What Is a Bonding Jumper? Grounding vs Bonding Explained

Grounding and bonding do different jobs

Grounding connects the electrical system to the earth. Its purposes include limiting voltage from lightning and line surges, and giving the system a reference. The conductor that makes that connection is the grounding electrode conductor.

Bonding connects metal parts to each other: enclosures, raceways, service equipment, and metal piping. Done properly, it creates a low-impedance path back to the source, so that when a fault puts voltage on a metal part, the overcurrent device sees enough current to open quickly. The conductor that carries that fault current on a branch circuit is the equipment grounding conductor.

The earth is a poor return path for fault current. That is why the bonding path to the source matters more for clearing a fault than the rod in the ground. Keep that in your head and a lot of exam questions answer themselves.

The kinds of bonding jumper

Your 2026 NEC names several, and the names point to the job:

• Main bonding jumper. Connects the grounded conductor and the equipment grounding conductor at the service, in the service equipment.
• System bonding jumper. Does the same at a separately derived system, such as a transformer, as covered by 250.30.
• Equipment bonding jumper. Connects two or more parts of the equipment grounding conductor, for example around a flexible connection.
• Supply-side bonding jumper. Used on the supply side of the service disconnect, and sized from its own table.

We are citing by name and section number only. Read the definitions in Article 100 in your own book, because the wording matters on the exam.

Which table sizes which conductor

This is where points are lost. Our grounding electrode conductor guide covers the sizing steps, and the short version is:

• Grounding electrode conductor: 250.66 and Table 250.66, entered with the size of the largest ungrounded service conductor (or the equivalent area for parallel sets).
• Equipment grounding conductor: 250.122 and Table 250.122, entered with the rating of the overcurrent device ahead of the circuit.
• Supply-side and main bonding jumpers: 250.102 and Table 250.102(C)(1), which sends you back to the size of the ungrounded conductors.

Different table, different entering quantity. Before you turn a page, say which conductor the question is about and what you will enter the table with. It takes ten seconds and prevents the commonest error.

A worked sorting exercise

Without any numbers, name the table for each of these:

1. The conductor from a service panel to a ground rod. Table 250.66.
2. The conductor run with a 60 ampere branch circuit to a metal junction box. Table 250.122, from the 60 ampere device.
3. The jumper that ties the neutral bar to the enclosure in service equipment. The main bonding jumper under 250.102.
4. The conductor from a transformer's secondary neutral to the building steel. Look to 250.30 for a separately derived system.

If you can sort ten of these quickly, you have the structure. Then the sizing is a table read.

Why the exam keeps asking

Grounding and bonding live inside the services area, with 10 scored items across both portions, and inside the wiring methods and equipment areas too. The Calculations portion also asks for sizing from these tables. See the exam outline for how the items split.

Read the whole condition

Many options on grounding items are right for some other condition. A rod-only connection may cap the grounding electrode conductor size. A feeder upsized for voltage drop changes the equipment grounding conductor but not the grounding electrode conductor. When you work an item, read to the end of the stem before choosing a table. The bulletin also tells candidates not to apply exceptions unless a question directs them to, and to treat conductors as copper unless told otherwise.

Test yourself

Take the free 10-question check. It needs no signup, and each explanation tells you which article and table the answer relies on, so grounding and bonding gaps show up fast.

FAQ

What is a bonding jumper?

A conductor that connects metal parts so they are at the same potential and a fault current has a path back to the source.

What is the difference between grounding and bonding?

Grounding connects the system to the earth. Bonding connects metal parts to each other and to the source for fault clearing.

Which table do I use for a grounding electrode conductor?

Table 250.66. For an equipment grounding conductor, Table 250.122.