Grounding vs Bonding: The Difference That Decides Many Exams

The 30-second answer

Grounding is the connection to earth: the rod, the water pipe, the building steel. The conductor that does this job is the grounding electrode conductor (GEC).

Bonding is the connection between metal parts that shouldn’t carry current but might end up energized in a fault. The conductor that does this job is the equipment grounding conductor (EGC) inside circuits, and an equipment bonding jumper (EBJ) at specific connection points.

Both are required. They serve different purposes. They’re sized from different tables. Mixing them up loses easy points.

Why the exam tests this

On a 100-question exam you can expect 8 to 12 questions on Article 250. The trap pattern is consistent: the question describes a scenario, names a conductor purpose vaguely, then asks for a size. If you don’t know which conductor is which, you reach for the wrong table and the answer looks wrong by exactly one row.

The grounding electrode conductor (GEC)

The GEC carries the connection from your service grounded conductor (the neutral) to the grounding electrode: the rod, the metal water pipe, the building steel, or the concrete-encased electrode. It only carries current during a fault. Most of its life, it sits there as a low-impedance reference path.

Sizing: Table 250.66, indexed by the largest ungrounded service conductor.

• Service conductor 2 AWG copper or smaller → 8 AWG GEC
• 1/0 through 350 kcmil copper → 2 AWG GEC
• Over 1100 kcmil copper → 3/0 GEC
• For metal water pipe: GEC capped at 3/0 copper regardless of service size (250.66(A)).
• For rod, pipe, or plate electrode: GEC capped at 6 AWG (250.66(A) note).
• For concrete-encased electrode: GEC capped at 4 AWG.

The caps for specific electrode types catch a lot of candidates. The full table answers "what do I need based on service size" but the caps say "you don’t need more than X for this electrode."

The equipment grounding conductor (EGC)

The EGC runs inside every circuit. Its job: provide a low-impedance path back to the service neutral so a ground fault trips the overcurrent device fast. Without an EGC, a ground fault can energize the metal frame of equipment without tripping the breaker. The EGC is the safety conductor.

Sizing: Table 250.122, indexed by the rating of the overcurrent device protecting the circuit.

• 20A OCPD → 12 AWG copper EGC
• 60A OCPD → 10 AWG copper
• 100A OCPD → 8 AWG copper
• 200A OCPD → 6 AWG copper
• 400A OCPD → 3 AWG copper

The upsize rule: If you upsize the phase conductors for voltage drop, you must upsize the EGC proportionally per 250.122(B). This is a frequent exam trap.

The equipment bonding jumper (EBJ)

The EBJ is the conductor used to bond metal that’s outside the standard EGC path. Typically across raceway expansion fittings, between metal conduit sections that aren’t mechanically continuous, or around equipment where the EGC needs help.

Sizing: Per 250.102, generally not smaller than the EGC sized to Table 250.122 for the OCPD ahead of it. For larger applications it follows the same logic but with practical rules for parallel runs and large feeders.

The supply-side bonding jumper (SSBJ)

The SSBJ is the bonding conductor used on the supply side of the service, between the meter, the disconnect, and the service equipment. There’s no overcurrent device on this side yet, so the EGC table doesn’t apply. Use Table 250.102(C)(1), indexed by the ungrounded supply conductor size.

This is a separate table from 250.66 and 250.122. Memorize that three different tables exist. That fact alone prevents the most common exam mistake.

The decision tree

When you see an Article 250 question, ask:

1. Is the conductor going to earth (electrode)? → GEC, Table 250.66.
2. Is the conductor inside a circuit running with the phase conductors? → EGC, Table 250.122.
3. Is the conductor bonding metal across a discontinuity downstream of an OCPD? → EBJ, 250.102 (similar to EGC).
4. Is the conductor bonding metal upstream of the service OCPD? → SSBJ, Table 250.102(C)(1).

What candidates miss most

• Mixing up Table 250.66 (GEC) and Table 250.122 (EGC). Different tables. Different inputs.
• Forgetting the electrode-type caps in 250.66 (rod = 6 AWG max, water pipe = 3/0 copper max).
• Forgetting to upsize the EGC when phase conductors are upsized for voltage drop (250.122(B)).
• Treating the SSBJ like an EGC. The supply side has its own table.
• Calling everything a 'ground' on the job site and assuming the exam uses the same loose vocabulary.