Master Electrician Practice
Chapter III

Grounding & Bonding

Master Electrician Practice study guide with diagrams.

Grounding & Bonding — Colorado Master Electrician Exam Study Chapter

Learning Objectives

By the end of this chapter, you will be able to:

Distinguish between grounding (system and equipment) and bonding, and apply each concept to the correct NEC article.
Size the grounded conductor, grounding electrode conductor (GEC), and bonding jumper for 3-phase, 4-wire services using the correct tables and conditions.
Identify the five recognized grounding electrode systems and the minimum-size requirements for each.
Apply the special rules for separately derived systems (transformers, generators) including the first-disconnect rule and the neutral-to-case bond location.
Calculate feeder and service conductor sizes with the correct demand factors, voltage drop considerations, and overcurrent protection coordination.
Recognize common inspection failures and exam traps related to bonding jumpers, main bonding jumpers, and equipment grounding conductors.

1.1 The Foundational Difference: Grounding vs. Bonding

The single most common source of confusion on the master exam is the distinction between grounding and bonding. They are not interchangeable terms.

Grounding is the intentional connection to the earth (or a body that serves as earth) through a grounding electrode. Its purpose is to limit voltage surges (lightning, line surges) and stabilize the system voltage relative to earth.
Bonding is the intentional connection between metallic parts to establish electrical continuity and conductivity. Its purpose is to clear faults by providing a low-impedance path back to the source, ensuring overcurrent devices operate.

Article 100 definitions are your first stop. The exam will test your ability to apply the correct term to the correct scenario. A classic trap: a green screw connecting the neutral bar to the metal enclosure of a panelboard is a main bonding jumper, not a "grounding screw." The connection to the earth via a ground rod is grounding, not bonding.

1.2 System Grounding: The Service Point (Article 250, Part I & II)

For a 3-phase, 4-wire, wye-connected service (the most common commercial configuration), the system is required to be grounded. The grounded conductor (the neutral) is the conductor that is intentionally connected to earth at the service.

Key thresholds you must memorize:

250.20(B): A 3-phase, 4-wire system supplied from a 480/277V or 208/120V source must have the neutral grounded.
250.24(C): The grounded conductor must be routed with the ungrounded conductors to the service disconnecting means. You cannot "drop" the neutral at the pole or transformer and rely on the earth as a return path — this is a code violation and a common exam trap.

Sizing the Grounded Conductor (Neutral):

Per 250.24(C)(1), the neutral must not be smaller than the required grounding electrode conductor (Table 250.66) but must also be sized to carry the maximum unbalanced load per 220.61.
For a service, the neutral is typically sized at 100% of the unbalanced load, but in no case smaller than the GEC size from Table 250.66.

Sizing the Grounding Electrode Conductor (GEC):

The GEC connects the grounded conductor (neutral) at the service to the grounding electrode (ground rod, water pipe, etc.).
Table 250.66 is your sizing tool. It is based on the size of the largest ungrounded service conductor. For example, if your service conductors are 500 kcmil copper, the GEC must be no smaller than 1/0 copper (or 3/0 aluminum).
Trap: The GEC is sized from the largest ungrounded conductor, not the neutral. If you have parallel sets of 600 kcmil, you use the sum of the circular mil areas (or the equivalent single conductor size) to enter Table 250.66.

1.3 The Main Bonding Jumper and System Bonding Jumper

Main vs System Bonding Jumper — Master Depth Main vs System Bonding Jumper — 250.28 / 250.30(A)(2) Service Disconnect vs Separately Derived System — 2026 NEC / NFPA 70 SERVICE — Main Bonding Jumper NEC 250.28 — at service disconnect Service conductors L1, L2, L3 Grounded (neutral) Service Disconnect Enclosure (first OCPD) MBJ GEC Electrode Sizing: Table 250.102(C)(1) Largest ungrounded conductor → jumper size NOT Table 250.122 Example: 500 kcmil Cu → 1/0 Cu jumper TRAP: Only ONE MBJ per service — never bond neutral to enclosure at more than one point. SDS — System Bonding Jumper NEC 250.30(A)(2) — at SDS first disconnecting means SDS Transformer or gen. L1, L2, L3 Grounded (neutral) SDS Disconnect Enclosure (first OCPD) SBJ EGC Equipment Sizing: Table 250.102(C)(1) Largest ungrounded feeder conductor NOT Table 250.122 Example: 500 kcmil Cu → 1/0 Cu jumper TRAP: SBJ must land at SDS first disconnecting means — never downstream of SDS OCPD. Both use Table 250.102(C)(1) — keyed to largest ungrounded conductor — never equipment grounding Table 250.122 Master Electrician Practice — CO-MST Ch.3 Grounding & Bonding — NEC 250.28 / 250.30(A)(2) — 2026 NEC

The main bonding jumper (MBJ) is the connection between the grounded conductor (neutral) and the equipment grounding conductor (EGC) bus at the service equipment. This is the only place (normally) where the neutral is bonded to the enclosure and the ground bus.

