6.2 Grounded Conductors, Main Bonding Jumpers & System Bonding Jumpers

6.2 Grounded Conductors, Main Bonding Jumpers & System Bonding Jumpers

Exam Fast Fact: When taking the Colorado Journeyman Electrician examination, remember that Table 250.102(C)(1) is your master reference for sizing supply-side bonding jumpers, main bonding jumpers (MBJs), system bonding jumpers (SBJs), and service grounded conductors under NEC 250.24(C). Never use Table 250.122 (Equipment Grounding Conductors) or Table 250.66 (Grounding Electrode Conductors) to size a main bonding jumper!

In every electrical distribution system, the grounded conductor (neutral) serves a dual personality. Under normal operating conditions, it is a current-carrying circuit conductor carrying unbalanced return current back to the source. Under ground-fault conditions, it becomes the primary fault-clearing conductor, carrying massive short-circuit current from the service enclosure back to the utility transformer winding. Managing this dual role requires rigorous adherence to conductor identification, supply-side routing, bonding jumper installation, and the strict prevention of downstream neutral-to-ground bonds.

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Grounded Conductor Identification (NEC Article 200)

NEC Article 200 governs the identification of terminals and conductors intended to be connected to the grounded neutral of an electrical system. On the PSI exam, questions regarding re-identification and color codes appear frequently.

Small Conductors: 6 AWG and Smaller (NEC 200.6(A))

An insulated grounded conductor of 6 AWG or smaller must be identified by one of the following means along its entire length:

1. A continuous white outer finish.
2. A continuous gray outer finish.
3. Three continuous white or gray stripes along the conductor's entire length on other than green insulation.
4. For aerial cable, a ridge or groove on the exterior of the conductor.

Critical Rule: You cannot take a black, red, or blue 10 AWG or 8 AWG wire in a conduit and wrap white phase tape on the ends to make it a neutral! Conductor re-identification in field raceways is strictly reserved for conductors 4 AWG and larger.

Large Conductors: 4 AWG and Larger (NEC 200.6(B))

An insulated grounded conductor of 4 AWG or larger may be identified by:

1. A continuous white or gray outer finish.
2. Three continuous white or gray stripes along its entire length.
3. A distinctive white or gray marking (typically white phase tape) encircling the conductor at its terminations and at all accessible points where the conductor is visible (pull boxes, junction boxes, and panelboard gutters).

Rules for Cable Assemblies (NEC 200.7(C))

In nonmetallic-sheathed cable (Type NM) or armored cable (Type AC/MC), a conductor with white insulation is permitted to be re-identified as an ungrounded (hot) conductor only in specific applications:

• Switch Loops: Where the white conductor is permanently re-identified with black tape at both ends and used as the supply feed to the switch (never as the switched return conductor from the switch to the luminaire).
• 240-Volt Single-Phase Loads: Feeding straight 240V appliances (water heaters, baseboard heaters, well pumps) where both circuit conductors are ungrounded, provided the white conductor is re-identified with colored tape at every accessible point.

Absolute Prohibition: A green conductor or bare wire can never be re-identified as a grounded neutral or ungrounded hot wire, and a white conductor can never be used as an equipment grounding conductor!

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The Mandatory Service Grounded Conductor (NEC 250.24(C))

One of the most heavily tested questions on state journeyman exams involves the requirement to bring a grounded conductor to a service disconnecting means, even when no neutral loads exist.

UTILITY TRANSFORMER (480Y/277V)             SERVICE DISCONNECT ENCLOSURE (480V 3-Phase)
┌────────────────────────────┐              ┌──────────────────────────────────────────┐
│ Phase A ───────────────────┼─────────────>│ Phase A Bus ─────────────> 480V 3-Ph Load│
│ Phase B ───────────────────┼─────────────>│ Phase B Bus ─────────────> (No Neutral)  │
│ Phase C ───────────────────┼─────────────>│ Phase C Bus ─────────────>               │
│ Neutral (X0)               │              │                                          │
│     │                      │              │ Neutral Terminal Block                   │
│     └──────────────────────┼─────────────>│       │                                  │
│       MANDATORY GROUNDED   │   NEC        │       │ <── MAIN BONDING JUMPER (MBJ)    │
│       CONDUCTOR            │ 250.24(C)    │       ▼                                  │
└────────────────────────────┘              │ Enclosure Chassis (Ground Bus)           │
                                            └──────────────────────────────────────────┘

The Rule of NEC 250.24(C)

Where an alternating-current system operating at 1000 volts or less is grounded at any point (such as at the utility pole-mount or pad-mount transformer), the grounded conductor must be run to each service disconnecting means and bonded to the enclosure.

