NEC 250.24(A)(5). In other words, the Neutral must be keep separate from the equipment ground on the load side of the service disconnect. It must also not be re-connected to the grounding electrode or grounding electrode conductor on the load side of the service disconnect. 

The way we connect the Neutral conductor to the equipment grounding conductor(s) and the service disconnecting means enclosure is by using the main bonding jumper. As per NEC 250.28 the bonding jumper can be either a screw, bus, or wire. Many times, equipment will come with a green screw that will go through the neutral bus and thread into the back of the enclosure. This is your main bonding jumper and creates the connection between the equipment ground bus and neutral bus through the enclosure itself.

System Bonding Jumper

System Bonding jumpers are very similar to main bonding jumpers as they provide the same function. System bonding jumpers are used when dealing with separately derived systems. In most facilities or building a separately derived system will usually be either a transformer or an onsite generator. System bonding jumpers are permitted to be made at the source or at the first disconnecting means or overcurrent protection device as per NEC 250.30(A)(1).

The system bonding jumper connects the grounded conductor to the supply-side bonding jumper and the normally non-current-carrying metal enclosure when installed at the source. 

The system bonding jumper connects the grounded conductor to the supply-side bonding jumper, the disconnecting means enclosure and the equipment grounding conductor(s) when installed at the first disconnecting means.

Supply-Side Bonding Jumper

If the source and of a separately derived system and first disconnecting means are located in separate enclosures a supply-side bonding jumper shall be installed with the circuit conductors from the source enclosure to the first disconnecting means enclosure. The Supply-side bonding jumper is essentially an equipment grounding conductor; it just has a slightly different name since it is used for a separately derived system. Below are 2 examples showing different configurations of supply-side bonding jumpers.

Equipment Bonding Jumper

An Equipment bonding jumper is a conductor used to bond multiple equipment housings together. For example, a motor bonded to a motor controller housing. It is sized the same way as an equipment grounding conductor using NEC 250.122.

Sizing of Bonding Jumpers

All bonding jumpers besides equipment bonding jumpers are sized using NEC Table 250.102(C)(1). For parallel sets of conductors, we size based on the sum of the single largest conductor in each set. Let’s look at an example.

Let’s say we had to size a single supply side bonding jumper for five sets of four 350kcmil copper conductors in a three inch conduit.  

Step 1: Take the largest size of conductor in each set and add them up: 1 x 350kcmil x 5 sets = 1750kcmil.

Step 2: The total exceeds 1100kcmil for copper as per Table 250.102(C)(1), so we follow note 1 found below the table, and multiply the total by 12.5 percent: 1750kcmil x .125 = 218.75kcmil. This means that we go to the next size up, which is 250kcmil copper.

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