250.28 requires the MBJ to be sized using Table 250.102(C)(1). This table is based on the area of the largest ungrounded conductor or the sum of parallel conductors.
Trap: Do not use Table 250.66 for the MBJ. Table 250.102(C)(1) is a separate table with different values. For a 500 kcmil copper service, Table 250.66 requires a 1/0 GEC, but Table 250.102(C)(1) requires a 2/0 copper bonding jumper. They are not the same.

Where the MBJ is located:

At the service disconnecting means (the main breaker panel).
If the service disconnect is a switchboard with multiple disconnects, the MBJ is located in that switchboard enclosure.

Critical Inspection Point: In a service panel, the neutral bar and the ground bar are bonded together with the MBJ. In a subpanel (downstream of the service disconnect), the neutral bar must be isolated from the enclosure, and the ground bar is bonded to the enclosure. Removing the green bonding screw in a subpanel is a standard inspection failure.

1.4 Grounding Electrode System (Article 250, Part III)

Grounding Electrode Assembly 250.50-250.66 — Master Depth Grounding Electrode Assembly — NEC 250.50 to 250.66 All electrodes present must be bonded together into one system SERVICE EQUIPMENT (first disconnect) GEC termination 250.24(A)(1) GEC 250.66 JB DIELEC UNION ⚠ Metal water pipe 10 ft+ underground 250.52(A)(1) ROD 250.52(A)(5) 25 Ω single-rod rule 250.53(A)(2): if >25 Ω, drive 2nd rod ≥6 ft apart 2nd rod if needed ≥ 6 ft (1.83 m) Concrete-encased electrode 250.52(A)(3) — 20 ft+ rebar Ground ring 250.52(A)(4) GEC Sizing — Table 250.66 Size from largest ungrounded service conductor (per Table 310.16): Service conductor GEC size 2 AWG Cu or smaller 8 AWG Cu 1 AWG to 3/0 AWG 6 AWG Cu ⚠ 250.66(A) rod allowance GEC to a ground rod need not be larger than 6 AWG Cu — even for huge services ⚠ TRAP — 250.68(A) accessibility GEC connections must be accessible — no burying splices in walls or underground Exception: concrete-encased, buried, or direct-burial connections per 250.68(A) Ex ⚠ TRAP — dielectric union / meter Bond on street side of dielectric union. Bonding only at meter is a violation — the water-pipe electrode is then isolated Master Electrician Practice — NEC 2026 Grounding & Bonding: GEC assembly per 250.50–250.66 ⏚

The NEC requires you to bond all electrodes present at the building to form a grounding electrode system. You cannot pick and choose; if a metal underground water pipe exists, it must be used.

The five recognized electrodes (250.50):

44.Metal Underground Water Pipe (250.52(A)(1)) — must be in direct contact with the earth for 10 feet or more. If less than 10 feet, you must supplement with another electrode (typically a ground rod).
45.Metal Frame of the Building (250.52(A)(2)) — must be bonded to the GEC.
46.Concrete-Encased Electrode (Ufer) (250.52(A)(3)) — at least 20 feet of 4 AWG bare copper wire encased in 2 inches of concrete, located near the bottom of a footing. This is the most effective electrode and is required for new construction.
47.Ground Ring (250.52(A)(4)) — at least 20 feet of bare copper, not smaller than 2 AWG, buried at least 2.5 feet deep, encircling the building.
48.Rod and Pipe Electrodes (250.52(A)(5)) — at least 8 feet in length, driven vertically. If rock is hit, you may drive at an angle (not more than 45 degrees) or bury horizontally at least 2.5 feet deep.

Sizing the GEC for a Rod Electrode:

250.66(A) — If the only electrode is a rod, pipe, or plate, the GEC is not required to be larger than 6 AWG copper (or 4 AWG aluminum). This is a common exception that saves material and is frequently tested.