Why Bring the Neutral if the Building Has No 120V or 277V Loads?

Consider an industrial pumping station supplied by a 480Y/277V three-phase utility service. The building contains only 480V three-phase motors and three-phase industrial heaters. No 277V lighting or 120V receptacles exist. Why must the contractor pay to pull a large copper neutral wire that terminates on an isolated block in the main disconnect?

Because if a motor coil shorts to the metal conduit or equipment chassis, that fault current must return to the X0 neutral terminal of the utility transformer winding to complete the circuit. If the grounded conductor is omitted from the service drop/lateral:

1. The ground-fault current would have to flow from the building's ground rods through the soil to the utility pole ground rod.
2. As proven in Section 6.1, soil resistance limits current to just a few amperes, which will never trip a 400A or 800A service circuit breaker.
3. The entire building's conduit system, switchgear enclosures, and motor frames would remain energized at 277V to ground indefinitely.

Minimum Sizing of Service Grounded Conductor (NEC 250.24(C)(1))

The grounded conductor brought to the service equipment must be sized according to NEC Table 250.102(C)(1) based on the size of the largest ungrounded service-entrance conductor.

The 12.5% Calculation Rule for Large Services

Where the ungrounded service-entrance conductors exceed 1100 kcmil copper or 1750 kcmil aluminum, Table 250.102(C)(1) directs the electrician to Note 1: the grounded conductor and main bonding jumper cannot have an area less than 12.5% of the total cross-sectional area of the largest ungrounded phase conductor.

Worked Sizing Example: A 2500A commercial service entrance consists of five (5) parallel sets of 600 kcmil copper conductors per phase. What is the minimum size required for the service grounded conductor?

1. Calculate total ungrounded area per phase: $\text{Total Area} = 5 \times 600\text{ kcmil} = 3000\text{ kcmil}$
2. Because 3000 kcmil exceeds 1100 kcmil, apply the 12.5% rule: $\text{Minimum Area} = 3000\text{ kcmil} \times 0.125 = 375\text{ kcmil}$
3. Referring to NEC Chapter 9, Table 8 (Conductor Properties), 375 kcmil is not a standard conductor size. Sizing must be rounded up to the next standard wire size: 400 kcmil copper.

Grounded Conductor in Parallel Raceways (NEC 250.24(C)(2))

When service conductors are installed in parallel in two or more raceways, the grounded conductor must also be paralleled. The grounded conductor in each raceway must be sized based on the size of the ungrounded conductors in that specific raceway from Table 250.102(C)(1), but in no case smaller than 1/0 AWG copper or 3/0 AWG aluminum.

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Main Bonding Jumper (MBJ) Engineering (NEC 250.28)

The Main Bonding Jumper (MBJ) is the single most critical component in any electrical service. It is the physical conductor, screw, or busbar that connects the equipment grounding conductors and the service-disconnect enclosure to the grounded circuit conductor (neutral).

                      SERVICE MAIN DISCONNECT
┌─────────────────────────────────────────────────────────────────┐
│                                                                 │
│   [GROUNDED NEUTRAL BUS]              [GROUND BUS / CHASSIS]    │
│            │                                    │               │
│            │                                    │               │
│            └───────────────[ MBJ ]──────────────┘               │
│                       (NEC 250.28)                              │
│                 • Table 250.102(C)(1)                           │
│                 • Wire, Busbar, or Green Screw                  │
│                 • Visible Green Finish (250.28(B))              │
│                                                                 │
│    Incoming Neutral                     Equipment Grounding     │
│    from Utility                         Conductors (EGCs)       │
│                                                                 │
└─────────────────────────────────────────────────────────────────┘

Construction and Material Requirements (NEC 250.28(A)-(D))

• Permitted Types (250.28(A)): May be a wire, busbar, screw, or similar suitable conductor.
• Green Screw Identification (250.28(B)): Where a screw is used as the main bonding jumper, it must have a green finish that is clearly visible when installed.
• Material (250.28(C)): Must be copper, aluminum, copper-clad aluminum, or other corrosion-resistant material.
• Sizing (250.28(D)): Sized from Table 250.102(C)(1) using the largest ungrounded service conductor (or equivalent area of parallel conductors).

