8.4 Bonding Requirements, Main/System Bonding Jumpers & Separately Derived Systems (NEC 250.28, 250.30 & 250.102)

Bonding Requirements, Main/System Bonding Jumpers & Separately Derived Systems (NEC 250.28, 250.30 & 250.102)

Bonding is the deliberate, permanent physical joining of metallic parts to establish electrical continuity and conductivity. Its mission is two-fold: prevent dangerous touch potential voltages on metal raceways and piping, and provide a heavy-duty fault path capable of conducting immense short-circuit currents without burning open. On the Oklahoma Journeyman examination, mastering the Main Bonding Jumper (MBJ) sizing rules, Separately Derived System (SDS) transformer connections under NEC 250.30, and piping bonding under NEC 250.104 is essential for high test scores and safe field installations.

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1. The Main Bonding Jumper (MBJ; NEC 250.28)

The Main Bonding Jumper is the vital link inside the service disconnecting means that connects the grounded circuit conductor (neutral) to the equipment grounding conductor and the service enclosure chassis.

+---------------------------------------------------------------------------------------------------+
|                         MAIN BONDING JUMPER (MBJ) SPECIFICATIONS (NEC 250.28)                     |
|                                                                                                   |
|   CONSTRUCTION TYPES (250.28(A))                    IDENTIFICATION (250.28(B))                    |
|   - Wire conductor (copper or aluminum)             - Green bonding screw with hexagonal head     |
|   - Copper or aluminum busbar                         that is clearly visible after installation. |
|   - Listed screw (green-finished)                                                                 |
|                                                                                                   |
|   SIZING METHOD (250.28(D) & NEC Table 250.102(C)(1))                                             |
|   - Phase conductors ≤ 1100 kcmil Cu: Direct lookup in Table 250.102(C)(1).                       |
|   - Phase conductors > 1100 kcmil Cu: Mandatory 12.5% Circular Mil Rule.                          |
+---------------------------------------------------------------------------------------------------+

Sizing the MBJ & The 12.5% Rule (NEC 250.102(C)(1) Note 1):

$\text{If } \text{Area}_{\text{phase}} > 1,100\text{ kcmil Cu} \implies \text{Area}_{\text{MBJ}} \ge \text{Area}_{\text{phase}} \times 0.125\ (12.5\%$

Worked Sizing Example: A 2,500-Ampere, 480V service has six 500 kcmil copper conductors per phase in six parallel conduits ($6 \times 500 = 3,000\text{ kcmil}$ total per phase):

1. Total area per phase = $3,000\text{ kcmil}$ ($3,000,000\text{ cmil}$).
2. Because $3,000\text{ kcmil} > 1,100\text{ kcmil}$, apply the 12.5% rule: $\text{Required MBJ Area} = 3,000\text{ kcmil} \times 0.125 = 375\text{ kcmil}$
3. Looking up standard conductor sizes in Chapter 9, Table 8:
   • 350 kcmil = $350,000\text{ cmil}$ (Too small)
   • 400 kcmil = $400,000\text{ cmil}$ (Compliant: $400\text{ kcmil} \ge 375\text{ kcmil}$).
4. Result: The Main Bonding Jumper must be not smaller than 400 kcmil Copper.

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2. Separately Derived Systems (SDS; NEC 250.30)

A Separately Derived System is an electrical premises source whose power is generated on-site, having no direct electrical connection (including a solidly connected neutral conductor) to supply conductors originating in another system.

• Common Examples: Step-down transformers (e.g., 480V delta primary to 208Y/120V wye secondary), solar PV standalone inverters, and backup generators equipped with a 4-pole transfer switch that switches the neutral conductor.
• Non-SDS Generator: If a backup generator feeds a 3-pole transfer switch where the neutral is solidly connected through to the main service, the generator is NOT a separately derived system and must NOT have a bonding jumper installed at the generator frame.

+---------------------------------------------------------------------------------------------------+
|                SEPARATELY DERIVED SYSTEM (TRANSFORMER) GROUNDING ARCHITECTURE                     |
|                                                                                                   |
|        480V 3-Phase Primary Feeder                                                                |
|        ================================+                                                          |
|                                        |                                                          |
|                                        v                                                          |
|                      +---------------------------------------------+                              |
|                      |       480V - 208Y/120V DRY TRANSFORMER      |                              |
|                      |                                             |                              |
|                      |   [Secondary Wye Coils]                     |                              |
|                      |         |                                   |                              |
|                      |         +----> Neutral (X0 Terminal)        |                              |
|                      |                   ||                        |                              |
|                      |                   || <=== SYSTEM BONDING    |                              |
|                      |                   vv      JUMPER (SBJ)      |                              |
|                      |         +--[Enclosure Ground Bar]           |                              |
|                      |         |         |                         |                              |
|                      +---------|---------|-------------------------+                              |
|                                |         |                                                        |
|         Supply-Side Bonding    |         | Grounding Electrode Conductor (GEC)                    |
|         Jumper (SSBJ)          |         | (Sized per Table 250.66)                               |
|         (Table 250.102(C)(1))  |         |                                                        |
|                                v         v                                                        |
|                     [208Y/120V Panel]  { Building Steel / Metal Water Pipe within 5 ft }          |
+---------------------------------------------------------------------------------------------------+