Inspection Point: All electrodes must be bonded together with a single GEC running to the service. You cannot run a separate GEC to each electrode; they must be spliced (using an irreversible compression fitting or exothermic weld) into one continuous conductor or connected via a grounding bus.

1.5 Separately Derived Systems (SDS) — Transformers and Generators

This is the most heavily tested area for the master exam. An SDS is a source of power with no direct electrical connection to the supply conductors (e.g., a transformer with a magnetic isolation, or a generator with a transfer switch that opens the neutral).

Key Rules (250.30):

The Grounding Connection: The system neutral must be grounded at the SDS source (the transformer secondary or the generator) OR at the first disconnecting means of the SDS. You cannot ground it at both locations (this creates a parallel neutral path and circulating currents).
The System Bonding Jumper (SBJ): This is the equivalent of the MBJ for an SDS. It connects the neutral to the equipment grounding conductor at the SDS source. It is sized per Table 250.102(C)(1) based on the largest ungrounded secondary conductor.
The GEC for the SDS: The GEC connects the neutral (at the SBJ location) to the nearest grounding electrode (typically the building steel or a water pipe). Sizing is per Table 250.66 based on the largest ungrounded secondary conductor.

The Neutral-to-Case Bond:

For a transformer feeding a panelboard, the neutral is bonded to the transformer case and the EGC bus at the transformer enclosure. The downstream panelboard must have an isolated neutral.
For a generator used as an SDS (via a transfer switch that switches the neutral), the bond is made at the generator. If the transfer switch does not switch the neutral (solid neutral), the generator is not an SDS, and the neutral-to-case bond is made at the service, not the generator.

Exam Trap: A generator with a 4-pole transfer switch (switching the neutral) is an SDS. A generator with a 3-pole transfer switch (solid neutral) is not an SDS. The bonding location changes accordingly.

1.6 Equipment Grounding Conductors (EGC) — Article 250, Part VI

EGC Sizing & Upsizing per 250.122 — Master Depth EGC Sizing & Upsizing — NEC 250.122 Ground-Fault Current Path • Table 250.122 + 250.122(B) Proportional Upsizing OCPD 100 A Breaker Table 250.122 base EGC: 8 AWG Phase A: 1/0 → 3/0 AWG Cu 105,600 → 167,800 cmil (ratio 1.59) Neutral (not upsized) LOAD Commercial Industrial Equipment EGC (Equipment Grounding Conductor) Ground Ground Fault current returns via EGC (not raceway alone — see 250.118) 250.122(B) Upsizing Calc Phase ratio: 167,800 / 105,600 = 1.59 8 AWG area: 16,510 cmil × 1.59 = 26,240 cmil Next size up: 6 AWG EGC (26,240 cmil ≥ required) Trap: 8 AWG from breaker only = wrong! ⚠ MASTER TRAP Reading EGC only from OCPD size (Table 250.122) and skipping 250.122(B) upsizing ratio → undersized EGC. Metal raceway alone is NOT a compliant EGC unless listed per 250.118 — always verify the EGC path. Master Electrician Practice — NEC 250.122(B) EGC upsizing for voltage drop (CO-MST ch3 Grounding & Bonding) STEP 1 STEP 2

The EGC is the fault-current return path. It runs with the circuit conductors and bonds all non-current-carrying metal parts (conduit, boxes, panel enclosures, motor frames).

Sizing the EGC:

Table 250.122 sizes the EGC based on the rating of the overcurrent device protecting the circuit, not the size of the circuit conductors.
Trap: If you have a 30A breaker protecting a circuit with 10 AWG conductors, the EGC must be sized for a 30A overcurrent device (10 AWG copper). If you upsize the circuit conductors for voltage drop, you must proportionally upsize the EGC per 250.122(B).

Parallel EGCs:

For circuits over 800A, or parallel conductors, the EGC must be run in parallel as well. Each parallel set must include an EGC sized per Table 250.122 based on the overcurrent device rating.

Motor Circuits:

For motor circuits, the EGC is sized based on the rating of the branch-circuit short-circuit and ground-fault protective device (the breaker or fuses), not the running overloads. If you have a 50A breaker protecting a 20A motor, the EGC is sized for 50A.

1.7 Feeder Sizing and Overcurrent Protection Coordination

A master must verify that feeders are properly sized for both ampacity and voltage drop, and that the overcurrent protection is coordinated.