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System Bonding Jumpers (SBJ) for Separately Derived Systems (NEC 250.30(A)(1))

A Separately Derived System (SDS) is an electrical premises wiring system whose power is derived from a generator, transformer, or converter windings and that has no direct electrical connection (including a solidly connected neutral) to supply conductors originating in another system.

Sizing the System Bonding Jumper

Under NEC 250.30(A)(1), the system bonding jumper connects the equipment grounding conductors of the SDS to the derived grounded conductor (X0 neutral). Like the MBJ, it is sized strictly from Table 250.102(C)(1) based on the largest derived phase conductor.

The Single-Location Rule

The SBJ must be installed at only one point on a separately derived system. It may be installed at:

1. The source (inside the transformer secondary terminal compartment), OR
2. The first system disconnecting means or overcurrent protective device (the secondary panelboard).

Critical Rule: Installing the SBJ at both the transformer and the secondary panelboard creates a prohibited parallel neutral path that violates NEC 250.6.

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The Fatal Downstream Neutral-to-Ground Bond (NEC 250.24(A)(5) & 250.6)

Every electrical inspector, master electrician, and PSI exam question enforces this absolute command: Never bond the neutral to ground downstream of the main service disconnect or SDS first disconnect!

         SERVICE DISCONNECT                             REMOTE SUBPANEL
┌─────────────────────────────────┐           ┌─────────────────────────────────┐
│ Neutral Bus ─────── Ground Bus  │           │ Neutral Bus        Ground Bus   │
│      │                  │       │           │      │                  │       │
│      │   [MBJ BONDED]   │       │           │   [FLOATING]         [BONDED]   │
│      │                  │       │           │   NO BONDING         CHASSIS    │
│      │                  │       │           │     SCREW!                      │
│      │                  │       │           │      │                  │       │
└──────┼──────────────────┼───────┘           └──────┼──────────────────┼───────┘
       │                  │                          │                  │
       │ Feeder Neutral   │                          │                  │
       └──────────────────┼──────────────────────────┘                  │
                          │ Feeder Equipment Ground (EMT / Wire)        │
                          └─────────────────────────────────────────────┘

What Happens if a Green Screw is Left in a Subpanel?

If an installer mistakenly threads the green bonding screw into the neutral bar of a 100A or 200A downstream subpanel:

1. Parallel Return Paths: The neutral bar is now electrically connected to the metal panel enclosure. The metallic conduit (EMT) and wire-type EGC run back to the main service panel in parallel with the insulated neutral wire.
2. Circulating Neutral Currents: Under normal conditions, single-phase 120V loads produce return current. Instead of returning 100% through the insulated neutral conductor, the return current divides between the neutral wire, the metal conduit, the building steel, water pipes, and metal junction boxes in inverse proportion to their impedances.
3. Continuous Touch Potential: Exposed metal conduits and appliance covers carry continuous amperage, creating persistent shock hazards.
4. Fire from Arcing Couplings: Electrical conduit couplings are designed to carry short-circuit fault current for fractions of a second; they are not rated to carry continuous neutral return current. Corroded or loose set-screw couplings will arc, overheat, and ignite surrounding building framing.
5. Nuisance Tripping: Ground-Fault Circuit-Interrupters (GFCI) and Arc-Fault Circuit-Interrupters (AFCI) monitor current balance between hot and neutral. Stray currents returning via metal conduit cause immediate nuisance tripping.

In all subpanels, the neutral bar must float (remain completely insulated from the steel cabinet), and all EGCs must terminate on a separate ground bar screwed directly into the chassis.

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Jobsite Scenarios & Common Exam Traps