Key Separately Derived System Rules (NEC 250.30(A)):

1. System Bonding Jumper (SBJ; NEC 250.30(A)(1)): An unspliced SBJ connects the derived neutral ($X_0$) to the equipment grounding conductor and metal enclosure. It must be installed at only one single location—either at the transformer source or at the first derived disconnect/panelboard, but NEVER at both.
2. Supply-Side Bonding Jumper (SSBJ; NEC 250.30(A)(2)): Where the SBJ is located in the first panelboard rather than inside the transformer, a metal raceway or wire SSBJ sized per NEC Table 250.102(C)(1) must connect the transformer enclosure to the panelboard enclosure.
3. Grounding Electrode Conductor (GEC; NEC 250.30(A)(5)): Each SDS must connect to a grounding electrode via a GEC sized per NEC Table 250.66 based on the derived secondary phase conductors. The GEC must connect to the nearest accessible building structural steel or metal water pipe within 5 feet of building entry.

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3. Bonding of Piping Systems & Structural Metal (NEC 250.104)

Metallic piping systems and exposed structural steel frames throughout a building are subject to accidental energization from frayed wiring, lightning surges, or ground faults. NEC 250.104 establishes strict sizing criteria:

+---------------------------------------------------------------------------------------------------+
|                         BONDING OF BUILDING SYSTEMS MATRIX (NEC 250.104)                          |
|                                                                                                   |
|  SYSTEM / STRUCTURE               SIZING CODE TABLE          SIZING BASIS                         |
|  -------------------------------  -------------------------  -----------------------------------  |
|  1. Interior Metal Water Piping   NEC Table 250.102(C)(1)    Largest ungrounded service conductor |
|     (NEC 250.104(A))                                         (Capped at 3/0 Cu in Table)          |
|                                                                                                   |
|  2. Other Metal Piping            NEC Table 250.122          Rating of the specific branch circuit|
|     (Gas, compressed air, etc.)                              likely to energize the piping.       |
|     (NEC 250.104(B))                                                                              |
|                                                                                                   |
|  3. Structural Metal Frame        NEC Table 250.102(C)(1)    Largest ungrounded service conductor |
|     (NEC 250.104(C))                                                                              |
+---------------------------------------------------------------------------------------------------+

Gas Piping Bonding Distinction (NEC 250.104(B)):

• Standard Rule: Interior metal gas piping is considered bonded if it is connected to equipment (such as a gas furnace, gas water heater, or gas range) that is supplied by an electrical branch circuit with a properly sized equipment grounding conductor (per Table 250.122).
• Corrugated Stainless Steel Tubing (CSST): Yellow CSST gas piping systems have thin walls susceptible to puncture from lightning-induced arcing. Manufacturer installation instructions and local Oklahoma amendments frequently mandate a dedicated #6 AWG copper direct bonding jumper connected between the rigid gas manifold and the service equipment enclosure.

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4. Service Raceway & Enclosure Bonding (NEC 250.92)

Because service conductors operate ahead of the main circuit breaker, a ground fault occurring in a service raceway is backed by the full short-circuit fault current of the utility grid. Mechanical locknuts cannot withstand these massive fault energies without blowing apart.

+---------------------------------------------------------------------------------------------------+
|                       SERVICE RACEWAY BONDING METHODS (NEC 250.92(B))                             |
|                                                                                                   |
|   PERMITTED BONDING METHODS                         PROHIBITED FOR SERVICE RACEWAYS               |
|   -----------------------------------------------   --------------------------------------------  |
|   1. Threaded hubs on enclosures made up wrench-    1. Standard locknuts with concentric or       |
|      tight.                                            eccentric knockouts.                       |
|   2. Threadless couplings/connectors made up tight. 2. Standard double locknuts on service conduit |
|   3. Listed bonding bushings with bonding jumpers      punched with oversized knockouts.          |
|      sized per Table 250.102(C)(1).                                                               |
|   4. Bonding locknuts (on clean punchings without                                                 |
|      eccentric/concentric knockouts).                                                             |
+---------------------------------------------------------------------------------------------------+

The Concentric / Eccentric Knockout Rule (NEC 250.92(B) & 250.97):

Where service conduits enter a sheet metal enclosure through concentric or eccentric knockouts (multi-ring factory punchings), the electrical continuity of the remaining metal rings is unreliable under severe fault currents. Electricians must install a listed bonding bushing equipped with a bonding jumper wire routed to the service grounded neutral bus to guarantee an unbroken, low-impedance fault clearing path.