Feeder Sizing (Article 215):

Feeders must have an ampacity of not less than the sum of the noncontinuous loads plus 125% of the continuous loads (215.2(A)(1)).
Continuous Load: A load where the maximum current is expected to continue for 3 hours or more. In commercial settings, lighting and some motors are continuous.

Voltage Drop (215.2(A)(4) Informational Note):

The NEC recommends (not mandates) that feeders and branch circuits have a voltage drop of no more than 3% for the feeder and 3% for the branch circuit, with a total of 5% from the service to the final load. This is an informational note, not a code requirement, but the exam will expect you to know the 3%/5% rule for design purposes.

Overcurrent Protection (240.21):

Tap Rules: A feeder tap is a conductor that connects to a larger feeder without an overcurrent device at the tap point. The most common is the 10-foot tap rule (240.21(B)(1)) — the tap must have an ampacity not less than the combined load, must not exceed 10 feet, and must terminate in a single overcurrent device rated no higher than the tap conductor's ampacity.
25-foot tap rule (240.21(B)(2)) — allows a 25-foot tap if the ampacity is not less than 1/3 of the rating of the overcurrent device protecting the feeder.

Inspection Point: Verify that the sum of the ratings of the branch-circuit breakers in a panelboard does not exceed the rating of the main breaker or the feeder. This is not a code violation (NEC allows it), but it is a design flaw that causes nuisance tripping. The exam will test your understanding of demand factors — you do not need to size the feeder for the sum of all breakers; you size it for the calculated load per Article 220.

1.8 Code Navigation — Where to Find It

ConceptNEC Location
Definitions (Grounding vs. Bonding)Article 100
System Grounding Requirements250.20
Service Grounding (Neutral, GEC)250.24
Main Bonding Jumper250.28
GEC Sizing TableTable 250.66
Bonding Jumper Sizing TableTable 250.102(C)(1)
Grounding Electrode System250.50, 250.52
Separately Derived Systems250.30
Equipment Grounding Conductors250.118, 250.122
EGC Sizing TableTable 250.122
Feeder Sizing215.2
Continuous LoadsArticle 100, 210.19(A)(1)
Tap Rules240.21(B)
Neutral Sizing (Unbalanced Load)220.61
Motor EGC Sizing250.122(D)

1.9 Inspection and Supervision Points

When you are the master on the job, you are responsible for the final sign-off. Here is your on-site checklist:

92.Service Entrance: Is the neutral bonded to the enclosure via the MBJ? Is the GEC connected to the neutral bus and routed to the electrode without splices (unless using an irreversible connector)?
93.Subpanels: Is the neutral isolated from the case? Is the green bonding screw removed? Is the EGC bus bonded to the case?
94.Transformers: Is the secondary neutral bonded to the case and the GEC? Is the primary conduit bonding (if metal) provided at both ends?
95.Bonding of Metal Parts: Are all metal raceways, boxes, and enclosures bonded? Are there any "floating" metal parts that could become energized?
96.Ground Rods: Is the rod driven to 8 feet? Is the clamp listed for direct burial? Is the connection accessible?

1.10 Common Exam Traps

Trap 1: Sizing the GEC from the neutral. Always size from the largest ungrounded conductor.
Trap 2: Using Table 250.66 for bonding jumpers. Use Table 250.102(C)(1) for MBJ and SBJ.
Trap 3: Bonding the neutral in a subpanel. This creates a parallel path for neutral current on the EGC, which is a fire hazard.
Trap 4: Assuming a generator is always an SDS. Check the transfer switch. If the neutral is solid, it is not an SDS.
Trap 5: Sizing the EGC from the conductor size. Always size from the overcurrent device rating.
Trap 6: Forgetting the 125% continuous load factor on feeders and branch circuits.
Trap 7: Using a ground rod as the sole electrode when a concrete-encased electrode exists. You must bond all electrodes present.

1.11 Summary for the Master

Grounding and bonding is the most frequently tested domain on the master exam because it is the most safety-critical. The key to success is not memorizing every table but understanding the function of each conductor and connection. When you see a scenario, ask yourself: "What is the purpose of this connection? Is it to limit voltage (grounding) or to clear a fault (bonding)?" Once you answer that, the correct article and table will follow.

Remember: The service is the point of demarcation. Everything upstream (the utility) is not your responsibility. Everything downstream (the service conductors, the panel, the feeders, the branch circuits) is yours. Know where the neutral is bonded, where it is isolated, and how the fault current will return to the source. If you can trace that path in your mind, you will pass this section.

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